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Published Project ReportPPR348

A Review of Literature on the Nature of the Impact of Roadworks on Traffic Movement and Delay

N BourneW GillanS O NotleyN TaylorD Webster

A Review of Literature on the Nature of the Impact of Roadworks on Traffic

Movement and Delay

by N Bourne, W Gillan, S O Notley, N Taylor and D Webster

PPR 348

2/462_109

PUBLISHED PROJECT REPORT

Transport Research Laboratory

PUBLISHED PROJECT REPORT PPR 348


by N Bourne, W Gillan, S O Notley, N Taylor and D Webster (TRL)

Prepared for: Project Record: 2/462_109

Analysis of Benefits of Maintaining the Network Following JTR Constraints (Transportation Review)

Client: Highways Agency, Network Services

(Ramesh Sinhal)

Copyright Transport Research Laboratory July 2008

This report has been prepared for Highways Agency. Published Project Reports are written primarily for the Client rather than for a general audience and are published with the Client’s approval.

The views expressed are those of the author(s) and not necessarily those of Highways Agency.

Name Date

Approved

Project Manager

Simon Notley 1/07/2008

Technical Referee

Tim Rees 1/07/2008

Published Project Report

TRL PPR 348

If this report has been received in hard copy from TRL, then in support of the company’s environmental goals, it will have been printed on recycled paper, comprising 100% post-consumer waste, manufactured using a TCF (totally chlorine free) process.


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Contents

Executive summary 1

Abstract 3

1 Introduction 4

2 Study themes 5

3 Economic impact of roadworks delays 6

4 Causes of roadworks delays 8

4.1 Queuing delays 8 4.2 Speed reduction 9 4.3 Incidents 10

5 Reducing roadworks delays 11

5.1 Understanding capacity and delay at works sites 11 5.2 Optimising capacity 14

5.2.1 Driver behaviour 14 5.2.2 Controlling merge behaviour 15 5.2.3 ITS control measures 17 5.2.4 Dynamic site management 18

5.3 Incident management 18 5.4 Efficient Completion of the Works 19 5.5 Diversion and Demand Management 20

5.5.1 Overview 20 5.5.2 Tactical Diversion 21 5.5.3 Strategic diversion 22 5.5.4 Inhibiting or Time Shifting Travel 23

6 Conclusions 24

6.1 Discussion of findings 24 6.2 Concluding remarks 25

Acknowledgements 26

References 26


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Executive summary Roadworks on the motorway and trunk road network generate traffic delays, often significantly greater than those which would have occurred had the road remained unobstructed. This results in costs to road operators and road users, as well as the economy as a whole. Literature places the total cost of congestion on UK roads at £20 billion per year. The amount of this which is due to roadworks on the HA network is not well established, but may be in the region of £250 million (2002 prices). Assuming traffic continues to grow, these costs are likely to increase in future. The non linear relationship between queuing and traffic demand means that the increase in percentage terms is likely to be greater than the traffic increase. On the other hand, it is essential that the road network is maintained and updated. The key question is how that can be achieved while minimising the negative impact of the works. In this report, current knowledge of the processes that give rise to delay as a result of roadworks is reviewed.

A large body of literature exists on roadworks and the effect they have on traffic movements and delay. By studying this literature, looking both at work on the impact of roadworks, and some of the measures proposed to mitigate that impact, it has been possible to gain an insight into the underlying causes of delay at roadworks.

The delay experienced by vehicles passing through a works site can be broken down into two components: delay caused by reduced speed in the works (caused either by increased traffic density or the enforcement of a speed limit) and the delay caused by queuing to enter the works. The latter can be considerably exacerbated by the occurrence of accidents within the works; some sources suggest that incidents are the primary cause of congestion at works sites. It is also possible that delay may be experienced by vehicles which divert onto alternative routes to avoid the roadworks. The subsequent increase in traffic on the diversion route may also cause delays to other vehicles on these routes.

Evidence suggests that whilst accidents have potential to cause severe delays at any time, delay due to queuing is greater than that due to speed reduction during busy periods, but during less busy off-peak periods speed reduction remains a source of significant delay whereas queuing is less apparent.

Queuing is the result of demand exceeding capacity, therefore the fact that works sites have a lower capacity than the unobstructed road can be cited as one of the key causes of delays at roadworks. However, capacity is found to be an elusive quantity dependent on the collective effect of individual driver behaviour. It can be measured in a number of ways and is affected by numerous other variables including weather conditions, proportion of heavy vehicles and the presence of police.

A number of sources attempt to quantify the capacity of works sites in terms of their physical characteristics. The capacity of a site is typically found to be primarily dependent on the number of lanes, with capacity being broadly proportional to the number of running lanes. Evidence also shows that narrow lanes have smaller capacity than full-width ones. This suggests that there must be an optimum number of lanes for a given width of road which maximises capacity. On a three-lane carriageway where one lane is closed, it is generally only feasible to run two full-width lanes or three narrow lanes. However, on the wider carriageways that are now becoming more common, there will be a greater number of options available.

The capacity of a single lane is primarily determined by drivers’ perceived safe headway at the prevailing speed. There is evidence that influencing driver speed and headway, using either VMS or in-car technology can increase capacity.

Evidence reveals the merge area, where the number of lanes is reduced, to be the major cause of capacity restriction in works. The effect of this lane drop is to disrupt the fairly homogenous nature of normal traffic flow and drivers appear to adopt a number of behaviours not seen in normal traffic flow. For instance, dense platoons of queuing traffic have been observed to form as drivers already in the queue attempt to prevent others


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from joining in front of them. When this platoon speeds up, drivers attempt to space out to achieve a perceived safe headway, resulting in stop-start traffic.

The idea that the merging behaviour restricts capacity is supported by a number of studies of controlled merges. Significant improvements in capacity and reductions in delay were achieved using both early-merge (where vehicles are encouraged to merge and cease overtaking behaviour some distance in advance of the site) and late-merge (where vehicles are encouraged to use all lanes until the beginning of the closure) systems. The fact that both systems were found to deliver benefits indicates that it is the inconsistency between drivers in uncontrolled merge behaviour that causes capacity to drop. The increase in consistency introduced using these methods was also shown to reduce accident risk and therefore the associated delays.

There are contradictory findings on whether the side of the lane drop (i.e the off-side of near-side lane is closed) affected capacity. There is some evidence that capacity is greater when the off-side (fast) lane is closed and vehicles merge to the near-side, particularly where the traffic consists of a large proportion of heavy vehicles. However, other findings indicate that merging to the off-side results in higher capacity. This indicates that the nature of the traffic approaching the site and the way it is distributed across the carriageway may have a bearing on capacity and therefore affect delay. There is also evidence that segregating different vehicle types within the site can improve capacity.

It is not clear to what extent the capacity within the works site is greater than that of the merge area. One source (Orth-Rodgers and Associates Inc., 1999) found a 15% increase in capacity using a late-merge system, suggesting that in this case there was significant residual capacity within the works before this scheme was implemented. If the loss of capacity due to merging behaviour could be removed, then the full capacity of the works site could be utilised. This would seem to be a strong argument for the use of narrow lanes (already very common in the UK) as they often allow the full number of running lanes to be retained, therefore removing the need for a merge.

The delay due to queuing at works may be mitigated to some extent if demand is reduced. There is a considerable body of literature on the subject of demand management, often based on influencing travel behaviour by providing information to the public. However, drivers may also choose to divert or not to travel without any intervention if they are aware that the works cause delays. This may mean that delay is self-limiting to some extent, with fewer journeys occurring as delay gets worse. The effect of diversion is less obvious, as drivers will still experience delay compared to their normal journey on the main-route, but this delay will be distributed across many alternative routes.

It is the conclusion of this review that the causes of delay at roadworks are bound up with the complexities of driver behaviour. It is the drivers’ need to maintain a perceived safe headway that limits the capacity within the site (and indeed on roads in general) and the drivers’ behaviour at the merge point which can often limit the capacity of a site as a whole. The creation of a capacity restriction results in queuing which in turn gives rise to delay. Even in the event that demand is less than capacity, drivers can choose to reduce their speed when traversing the works site either because a speed limit is enforced, or because increased traffic density requires a drop in speed to maintain perceived safe headway, again giving rise to delay. Further complexity is introduced when the drivers’ travel choices are considered as they may choose to divert or change their travel plans. This means that delay due to a given site may stretch beyond the immediate area of that site; the nature and extent of this is again a function of behavioural patterns. It is by discovering the determining factors of driver behaviour, and understanding how these influence traffic flow, that one may begin to understand and address the underlying causes of congestion.


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Abstract Roadworks on the motorway and trunk road network generate traffic delays, often significantly greater than those which would have occurred had the road remained unobstructed. This results in costs to road operators and road users, as well as the economy as a whole. On the other hand, it is essential that the road network is maintained and updated. The key question is how that can be achieved while minimising the negative impact of the works. In order to address this question, it is necessary to gain an understanding of the processes which give rise to delay at roadworks.

A large body of literature exists on roadworks and the effect they have on traffic movements and delay. By studying this literature, looking both at work on the impact of roadworks, and some of the measures proposed to mitigate that impact, it has been possible to gain an insight into the underlying causes of delay at roadworks. This report presents a review of currently available literature and attempts to bring together current understanding of the field.


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1 Introduction Roadworks on the motorway and trunk road network generate traffic delays, often significantly greater than those which would have occurred had the road remained unobstructed. This results in costs to road operators and road users, as well as the economy as a whole. Assuming traffic continues to grow, these costs are likely to increase in future. The non linear relationship between queuing and traffic demand means that the increase in percentage terms is likely to be greater than the traffic increase. In addition, the delay at roadworks is sensitive to traffic parameters and detailed changes in the design of the roadwork. That means that the variability of journey times increases in the presence of roadworks.

Hence the existence of roadworks adds to congestion and network inefficiency. On the other hand, it is essential that the road network is maintained and updated. The key question is how that can be achieved while minimising the negative impact of the works. In this report, current knowledge of the processes that give rise to delay as a result of roadworks is reviewed.

Roadworks, and the delays and accidents they cause, are not a new problem. Octavius (c. 100), writing to his brother Candidus at the Roman fort of Vindolana, used the state of the road to Catterick as a reason for not collecting a wagon load of hides. More recently, road construction and repair methods have been a feature of the TRL research programme since it was founded, as TRRL, in the 1930s. That research led eventually to the production of Chapter 8 of the Traffic Signs Manual (Department for Transport, 2008c) which gives extensive guidance on the implementation of works. It is now required that contractors take account of the delays at works when planning their activities. That may well become of increasing importance as new DBFO contracts come into play. These are likely to contain performance related payment elements linked to the Highways Agencies objectives.

There is currently greater focus on traffic delays and journey time variability than ever before. Traffic levels across the network are approximately double the values in the late 1980s and congestion is a major issue. For example Eddington (2006) has estimated the national economic cost of traffic congestion as £20 Billion.

Roadworks are also likely to be a key determinant in whether the Department for Transport (DfT) and Highways Agency (HA) meet the SR2004 Public Service Agreement target (PSA1) to reduce congestion on the interurban road network in England below 2004/5 levels by 2007/8. The PSA1 congestion target is designed to measure journey time reliability, and is based on the 10% most delayed daytime journeys for a defined set of routes and departure times across the HA network. As such, it is strongly influenced by abnormally long journeys, such as those during roadworks (DfT, 2007).

Hence it is an appropriate time to review the research on the delays and accidents that roadworks cause, to determine whether there are new ideas or approaches that might lead to savings. The results of this review could lead to new approaches or procedures for managing roadworks, including ideas for use of ITS (Intelligent Transport Systems) technology.


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2 Study themes The literature review reported herein has been based on a search of the International Transport Research Documentation (ITRD) database at TRL, supplemented by a range of unpublished TRL reports and scans of internet search engines. In the case of the latter the aim was to identify “grey” literature i.e. typically short articles and press reports which might not be discovered in the search of formal publications.

The following key study themes were identified in the literature:

(a) Literature concerned with setting a value on the “problem”, determining the delays that roadworks cause and the impact on the economy and road users.

(b) Methods of completing the works more quickly than before. That might involve the use of innovative materials or procedures.

(c) Methods of minimising the road width required for the works and of minimising the capacity1 loss.

(d) Dynamic site management schemes, which modify the layout to maximise the residual capacity of the works: this includes tidal flow layouts and systems which move barriers quickly.

(e) Methods of quickly clearing accidents and breakdowns at roadwork sites to restore lost capacity.

(f) Maximising works capacity, or throughput, by modifying driver behaviour, for example at the merge and the approaches to the works.

(g) Minimising delays by managing demand on the route in ways which do not involve diversion.

(h) Managing demand by encouraging drivers to avoid the stretch of road where queuing occurs.

Section 3 of this report contains a brief discussion of the literature in area (a). This gives an impression of the impact of roadworks on a national scale. The impact of roadworks on a more local scale is considered in Section 4, which addresses the general ways in which roadworks can influence traffic movement and delay. The ideas established in Section 4 are explored in more detail in Section 5, which looks in detail at a number of the themes listed above. The majority of the literature discussed in Section 5 is focussed on the development or assessment of specific measures intended to reduce delay and/or improve safety at works sites. Whilst it is not within the scope of this report to recommend such measures, the extent to which they were successful is an indicator of how accurately they addressed the underlying issues. Therefore, by taking a holistic view of research into reducing delay at works, it is possible to gain greater insight into the underlying causes of delay.

1 The term “capacity” is often used to describe the maximum possible traffic flow rate (or throughput) on a network link. That definition is misleading, because it implies that it is solely a property of the link. In practice the maximum throughput is a function of many variables including the prevailing weather conditions, traffic composition and driver behaviour, as well as the link geometry. In the case of roadworks reducing carriageway width however, it is generally true that the resulting capacity reduction is substantially greater than these variations. Nevertheless, these caveats must be considered when this quantity is treated as a constant in roadworks assessments.


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3 Economic impact of roadworks delays There are alternative methods of quantifying the impact of the delays caused by congestion at roadworks. The traditional concern of Government is the economic impact, the value of delays, at individual works level and nationally. That is generally assessed by calculating the relevant lost time and applying the value of time figures published by the DfT in WebTAG Unit 3.5.6 (DfT, 2008b). Individuals, either professional drivers or private motorists, tend to be more concerned with the delay and uncertainty introduced into their individual journey. That concern led to the Development of the PSA1 congestion target for DfT and HA, to make journeys on the strategic road network more reliable by 2007/8 (DfT, 2007).

There is a substantial body of literature on methods of valuing time. That is of marginal relevance in this study and has not been included. The most comprehensive estimate of the overall cost of congestion is that included within the Eddington Transport Study (Eddington, 2006), which put the total cost of all congestion due to all causes on UK roads at £20 billion. That is a theoretical figure; it does not mean that if that congestion were eliminated the economy would be better off by that amount. However, it would justify significant changes in vehicle use and the demand for alternative means of transport.

This macroeconomic approach to valuing congestion is interesting but, again, is of marginal relevance to this study. However there are useful reviews of the methodologies and background to this type of study by Goodwin (2004) and commissioned by the Scottish Executive (Grant-Muller and Laird, 2005).

There are few references to work on valuing the delays caused by roadworks on the strategic road network. One early example was a TRL Working paper published in 1988 but based on 1986 data which was widely quoted by DfT, including the Chief Highway Engineers Department (the forerunner of the HA). That concluded that the total congestion cost due to motorway roadworks was about £45 million at the 1987 time value of £5.50 per hour (Robertson, 1988). A further calculation suggested that the total cost of delays at roadworks on all purpose roads, rural and urban, was £180 million. For the purpose of this paper we have assumed that half of those delays occurred on the non motorway strategic road network. The total cost of delays at roadworks on the full strategic network would then have been £135 million pa. The value of time in WebTAG 3.5.6, using national average vehicle proportions for 2002, is £11.28 per hour for the average vehicle, just over double the 1987 value. Hence, on that basis, the scaled value of the cost of roadworks on the strategic network would be about £275 million.

This figure of £275 million is indicative but needs to be treated with some caution. Traffic levels have approximately doubled since 1986, which would tend to increase the delays at works. On the other hand there has been a considerable focus on managing works efficiently in the period since then, in part spurred on by the introduction of QUADRO2;the first version of that was only coming into general use at the start of the period.

The first on-line measure of traffic delays across the network became possible with the development of queue measuring systems, notably the HA MIDAS system, concentrated on motorways, and the Trafficmaster queue detector network, which also included major non motorway roads. For example in 1997 a Trafficmaster press release indicated ”The problem of congestion on Britain's motorways is getting worse. There has been a 69% increase in traffic on motorways over the last ten years and 32 million man hours were lost through motorway congestion in the first quarter of 1997 alone (source: Trafficmaster Congestion Index)”. Details of the methodology used to derive these figures are no longer available. If taken at face value, however, they suggest that the annual delays at the time were about 128 million man hours, or about 110 million

2 QUADRO (Queuing and Diversion at Roadworks) is a software tool developed by TRL for the DfT, for the economic assessment of roadworks. It is the Highways Agency’s and DfT’s standard roadworks assessment tool. QUADRO is described by DfT (2002).


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vehicle hours per annum at typical occupancies. A separate TRL project (Frith, 1999), based on examination and analysis of the Trafficmaster data concluded that about 20% of the losses for all journeys in 1995/96 were due to roadworks. On that basis the cost of congestion due to roadworks on the strategic network would be about £150 million using the prevailing value of time, about £7 per vehicle hour, or about £250 million at 2002 prices. However the work also suggested that the proportion of delays due to roadworks was dropping rapidly over the period of the study, to about 10% by 1998/99. Unpublished work by TRL (Isherwood and Frith, 2005) found that roadworks were responsible for around 40% of delay on the journeys that would have contributed to the PSA1 target3 on the M25 J7-16. If this section is representative of the network, this would suggest that roadworks tend to cause greater delay to a smaller number of vehicles than other sources of delay and therefore have a proportionally greater impact on the PSA1 target than on total delay for all journeys.

One surprise in this study is that we have not detected any current results on the costs of delay focussed on the strategic network although Goodwin (2005) has considered the costs of congestion at utilities street works and has discussed other literature. The estimates of the costs of the congestion he identified range from £350 million pa to £4.3 billion, the latter from a DfT commissioned study by Halcrow. At first sight the latter figure seems extremely high but in reality it corresponds to the average vehicle spending about 20 minutes every week queuing at roadwork sites. That may still be more than most people’s experience.

Hence, there is a wide range of estimates of the congestion costs at roadworks. As indicated above it is slightly surprising that more effort has not been made to produce an accurate, up to date figure. The delays are estimated up front for works which need to be assessed by QUADRO, and a national compilation of those results would be possible. In principle modern technology in the form of ANPR (Automatic Number Plate Recognition) linked to traffic detectors should be able to produce an on-line running estimate of the congestion costs while the works are in progress. Indeed the HA is developing a project which includes some of the equipment which would be required. That is entitled “Information for motorists on transit times through long term roadworks”. Such a project could be modified to produce an online record of the congestion costs, as well as the traffic delays. The Agency also holds a comprehensive database of journey times and flows on motorways and trunk roads within HATRIS (Highways Agency Traffic Information System), which could potentially be combined with knowledge of recent or historical roadworks to measure their impact.

One application of accurate congestion costs would be that they would give an indication of the accuracy of QUADRO. More critical, however, is that they would improve the accuracy of the overall costs of congestion, which ultimately feed into the figures such as the £20 billion quoted by Eddington (2006). That is shaping major policy decisions in areas such as demand management; accurate estimates of the delays and consequent congestion costs in the components of that figure would help focus the debate.

Roadworks are also a key determinant in whether DfT and HA meet the PSA1 target to reduce congestion on the interurban road network in England below 2004/5 levels by 2007/84. The DfT annual report recognises that “some routes experience large changes in either direction, some of which might be explained by roadworks being started or completed”. Hence it seems likely that the format of the PSA target places a greater focus on reducing the delays at roadworks than consideration based purely on the costs of congestion.

3 Note that this work used an early version of the PSA1 definition under which only the most delayed 5% or journeys rather than 10% contributed to the target. 4 Since this report was drafted, it has been announced that the HA have failed to meet the PSA1 target. The DfT annual report (2008d) suggests that the greater number of schemes underway in the target year as opposed to the baseline year may have contributed to this.


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4 Causes of roadworks delays Roadworks introduce traffic delays on the road network through three principal mechanisms:

1. Delay which occurs because the speed through the roadworks is less than the free speed of traffic. For example a vehicle which traverses a 1 mile length of road at 50 mph, a typical speed in a roadworks, takes 20 seconds longer than if it were travelling at the 70 mph speed limit prevailing on the unobstructed route.

2. Queuing delays at pinch points, generally the start of the roadworks, whenever the traffic demand – typically measured in vehicles per hour or passenger car units (pcu) per hour – exceeds the residual capacity of the works site.

3. Incidents either caused by the presence of the works (for example by sudden slowing of traffic joining the back of the queue) or exacerbated by the presence of the works (primarily those incidents within the reduced capacity section where there is no hard shoulder)

4.1 Queuing delays

At first sight roadwork delays are relatively simple. The key parameters which determine delays at a works site are:

• the duration of the works;

• the traffic demand approaching the works;

• the residual capacity of the works;

• the propensity of drivers to divert on to alternative route, if they know of one, which, in turn depends on;

• the availability and suitability of diversion routes.

The demand and capacity, both of which may vary with time, determine the characteristics of the upstream queue, which is the most obvious manifestation of delay, at least in peak times. That queue forms at the entrance of the roadworks, the point at which the road capacity reduces (known as the bottleneck). The queue forms when the demand traffic flow approaching the roadworks exceeds the capacity of the merge area. In simple models the instantaneous queue size, measured as the number of vehicles queuing, is formed by integrating the excess traffic over the time from which the demand exceeded the capacity to the measurement point. The front of the queue stabilises at the point where the capacity drops from that of the unobstructed road to that of the works site. The queue begins to clear when the demand drops below the capacity of the site and is fully cleared when the cumulative excess flow equals the cumulative spare capacity.

That is an extremely simple queuing model; effectively it assumes that the traffic stacks up vertically at the entrance to the works; these models are described as ‘vertical’ queuing models. In practice the queue grows upstream backwards from the works and as it grows it sweeps up more traffic. Hence more vehicles are involved in the queue than suggested by the simple model. Models that take this into account are known as ‘horizontal’ queuing models. These models typically predict up to 20% more delay per vehicle when demand is close to normal capacity (based on a deterministic queuing scenario using the vertical and horizontal models developed by Taylor, 2006).

In these models the queue is initiated at the bottleneck and its effects only propagate upstream. However, Hunt and Yousif (1994) observed that congestion propagated downstream as well, as drivers were attempting to re-establish safe stopping distances. Though the bottleneck exists only at the entrance to the works section, slow-moving or stop-start traffic was observed throughout the works section. This potentially provides


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another source of delay that is often not considered, because it depends on individual driver behaviour and is difficult to model. The various factors involved with queuing delay and capacity are discussed in sections 5.1 and 5.2.

A vertical queuing model forms the basis of the QUADRO software tool for predicting delays at roadworks and assessing their cost. QUADRO also includes a methodology for calculating the effective capacity of the route, and for modelling diversion routes. This tool is currently used widely within the UK industry to allow the cost of delay to be taken into account when planning works and considered alongside the direct works costs; as such it serves a valuable role. However, it is a relatively simple tool based on a vertical queue and single diversion route calibrated using the results of empirical investigations and research, some of it conducted almost twenty years ago. Whilst QUADRO addresses all the sources of delay identified in Section 4, it is not clear if the delays predicted are consistent with those measured by the abundant detection equipment now installed on the HA network. Furthermore, the results of any queuing model depend critically on the specified capacity. It is not clear whether changes in site layout, vehicle design or driver behaviour over the last twenty years have led to a significant change in the general capacity of works sites. This question is addressed in more detail in Section 5.1.

4.2 Speed reduction

Jiang (1999b) compares the estimated delay costs at work zones arising from a number of factors:

• Deceleration delay cost – the cost of lost time due to forced deceleration

• Reduced speed delay cost – the cost of lost time while travelling through the works zone at a lower speed

• Acceleration delay cost – the cost of lost time while accelerating back to the desire speed after exiting the works zone

• Vehicle queue delay cost – the cost of lost time spent queuing

• Excess cost of speed change – the additional operating cost to vehicles caused by accelerating and decelerating

• Excess running cost of vehicle at reduced speed – the change in vehicle running cost resulting from varying fuel consumption at different speeds

The highest component of cost during congestion was found to be the vehicle queue delay cost. However, the reduced speed delay cost was also substantial, and in uncongested times, when demand was less than capacity, the reduced speed delay cost was the greatest of the costs considered. Other costs were relatively insignificant, and the excess running cost was negative, since the running cost of vehicles is lower at the reduced speeds.

An estimate can be made of the scale of delays caused by speed reduction: for example, a vehicle which traverses a 1 mile length of road at 50 mph, a typical speed in roadworks, takes 20 seconds longer than if it were travelling at the 70 mph speed limit prevailing on the unobstructed route. At a flow of 60,000 vehicles per day (a typical figure), that represents a loss of 330 vehicle hours per day. This is offset by economic gains of vehicles travelling at lower speeds, due to the reduced fuel consumption – Jiang’s “excess running cost”. Typically the saving in fuel costs per km of travelling at 50mph rather than at 70mph is 23% for an average car, and 39% for an average LGV (DfT, 2008b).

The total delay losses in the works site will depend on its length, the number of vehicles affected per day, and the “reference speed” of traffic on the unobstructed road. It is not necessarily this simple however – as previously mentioned, slow moving and stop-start traffic can persist through the roadworks if flow breakdown occurs at the bottleneck. This


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is due to the behaviour of drivers merging with other traffic, which can depend on speeds, headways and variability within the traffic stream as it approaches and merges. Further discussion can be found in section 5.2.

4.3 Incidents

The effect of an incident on capacity is much greater in roadworks than at other locations, and incidents are more likely to cause congestion at roadworks than excess demand flow (Hunt and Yousif, 1994). Observations by Hunt et al. (1991) confirmed that incidents are the major cause of congestion at motorway maintenance sites. A reduction in throughput was also noted on the opposing carriageway as drivers slowed to view the incident. The effect of incident duration and capacity on the time taken for a queue to disperse is demonstrated by Hunt and Yousif (1994).

Approximately 4.6 million recorded and unrecorded incidents occur each year on the national motorway network (Roberts et al. 1994), and 81% of those viewed on a video would not have been recorded in police records due to their short duration and nature. Approximately 6% are estimated to have the potential to affect flows on the main running carriageway. Roberts et al. (1994) reported overall weighted mean incident duration was approximately 47 minutes. The weighted mean bottleneck capacity of 1670 pcu per lane per hour was 25% lower than the theoretical unobstructed link capacity. Both these figures are based on all incidents, not just those occurring within works.

The Federal Highway Administration (FHWA) Incident Manual (PB Farradyne, 2000) reports that traffic incidents are a major source of both highway congestion and safety problems in the US. Incidents are estimated to cause approximately half of all traffic delay. Crashes that result from other incidents account for approximately 16% of all crashes and cause 18% of freeway deaths.

Coombe and Turner (1989) reported on accident data and traffic flows, with and without roadworks present in Norfolk and Berkshire. The roads were trunk roads (26 sites), A-class (54 sites), B-class (45 sites) and C-class (30 sites). For the 80 sample sites on single carriageway trunk and A-class roads, roadworks increased the accident rate by 172%, which was statistically significant at the 99% level of confidence.

Hayes et al. (1994) reported that there was an increase in personal injury accident rate of 130% at the 22 motorway roadwork sites sampled compared to the same site without works. Hayes and Taylor (1993) reported an increase in the personal injury accident rate of 14.5% at 26 all-purpose dual-carriageway sites. They suggest that works at these sites have less impact on accident rates than on motorway sites due to the lower traffic levels and the fact that driver’s on all-purpose roads are “more used to changes in driving mode”.

In contrast, Freeman et al. (2004) concluded that the evidence produced by their study showed that the risk when roadworks are present is now similar to the risk when no roadworks are present. However this assessment is only associated with personal injury accidents and there is no evidence to suggest that the effect is the same for damage only accidents.

Tarko and Venugopal (2001) found that shorter work zones had a significantly higher risk of crashes on the approach compared with longer work zones and also that the greater the intensity of work (defined as cost/(duration x length)), the greater the accident risk.

Some of the studies discussed in section 5.2 have found solutions which reduce the risk of accidents. Section 5.3 includes discussion of incident management schemes, which aim to reduce the impact when an incident does occur.


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5 Reducing roadworks delays

5.1 Understanding capacity and delay at works sites

For the purposes of modelling and estimation of delays, the term capacity is often used to refer to a static single-valued parameter of the road. It is often defined as the maximum sustainable traffic flow5, but in fact capacity is a very elusive quantity which is dependent on a large number of variables and can be difficult to measure.

Hunt et al. (1991) reported on the merge and crossovers at motorway roadworks. Without incidents flow breakdown was found in the traffic flow range of 1600 to 2300 pcu/h/lane. The average sustainable flow was found to be 4050 pcu/h (4050-29H veh/h) for a 2 lane section which does not include the hard shoulder and 3900 pcu/h (3900-29H veh/h) for a 2 lane section consisting of the hard shoulder and one standard lane (where H is the percentage of heavy vehicles in the traffic stream).

In recent years narrow lanes have become common in an attempt to increase capacity, under the assumption that three narrow lanes have a greater capacity than two standard lanes. Very little research has been found to back this up. The only study found in this review that estimates narrow lane capacity (Marlow et al., 1992) found that single narrow lanes (<3m) have an operational capacity of 1300-1400 veh/h: this is a 9-16% reduction from Hunt’s et al. (1991) measured single lane capacity (1800-17H) assuming H=15%. Marlow et al. (1992) cite Maclean and Greenway’s (1977) finding that vehicle throughput on narrow lanes was about 15% lower than the lane capacity at conventional work sites.

Assuming the use of narrow lanes reduces capacity by 15% in each lane suggests that the capacity of three narrow lanes is likely to be up to 38% higher than that of two standard lanes (if total capacity is assumed to be the sum of the individual lane capacities, though this assumption probably yields a slight overestimate). However, Maclean and Greenway conducted their study more than three decades ago, when narrow lanes in motorway work sites were a novelty, and it is not known whether this result applies to conditions in modern works sites.

Hunt and Yousif (1994) describe the results of a survey of throughput at roadworks zones operating at or close to capacity. The available evidence identified the merge area as the major cause of congestion induced by excess vehicle demand flow. Data describing driver behaviour are presented. Hunt and Yousif (1994) also conducted a simulation of various lane closure options, comparing the effects of closure length, which lanes are closed and traffic composition and segregation. Results agreed closely with observations on maximum throughput, and suggested that HGV percentage had the greatest effect on capacity (in veh/hr), which is to be expected. It was also found that capacity was much higher when the nearside lane was closed than when the offside lane was (although this was noted to contradict the findings of study in the Netherlands by Papendrecht and Schuurman, 1991). Segregation of traffic by lane (restricting HGVs to the nearside lane) and prohibiting lane changing was reported to increase capacity further. The length of the roadworks zone was found to have no effect on capacity. Observations of maximum throughput suggested that it was 10-20% lower after flow breakdown than under normal conditions.

Jiang (1999a) studied various lane closures at work zones on Interstate highways in Indiana, and concluded (empirically) that capacity through the work zone was best defined as the traffic flow rate just before a sharp speed drop (which indicates flow breakdown). Jiang also observed that during congestion the flow through the works

5 Flow is defined as the rate of vehicles passing a fixed point, measured in vehicles per hour or passenger car units (pcu) per hour. Passenger car units are used to represent the equivalent number of cars in a traffic stream composed of a variety of different kinds of vehicles, and so take account of the different characteristics of HGVs, for example. The terms “volume” and “throughput” are sometimes used interchangeably with “flow”.


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(equal to the queue discharge rate) was on average lower than the normal capacity of the works.

Minderhoud et al. (1997) discuss four groups of capacity estimation methods based variously on headways, traffic volumes, speed and density. They emphasise the stochastic nature of capacity, due its dependence on continually varying quantities such as driver behaviour, traffic, road and weather conditions. As such a single value for capacity does not really exist, but one can be approximated from the distribution of measurements over a period at one or more points on the link in question.

The simplest and most commonly used method for capacity estimation is to base it on the observed maximum flow observed over a certain period (see Figure 1). Note that the maximum often occurs just before flow breakdown, as in Figure 1, where the sharp drop in flow during a peak period indicates flow breakdown. Other traffic volume methods use the expected maximum value from probabilistic interpretation of observed flows over a period.

Treated as a stochastic variable, the distribution of capacity measurements over time is sometimes assumed to be Gaussian, but a function can be estimated by measuring flows over a long period and plotting the frequency distribution – this is expected to be a bimodal shape consisting of the combined demand and capacity distributions (see Figure 2). The estimate of the capacity distribution function can be improved by categorising flow measurements as either congested or free-flow (using upstream speed measurements) to separate the two functions.

Headway models can be applied to a single lane to estimate the capacity from the time headway of vehicles, based on the assumption that vehicles constrained to drive at the same speed as the vehicle in front (i.e. queuing) have a fixed time headway distribution. These models were found to overestimate capacity and were not recommended by Minderhoud et al. (1997).

Capacity can also be estimated using observations of flow and density to plot the so called fundamental diagram of these two variables, and fitting a mathematical function for flow as a function of density – the maximum of this function is then the capacity (see Figure 3).


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Figure 1: Direct estimation of capacity as maximum of observed traffic flows

Figure 2: Estimating capacity distribution from frequency distribution of observed flows (adapted from Minderhoud et al., 1997)

Flow (veh/h)

Time (~days)

Flow (veh/h)

Frequency

Capacity distribution function?

Distribution of demand

below capacity

Frequency distribution of measured flow data

Mean capacity?

Flow (veh/h)

Time (~minutes)

Capacity?


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Figure 3: Fundamental diagram method of estimating capacity

5.2 Optimising capacity

5.2.1 Driver behaviour

The capacity of a roadworks site, either in the situation before flow breaks down or when traffic is discharging from the front of the queue, is governed by the behaviour of drivers, particularly vehicle following behaviour. Every driver has a “comfortable” following distance in free flow traffic which can vary according to whether the driver is expecting to encounter a queue ahead or expects the road to be unobstructed. Typical headways range from about 1.2 seconds to 3 seconds. The situation differs a little when drivers are pulling away from the front of a queue; that depends on the dynamics of the vehicle and the reaction time of the driver.

That variability produces significant changes in the capacity of a roadworks site, both in the period before a queue is established and during it. Measured figures in free flow conditions range from about 1,800 vehicle/lane/hour, the practical figure produced by the 2 second rule, to over 2,700 vehicles/lane/hour. A similar range is produced by vehicles pulling away from a stop line. In each case the capacity may drop depending on the lighting and weather conditions.

Hence, in principle, significant benefits could be derived by modifying drivers’ behaviour in ways which would increase capacity.

Queuing can also arise when capacity is restricted by factors other than roadworks. Gillan and Owens (1983) state that queues of up to 25 km long have been reported on the M5 in Somerset, lasting for several hours and caused by a stretch of motorway with an abnormally low capacity, measured at 4,700 vehicles per hour on a 3 lane motorway, or under 1,600 vehicles/lane/hour. The authors concluded that reason for the abnormally low capacity was the appearance of the lane ahead; essentially the layout of the road was such that traffic climbing Naish Hill (junctions 19-20) appeared foreshortened, and

Flow (veh/h)

Density (veh/km)

Capacity? Fitted function

Measured data


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gave the impression that traffic was queuing even when flowing freely. As a result drivers modified their behaviour because they expected a queue. The report recommends experiments using a visible barrier to reduce visibility of the hill ahead. Other than that, a major increase in capacity could only be achieved by civil engineering works. Adding an extra lane might be beneficial but might not justify the cost. Some increase in capacity might be achieved by controlling the demand traffic flow. (This problem was eventually addressed in 2006 with the addition of an extra lane).

Tarko and Venugopal (2001) found capacity to be reduced both by aggressive late merging behaviour and by traffic forming dense platoons to prevent aggressive drivers “cutting in”. The dense platoons were forced to spread out once inside the work zone, in order to achieve perceived safe stopping distances, and this resulted in stop-and-start traffic upstream on the approach. They also found capacity to be reduced by rain (a reduction of about 10%) and by heavy vehicles (a reduction of about 4.04 veh/hr per 1 point increase in heavy vehicle percentage). The latter result differs from that of Hunt et al. (1991) whose equations include a reduction in capacity of ~15 veh/hr per 1 point (see Section 5.1). Police presence also reduced capacity significantly (by 14%), probably because it encouraged drivers to leave larger gaps. The side of the lane drop (left or right of the carriageway) was found to have no significant effect on capacity.

Polus & Shwartzman (1999) studied the speeds, headways and the throughput of works in uncongested (but high density) conditions, and the impact of a visible police vehicle as a deterrent to speeding and dangerous driving. The effect of the police deterrent was to smooth out fluctuations in the traffic flow, and equalise volume between the two lanes (volume was unchanged in the “slow” lane although it was reduced in the “fast” lane). Time headways were observed to reduce (density increased due to the reducing speeds) but become less variable. Polus and Shwartzman concluded that speed enforcement reduced variability in speeds and headways, which had a positive impact on safety. This may be the case, although it may also be argued that the increased density is a hazard in itself, if safe stopping distances are not maintained, and may be more likely to initiate flow breakdown.

Meyer (1999) reviews a number of studies of the effect on speed and safety of transverse white bars and chevrons painted on the road (“optical speed bars”). Trials of various options were applied to highway work zones, and were found to reduce speeds and speed variation, which has a beneficial impact on safety. It is also possible that by reducing speed variation, the likelihood of flow breakdown is reduced and hence delays are reduced.

5.2.2 Controlling merge behaviour

Hunt and Yousif (1994), in their study of motorway roadworks, observed that the loss of a lane caused confusion and turbulent flow as high density queuing traffic was forced to merge. This was reportedly caused by inconsistent behaviour between different drivers: while most vehicles merged far upstream of the lane closure, a minority continued further down towards the closure, where their forced merge with the already dense traffic stream initiated flow breakdown, which then propagated downstream until perceived safe headways were restored. Hunt and Yousif suggest that this flow breakdown could be prevented, and the capacity of the merge thus increased, by part time signals controlling flow from each lane into the bottleneck, or variable message signs encouraging drivers to vacate the closed lane before reaching the queue.

The use of signs to control the lane merge situation has sparked some interest in the US, and a number of research projects have been published on the subject.

McCoy and Pesti (2001) provide a brief but comprehensive review of alternative merge strategies used in the US. These include static early-merge systems, using conventional signs to prohibit overtaking in the lane merge situation and to promote earlier merging on the run up to the roadworks, with the aim of reducing accidents and improving


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capacity through the works. Such schemes reduce the frequency of forced merges, but they tend to increase travel times as vehicles with a greater desire speed get stuck behind slower vehicles on the approach during uncongested times (McCoy and Pesti, 2001).

Datta et al. (2002) report on a typical dynamic early-merge system in Michigan, using variable message signs with an arrow pointing to the direction in which to merge and at what point on the road to merge, followed by a series of dynamic lane merge sign trailers and a series of static "Do Not Pass" signs. Their results indicated a significant reduction in aggressive driver behaviour, lane violations and a slight improvement in traffic flow. Mean travel time delay data indicated the mean delay “before” was significantly higher than the mean delay “after” implementation of the system. An economic analysis at one site indicated a benefit if a value of time of $3.80 per person per hour is assumed for travel time savings.

Bushman and Klashinsky (2003) report on the dynamic early-merge systems used in work zones across Indiana and Michigan. Their observations indicated that peak travel times were reduced by more than 30%, the average number and duration of stops were reduced and the number of aggressive manoeuvres was reduced by 50-75%, thereby reducing the accident risk (and resulting delays).

These dynamic early-merge systems reduce the problems suffered by static systems because the length of the “Do Not Pass” zone is adjusted to suit traffic volumes. However, Tarko and Venugopal (2001) reported that the Indiana Lane Merge System (a dynamic early-merge system) resulted in a 5% loss of capacity, though the authors attributed this to drivers’ unfamiliarity with the system.

Pesti et al. (1999) discuss the potential advantages of a late merge strategy at roadworks. In Pennsylvania Department of Transport’s late merge scheme, signs encouraged drivers to use all lanes up to the point where the closure begins, where they were instructed to merge in turn. The purpose of this scheme was to reduce road rage at congested times, caused by some drivers overtaking the queue in the lane to be closed, but it had the additional benefits of increasing capacity by as much as 15% (Orth-Rodgers & Associates, Inc., 1995), and of reducing queue length by the ratio of open lanes to all lanes. In addition, Pesti et al. (1999) conducted a study which found that the standard deviation of the vehicle speed distribution was reduced under the scheme by 18% for uncongested and 27% for congested flow, which has the potential of reducing accident risk. Pesti et al. (1999) also reported reduced traffic conflicts under the scheme - 75% fewer forced merges and 30% fewer lane straddles – a conclusion echoed by McCoy et al. (1999). Pesti et al. (1999) suggested in their report that benefits could be improved by higher compliance (particularly of HGVs) – something which may be achieved by more traveller information and more widespread use of the strategy.

However, McCoy and Pesti (2001) argue that the late merge option could increase accident risks in low-flow conditions, when high speed traffic is forced to merge in a relatively short space, where there may be ambiguity over right of way. They concluded that the late merge option is the best system during peak periods, but a traditional system (informing drivers of the closure in advance but allowing them to merge as they see fit) is preferable during off-peak periods. To achieve this, McCoy and Pesti propose a dynamic late-merge system, which activates variable message signs displaying late-merge advice, only when congestion is detected. It is noted that the accident risk could be increased under this system when it switches from traditional to late-merge, if drivers try to move out of the queuing lane into the faster moving lane.

Klein et al. (2004) studied (in Germany) the effect of a merge system in which traffic merged into the slow lane, only changing into the fast lane afterwards (since the slow lane was closed). In contrast with Tarko’s and Venugopal’s (2001) finding that capacity was unaltered by the side of the lane drop, Klein et al. (2004) found that this merge system had smoother merging, higher capacity and increased safety, compared with a system in which traffic merged into the fast lane. A capacity gain was observed at all


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flow levels and all times of day, and the gain depended on the proportion of heavy vehicles, ranging from 0% with less than 5% heavy vehicles, to 60% with 30% heavy vehicles.

5.2.3 ITS control measures

The Controlled Motorways scheme on the M25 (Rees et al. 2004) has been shown to generally smooth traffic flow, increase headways and lane utilisation, and reduce the risk of flow breakdown, by activating variable speed limits (VSL) at high flow levels. These speed limits can also be activated upstream of a queue (“queue protection”), significantly reducing the risk of accidents caused by high-speed traffic joining the back of the queue.

During 2004 and 2005, the M25 between Junctions 12 and 15 underwent major widening works, with a fixed 40mph speed limit applying within the roadworks area. Controlled Motorways could not operate in the roadworks area itself, as the gantries holding the signals were removed prior to the works commencing. However, Controlled Motorways continued to operate on the lead-in to and the exit from the roadworks. Several modifications were made to the operation of Controlled Motorways in the vicinity of the widening works to aid the progression of traffic into the roadworks (Rees, 2006). The main features were:

• During periods of high flow, a gradual reduction in speed limits approaching the roadworks.

• During periods of low flow, no additional speed limits were displayed.

• The queue protection system continued to provide protection for any queueing traffic on the approach to the roadworks.

This new signalling strategy was successful in reducing the variability of speeds and headways, allowing traffic to pass through the roadworks with a minimum of delay (Dixon and Rees, 2005).

Similarly, the Active Traffic Management (Stewart et al. 2006) and Hard-Shoulder Running (DfT, 2008a) schemes on the M42 have potential for effectively managing traffic approaching and traversing works.

The benefit of using VSL algorithms during roadworks was studied in a simulation by Lin et al. (2004). Two algorithms were tested: VSL-1 aimed to reduce queuing delay, and VSL-2 aimed to maximise total throughput. Both VSL algorithms reduced speed variability in the simulated traffic stream, though VSL-1 performed better. Both control algorithms succeeded in achieving their respective aims. In addition, Lin et al. (2004) cite field studies indicating that the risk of rear-end collisions in work zones can be reduced by VSL: Committee for Guidance on Setting and Enforcing Speed Limits (1998); and Coleman et al. (1996).

Additionally, Collins (2007) found that average speed enforcement (which has been used on major roadworks since 1999, and can be coupled with VSL with the appropriate technology installations) reduces speed variability, resulting in smoother traffic flows and higher, more reliable throughputs.

Stevens and Gillan (2008) consider an alternative approach to smoothing flow through roadworks, based on the general fitment of AICC (Autonomous Intelligent Cruise Control) in vehicles. Essentially a forward looking range finding radar on a vehicle measures the distance from the vehicle in front and automatically adjusts the headway, the inter vehicle gap, to a pre determined value, by adjusting the vehicle brake and accelerator.

AICC has been sold on a number of vehicles over the years, but has mainly been marketed as a driver aid on fairly expensive vehicles. For example it is available as an extra on the latest model of Ford Mondeo. However if it were installed on all, or at least


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the majority of vehicles, it would lead to the development of “road trains”, often suggested as a means of increasing motorway capacity. At present the guidance on manually set headways on motorways, the “two second rule” leads to a motorway capacity of just under 1,800 pcu (passenger car units) per lane per hour, although in reality lane flows are often higher on busy motorways. However, the Mondeo unit allows headway to be set to a minimum of 1.4 seconds, a figure selected on the basis that it occurs relatively frequently in reality. If all drivers had AICC set at that figure, the theoretical lane capacity would increase to 2,400 pcu while a 1 second headway would increase capacity to over 3,000 pcu/hour. Gains of such order would greatly improve the capacity of roadworks. The paper concludes that the constraints which limit take up of AICC are primarily due to the microeconomics of the purchase decision and not technology.

5.2.4 Dynamic site management

Typically the traffic flow approaching a roadworks site varies with time. Generally it will peak at certain times of the day. In many cases it will display tidal behaviour, particularly on radial routes leading in/out of conurbations. In an ideal world the size of the workspace available to the contractor would be maximised during periods of light traffic while more road space would be made available for traffic during busy periods.

Marlow et al. (1992) report that derivatives of the conventional contra-flow layout have been developed and successfully operated at a number of sites. The new layouts employ either a reversible (tidal) lane or narrow lanes and may be adapted to both the full and partial contra-flow system. The use of these layouts, in appropriate traffic conditions, shows potential overall savings, with the reduced cost of traffic delay offsetting the expenditure on extra signs and markings. As described in section 5.1, three narrow lanes are likely to have a significantly greater capacity than two standard lanes.

Rathbone (2000) discusses the implementation by many jurisdictions in the US and Canada of Movable Barrier Technology (MBT) to improve safety, capacity and efficiency during recent reconstruction projects. MBT is a type of separation technology that has the ability to provide positive protection continuously both to road users and construction workers while responding to traffic flow conditions and construction needs. MBT's flexibility allows it to be moved quickly and safety to open and close traffic lanes as required by varying work zone activities and traffic volumes. Design and construction of MBT systems, examples of successful use in various locations, and the minimal impact on lane closures and other benefits of MBT are described. The distance of the shift can be varied from 4 to 18 feet and can operate at up to 5 mph. Two examples are given in the paper. In both cases the contractor was able to move barriers to make available an extra working lane during off peak traffic, but restoring the space for traffic use in the morning and evening peak hours. In both cases it was calculated that the total construction time had been reduced by approximately 30%.

TRL is currently researching the use of an MBT-like “QuickChange Movable Barrier” (QMB), and optimising its use with the Costain Traffic Manager (CTM), in a project for Costain Ltd (Owlett et al. 2008). The CTM tool under development is aimed at providing intelligence-based decision support on when the barrier should be moved, both in off-line planning and on-line using real-time data.

5.3 Incident management

When there is a traffic incident, such as an accident or breakdown, in motorway roadworks, it is essential that it is cleared quickly to minimise delays caused to traffic, particularly in peak times. However, that normally demands resources such as stand-by recovery vehicles, so there is a balance to be struck in determining the resources needed. The relevant calculations are included in QUADRO. There have been a number of


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studies aimed at understanding the impacts of incidents and how best to reduce these impacts.

Coe and Evans (1992) recommend that consideration should be given to reviewing incident rates and durations used in QUADRO. They suggest that Flat bed transporter type recovery vehicles should be specified and at least one should be capable of carrying five passengers.

Stoneman et al. (2006) reported on a comprehensive investigation of 8 countries –Holland, France, Germany, Sweden, USA, Australia, Malaysia and Japan – which included operational procedures (two examples), technical innovations (seven examples such as Computer Aided despatch), organisational aspects (five examples) and training/working requirements (two examples).

Highways Agency (July 2002) reported on a comprehensive study which gathered information and convened workshops. The study recommendations were in eight categories covering operational, monitoring, organisational, guidelines/planning support, training and technological aspects. The focus was on unplanned incidents, ranging from minor vehicle spillages to multiple vehicle accidents. It does not make specific reference to roadworks or incidents at roadworks, other than to indicate that a nearby roadworks site could be a source of resources to help clear a major incident problem. The report contains a particularly comprehensive task analysis for incident management, which could be a useful pointer when developing new approaches for handling incidents which occur in roadworks.

The US experience in incident management is enshrined in the Federal Highway Administration (FHWA) Incident Manual (PB Farradyne, 2000), which emphasises that effective incident management is a team activity. Reducing traffic congestion and improving roadway safety are high priorities for the FHWA, which strongly endorses the establishment and use of good traffic incident management. Effective transportation system management and operations depends on the aggressive management of temporary disruptions (caused by traffic incidents, work zones, weather, special events, etc) in order to reduce the consequences of these disruptions and return the system to full capacity. This is aided by the Incident Command System (ICS), a systematic tool for the command, control, and coordination of an emergency response. ICS allows agencies to work together using common terminology and operating procedures for controlling personnel, facilities, equipment, and communications at an incident scene.

5.4 Efficient Completion of the Works

Another approach to minimising delays is to complete the works more quickly. This can be achieved by:

• efficient project management, ensuring that there is no slip in the critical path.

• efficient logistics, i.e. ensuring that all of the materials, labour and machinery required for each task is at the site ready to go when needed.

• Selection of appropriate materials, such as asphalt or concrete that does not require excessive setting times.

• Appropriate contracts: it is essential that the objectives of the HA and contractors are aligned in the contract for the works. The HA may have a PSA target aimed at minimising congestion but if the contractor is focussed on completing the work as cost effectively as possible that may be neglected in the works. Various approaches are possible; early examples included the use of lane rental in the contract, which produced reductions of 38% in the time taken to complete works. The need to minimise delay at roadworks is also likely to be a component of DBFO contracts.


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These actions are not confined to the UK. One example is work in Pennsylvania, which bundled these civil engineering approaches with traffic management. Schneider (1998) reports that Pennsylvania Department of Transport has implemented several new strategies aimed at reducing congestion, such as building auxiliary lanes, increasing night work to alleviate lane closures during peak hours, utilizing quick-cure concrete, and including contract incentives/disincentives that spur contractors to complete work as quickly as possible. Intelligent transportation systems (ITS) are also being added to the arsenal. These high-tech systems monitor traffic flow and provide real-time information to motorists before they enter the construction zone. Motorists are prepared for the traffic slow down, reducing the likelihood of rear-end collisions. They also have the opportunity to choose an alternate route.

Obviously there is interaction between the need to maintain traffic flow and allowing the contractor to work as efficiently as possible. One radical approach is to close the road completely for a period. This has been assessed in the US.

Grave et al. (2003) reported that full road closure, while not amenable to all construction situations, is a methodology many agencies are giving greater consideration to in order to perform required roadway maintenance and rehabilitation. They reported on six sites which showed the estimated time saved varied from 63% to 95% of the exposure. For the study sites, projected congestion impacts typically went unrealised as demand during the project was less than expected as a result of information dissemination. Worker productivity improved. Five of the projects studied cited traveller and worker safety as a benefit of the full closure.

Much of the above falls under the heading of civil engineering, rather than traffic management, and has been the subject of a substantial body of research, at TRL and elsewhere. Much of that is already embodied within the working methods employed by HA and its contractors. The different nature of this activity, and its scale, mean that it would merit a separate review and it has not been considered further here.

5.5 Diversion and Demand Management

5.5.1 Overview

As discussed previously, the delay at a roadworks site is governed by a number of parameters, including the demand flow (the traffic approaching the works site). A queue is generated when the demand flow exceeds the residual capacity of the works site. Simple arithmetic indicates that the rate of queue growth is sensitive to the level of traffic demand. For example if the residual capacity at the works site is 4,000 vehicles per hour and the demand is also 4,000 vehicles per hour there will be no queue. A 10 per cent increase in demand, to 4,400 vehicles per hour, will result in a queue of over 400 vehicles after one hour, using the simplest queuing models. The queue will be over 1km long (allowing 10m per vehicle and assuming that the queue is spread over four lanes at most).

The discussion in previous sections has centred on increasing capacity at the works site itself. Delays may also be reduced by cutting the demand flow at the site, either by suppressing demand or by diverting some of the approaching traffic. The literature indicates that a variety of methods have been adopted to achieve a reduction in demand. They can be divided broadly into the following groups:

• Local, or tactical, diversions around the works site. The driver is on a route which would take them through the site and they only become aware of the unexpected delay a short distance ahead, perhaps one or two junctions ahead in the case of a motorway works site. Drivers may be informed about the delays at the site and the potential to use an alternative route.


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• Remote, or strategic, diversions. In this case a driver becomes aware of the potential delay early on in a journey, or possibly at the trip planning stage before setting out. Normally drivers would be informed of the expected delay at the site and make their own decision about routing, ideally early on in a journey or even before embarking.

• Promoting shifts in the journey times or deterring journeys if they would result in travel past the works.

Inevitably there is overlap between tactical diversion and strategic diversion.

The process of diversion allows the effects of a works site to propagate beyond the road on which it is located to the surrounding network. In order to fully account for the delay caused by a given set of works, one not only needs to consider delay to the vehicles passing through it, but also the delay caused to vehicles diverting around the site and the delay caused to other vehicles on the roads used for diversion by the increased volume of traffic.

Whilst the ideal scenario is to minimise total delay, it is not always possible to understand how vehicles divert, as diversion often takes them away from the strategic network, to local roads where their journey time cannot be monitored. Excessive diversion could cause huge delays on the local network, and insufficient diversion will leave capacity unused on the local network whilst vehicles queue on the main route. Understanding and exploiting diversion is a significant challenge and is discussed in the following sections.

5.5.2 Tactical Diversion

Tactical diversion on roads near to the roadworks site is one approach to reducing demand. Essentially traffic upstream from the works is diverted onto an alternative route which bypasses the works and rejoins the main route downstream of the works site. This effectively utilises residual capacity of the nearby network to reduce overall delay.

Tactical diversion can occur due to drivers becoming aware of an unexpected queue in a variety of ways, for example by seeing it as they approach the tail, by hearing about it on a traffic broadcast or by a warning on a variable message sign (VMS). The word “unexpected” is critical; they may be aware of the works site and may even have passed it previously but on this occasion queues are significantly greater, possibly due to exceptionally high flows or an incident on the site. Increasingly in future they may become aware of delays on motorways due to the journey time information given on HA variable message signs as part of the Motorway Travel Time Service.

The possibility of tactical diversion is allowed for in QUADRO. QUADRO models diversion using a single diversion route on which journey time increases as flow increases. Procedures for calculating the capacity of the most appropriate diversion route are included within the QUADRO manual (DfT, 2002, parts 4 and 7). The amount of traffic that diverts is that which results in equal journey times on the diversion route and main route. This type of diversion reduces delay as it makes use of the ‘parallel’ capacity of the diversion route to allow some vehicles to bypass the queue. Of course, the diverting vehicles are also delayed by the same amount as those on the main route, but the total delay is less than without diversion.

It is inevitable that the process modelled within QUADRO is an imperfect representation of reality. A driver at the decision point has to make a choice about diverting or not based on the information available to them. They may have imperfect knowledge of the road conditions, the route and the delay on the diversion route. It seems likely that reliable travel time information available from information services plus, possibly, the use of in-vehicle navigation units will enable this to be improved in future.

The possibility of generating optimum diversion rates by linking an assignment model to VMS has been a significant component of the TRL Programme sponsored by the HA since


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1990. Gower and Taylor (1991a) examined the impact of different motorway diversion strategies and considered the application of the MCONTRM assignment model for control of networks involving a motorway (Gower and Taylor, 1991b). This concept has been developed into the Motorway On-Line Advisor (MOLA), an ITS installed in the South East Regional Control Centre to manage tactical diversions in real time around incidents on the M2, A2, M20 and A20.

One issue with such diversion management systems is that control depends on being able to predict the proportion of motorists that will follow a recommended diversion route. Too much diversion will lead to long queues on the diversion route and increase the overall congestion, measured as vehicle hours lost in a queue. The modelling work conducted by Gower and Taylor confirmed that conclusion and showed that for any works site which causes queuing, there is an optimum diversion rate which minimises the total queuing losses across the network. That diversion rate is site specific; it depends on the availability and capacity of the surrounding network.

It is worth noting that the optimum diversion rate identified by Gower and Taylor, which is aimed at minimising total vehicle hours lost on the network, is unlikely to coincide with the rate derived in a QUADRO assessment, which is based on equalising journey times on the main route through the works site and on the diversion route.

Hence optimal control of a diversion depends on being able to encourage a defined proportion of the traffic to use an available diversion route. That means that the information systems used need to be calibrated in terms of their impact on a driver’s propensity to divert; it is a behavioural issue. The majority of research on this topic seems to have taken place in the US. For example Bushman et al. (2004) systematically evaluated the effect of three different message styles displayed on VMS upstream from a roadworks site which caused queuing. They showed that the diversion rate increased systematically by varying the information. The lowest rate was achieved by including a basic warning about the works e.g. “Drive with caution/Road Work Ahead”. The diversion rate was increased by including delay information e.g. “Traffic stopped ahead/ 10 minute delay” while the highest rate was achieved by including diversion advice e.g. “Use exit 150 as Alt.”

5.5.3 Strategic diversion

Strategic diversion is becoming an increasingly viable option due to the availability of journey planning services such as Transport Direct (http://www.transportdirect.info/) – based on historical data – and the real-time travel information offered by the HA website (http://www.highways.gov.uk/) and by some private companies such as Trafficmaster (http://www.trafficmaster.co.uk/).

It is inevitable that much of the responsibility for determining whether or not to choose a strategic diversion route which avoids a roadworks site must rest with the individual driver or vehicle operator. The route choice may have to be taken well before the time the vehicle would approach the works site, had it remained on the original route, and it is inherently difficult to predict delays, not least because they will be modified by tactical diversion. The availability of suitable diversion routes will also depend on the characteristics of the road network in the region between the origin and the destination. The variability of the delay and the multiplicity of diversion routes make it difficult to give detailed diversion advice. It is perhaps intuitive that the best approach is to ensure that drivers are well aware of the potential for delay at the site before they embark on their journey, and to leave them to make their own decisions. However, this will not necessarily minimise total delays across the network, as drivers will still not have knowledge of the decisions made by other drivers travelling at the same time.

Nevertheless strategic diversion is attractive for road operators. If traffic diverts well away from the site it is likely to be distributed across the network and not cause perceptible extra delay on the diversion routes. Alternatively drivers may elect to modify


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their departure time to encounter the works site when traffic demand is such that there is minimal or no queuing at the site. If even a few per cent of the diverting traffic can be diverted in space or time in these ways, then the benefits at the site are likely to be significant and the negative impact will be imperceptible.

Lamont (2006) has described a model system aimed at promoting strategic diversion to minimise delays at a major roadworks, 12 km long, on the Peartree to Drybridge section of the A38, to the south of Exeter. Key features were:

• ANPR located at the start and finish to the works to continually monitor journey times through the works.

• An on-line simulation tool, based on the Paramics traffic model, which estimated future queuing delays at the site using historic data and current travel time estimates.

• A variety of methods for disseminating delay information, both current and predicted, including upstream VMS, a special page on the HA website and a service which allowed drivers to register to receive text updates of delay times on mobile phones.

• A publicity strategy, based on the media and leaflets available in a variety of locations, ranging from motorway services to public libraries. The aim was to inform the public both about the works and the means of receiving on-line information about the delays.

Unfortunately there has not been a quantified assessment of the system or other attempts elsewhere to minimise delays by informing travellers. It does seem likely, however, that this type of system will have an impact on minimising delays, and in particular in reducing the peak delays, those most likely to impact on the HA performance targets.

Strategic diversion has little bearing on the nature of behaviour at the site itself and is therefore outside the scope of this report. Effectively it is an expansion of the type of information already made available via the HA and Trafficmaster websites. Nevertheless it seems that systematic attention to the evaluation of approaches and methods of optimising systems could generate dividends.

5.5.4 Inhibiting or Time Shifting Travel

Inhibiting travel or promoting alternative means of transport is an approach popular in urban areas where journeys are short and alternatives are plentiful. For example, congestion in Central London has dropped 25% since the introduction of a “congestion charge” (Transport for London, 2007). Tolling the general motorway network was last aired in the 1993 Green paper, “Paying for Better Motorways”, but has only received indirect attention since then, as part of the DfT work on road pricing. Obviously if tolling were introduced it would be possible to modify the tolls at different times of day to encourage some drivers to avoid driving in peak hours, either by inhibiting journeys or choosing to drive off peak. That would minimise peak hour delays at works sites and would provide an extra lever for managing roadwork delays and the HA ability to meet its PSA objectives. Given that tolling is not on the policy agenda this topic is not considered further here.


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6 Conclusions

6.1 Discussion of findings

A large body of literature exists on roadworks and the effect they have on traffic movements and delay. By studying this literature, looking both at work on the impact of roadworks, and some of the measures proposed to mitigate that impact, it has been possible to gain an insight into the underlying causes of delay at roadworks.

Literature places the total cost of congestion on UK roads at £20 billion per year. The amount of this which is due to roadworks on the HA network is not well established, but may be in the region of £250 million (2002 prices, see Section 3).

The delay experienced by vehicles passing through a works site can be broken down into two components:

• delay caused by reduced speed in the works (caused either by increased traffic density or the enforcement of a speed limit) and

• delay caused by queuing to enter the works.

The latter can be considerably exacerbated by the occurrence of accidents within the works; some sources suggest that incidents are the primary cause of congestion at works sites (see Section 4.3). It is also possible that delay may be experienced by vehicles which divert onto alternative routes to avoid the roadworks. The subsequent increase in traffic on the diversion route may also cause delays to other vehicles on these routes.

Evidence suggests (see Section 4) that whilst accidents have potential to cause severe delays at any time, delay due to queuing is greater than that due to speed reduction during busy periods, but during less busy off-peak periods speed reduction remains a source of significant delay whereas queuing is less apparent.

Queuing is the result of demand exceeding capacity, therefore the fact that works sites have a lower capacity than the unobstructed road can be cited as one of the key causes of delays at roadworks. However, capacity is found to be an elusive quantity dependent on the collective effect of individual driver behaviour. It can be measured in a number of ways and is affected by numerous other variables including weather conditions, proportion of heavy vehicles and the presence of police.

A number of sources attempt to quantify the capacity of works sites in terms of their physical characteristics (see Section 5.1). The capacity of a site is typically found to be primarily dependent on the number of lanes, with capacity being broadly proportional to the number of running lanes. Evidence also shows that narrow lanes have smaller capacity than full-width ones. This suggests that there must be an optimum number of lanes for a given width of road which maximises capacity. On a three-lane carriageway where one lane is closed, it is generally only feasible to run two full-width lanes or three narrow lanes. However, on the wider carriageways that are now becoming more common, there will be a greater number of options available.

The capacity of a single lane is primarily determined by drivers’ perceived safe headway at the prevailing speed. There is evidence that influencing driver speed and headway, using either VMS or in-car technology can increase capacity (see Section 5.2.3).

Evidence reveals the merge area, where the number of lanes is reduced, to be the major cause of capacity restriction in works (see Section 5.2.1). The effect of this lane drop is to disrupt the fairly homogenous nature of normal traffic flow and drivers appear to adopt a number of behaviours not seen in normal traffic flow. For instance, dense platoons of queuing traffic have been observed to form as drivers in the queue attempt to prevent others from joining in front of them. When this platoon speeds up, drivers attempt to space out to achieve a perceived safe headway, resulting in stop-start traffic.


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The idea that the merging behaviour restricts capacity is supported by a number of studies of controlled merges (see Section 5.2.2). Significant improvements in capacity and reductions in delay were achieved using both early-merge (where vehicles are encouraged to merge and cease overtaking behaviour some distance in advance of the site) and late-merge (where vehicles are encouraged to use all lanes until the beginning of the closure) systems. The fact that both systems were found to deliver benefits indicates that it is the inconsistency between drivers in uncontrolled merge behaviour that causes capacity to drop. The increase in consistency introduced using these methods was also shown to reduce accident risk and therefore the associated delays.

There are contradictory findings on whether the side of the lane drop (i.e. whether the off-side or near-side lane is closed) affected capacity. There is some evidence that capacity is greater when the off-side (fast) lane is closed and vehicles merge to the near-side, particularly where the traffic consists of a large proportion of heavy vehicles. However, other findings indicate that merging to the off-side results in higher capacity. This indicates that the nature of the traffic approaching the site and the way it is distributed across the carriageway may have a bearing on capacity and therefore affect delay. There is also evidence that segregating different vehicle types within the site can improve capacity.

It is not clear to what extent the capacity within the works site is greater than that of the merge area. One source (Orth-Rodgers and Associates Inc., 1999) found a 15% increase in capacity using a late-merge system, suggesting that in this case there was significant residual capacity within the works before this scheme was implemented. If the loss of capacity due to merging behaviour could be removed, then the full capacity of the works site could be utilised. This would seem to be a strong argument for the use of narrow lanes (already very common in the UK) as they often allow the full number of running lanes to be retained, therefore removing the need for a merge.

The delay due to queuing at works may be mitigated to some extent if demand is reduced. There is a considerable body of literature on the subject of demand management, often based on influencing travel behaviour by providing information to the public (see Section 5.5). However, drivers may also choose to divert or not to travel without any intervention if they are aware that the works cause delays. This may mean that delay is self-limiting to some extent, with fewer journeys occurring as delay gets worse. The effect of diversion is less obvious, as drivers will still experience delay compared to their normal journey on the main-route, but this delay will be distributed across many alternative routes.

6.2 Concluding remarks

It is clear from this review that the causes of delay at roadworks are bound up with the complexities of driver behaviour. It is the drivers’ need to maintain a perceived safe headway that limits the capacity within the site (and indeed on roads in general) and the drivers’ behaviour at the merge point which can often limit the capacity of a site as a whole. The creation of a capacity restriction results in queuing which in turn gives rise to delay. Even in the event that demand is less than capacity, drivers can choose to reduce their speed when traversing the works site either because a speed limit is enforced, or because increased traffic density requires a drop in speed to maintain perceived safe headway, again giving rise to delay. Further complexity is introduced when the drivers’ travel choices are considered, as they may choose to divert or change their travel plans. This means that delay due to a given site may stretch beyond the immediate area of that site; the nature and extent of this is again a function of behavioural patterns. It is by discovering the determining factors of driver behaviour, and understanding how these influence traffic flow, that one may begin to understand and address the underlying causes of congestion.


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Acknowledgements The work described in this report was carried out in the Transportation Division of the Transport Research Laboratory. The authors are grateful to Tim Rees who carried out the technical review and auditing of this report.

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Abstract

Roadworks on the motorway and trunk road network generate traffic delays, often significantly greater than those which would have occurred had the road remained unobstructed. This results in costs to road operators and road users, as well as the economy as a whole. On the other hand, it is essential that the road network is maintained and updated. The key question is how that can be achieved while minimising the negative impact of the works. In order to address this question, it is necessary to gain an understanding of the processes which give rise to delay at roadworks.


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Roadworks on the motorway and trunk road network generate traffic delays, often significantly greater than those which would have occurred had the road remained unobstructed. This results in costs to road operators and road users, as well as the economy as a whole. On the other hand, it is essential that the road network is maintained and updated. The key question is how that can be achieved while minimising the negative impact of the works. In order to address this question, it is necessary to gain an understanding of the processes which give rise to delay at roadworks.


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