nhdc additional development assessment · technical note direct tel: +44 (0)121 262 1985 t +44...

Technical Note

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\\ukbhm2fp001\UKBHM2FP001-V1TP\TP\PROJECT\Transport Planning - Hitchin Urban Transport Plan\17 - NHDC Addtional Site\Reporting\TN-NHDC Additional

Development Assessment_200613_v3.docx

Project: Hertfordshire County Council Transport

Planning Contract

Job No: 60271338

Subject: North Hertfordshire District Council Additional Housing Assessment

Prepared by: Abigail Finch Date: 23/06/13

Checked/

revised by:

Adrian Hewitt Date: 25/06/13

Approved by: Nick Secker Date: 01/07/13

1 Introduction

AECOM was asked by North Hertfordshire District Council (NHDC) to look at future transport issues in

the Borough resulting from new housing development proposals. This assessment focuses on the

evaluation of two large development sites; one East of Luton and one north of Baldock along with a

small number of dwellings between Little Wymondley, Todds Green and Stevenage.

To inform the model testing the Stevenage and Hitchin UTP Model (SHUM) has been utilised. This

model covers the whole of the Stevenage Borough and the majority of the North Hertfordshire District

around Hitchin, Letchworth and Baldock. The locations of the two main development sites fall outside

the core model area, therefore it is not possible to assess the developments impacts on the immediate

road network surrounding the sites. Instead, the developments are assessed by way of their impact on

key routes in the vicinity of the sites, within the core SHUM network area. The impact will also be

evaluated at particular network pinch points that were identified in the previous work from the NHDC

Local Development Framework Housing Assessment undertaken in November 2012.

The development proposal identified one housing development scenario to be considered for the future

year 2031. This scenario is referred to as Scenario 6 following on from the previous housing

assessment undertaken in November 2012, discussed below. The scenarios development components

are:

Scenario 6: North of Baldock and East of Luton

a. 4,700 baseline dwellings (commitments plus priority 1 and 2 sites from the SHLAA

assessment);

b. 3,000 dwellings east of Luton;

c. 3,000 dwellings north of Baldock; and

d. 400 between Little Wymondley/Todds Green and Stevenage

This technical note serves two key purposes:

• Firstly, to explain the stages undertaken during the forecasting of the future year development

demand (Section 2 to 4).

• Secondly, to present details of potential transport issues on the highway network as a result of

additional housing development proposals (Section 5).

2 Background

The Stevenage and Hitchin Urban Transport Model (SHUM), covering Stevenage and some of the North

Hertfordshire area (primarily Hitchin, with elements of Letchworth included) was developed by AECOM

in 2009 and subsequently updated in October 2011 following a review and advice from the Highways

Agency (HA). SHUM was developed to assist the preparation of the Hitchin and Stevenage Urban

Transport Plans (UTP) and is validated to a 2008 base against observed traffic count data and journey

times. The HA signed off the 2008 base year model (October 2011 version) on 8th December 2011.

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It should be noted that the development that has taken place with the modelled study area between

2008 and 2011 has been incorporated within the model to ensure that it robustly reflects the today’s

base year situation. This therefore means there may appear to be slight discrepancies between

modelled development figures and those numbers included in the Local Plan. Using SHUM in

forecasting mode seeks to determine the impact on the future transport network as a consequence of

shifting patterns of demand over time, and forms the basis of the forecasting and analysis of the

additional housing development proposal.

In November 2012, AECOM was asked by North Hertfordshire District Council (NHDC) to identify future

transport issues in the district, in response to housing development proposals forecast as part of the

scenario testing to inform the development of Local Plan allocations. A series of 5 development

scenarios were tested and it was found that many of the resulting highway problems were consistent between

each situation, as particular pinch points on the network act as constraints to effective operation of the network. The

outcome was the establishment of some mitigation proposals to address the problems and enable the growth to

come forward. The latest assessment follows on from that work, evaluating the new development scenario in

reference to the pinch point.

3 Network Development

This section discusses the extent of the future year highway network, including constructed and

committed infrastructure.

As discussed, the basis for the model forecasting was SHUM, which covers North Hertfordshire and Stevenage, with the extent of the highway network shown in Figure 3.1.

Technical Note




Figure 3.1 SHUM Highway Network

3.1 Future Year Highway Networks

SHUM was validated to reflect the transport network operation in 2008. To provide a representative

transport network for 2031, constructed and committed transport improvements in the study area were

included. This resulted in the development of a do minimum network for 2031 which included the

following improvements:

• Hitchin Payne’s Park gyratory pedestrian crossing

• Glaxo Smith Kline junction improvements

• A1(M) Junction 7 signalised junctions

• A1(M) Junction 6 northbound all lane running, Welwyn, HA pinch point scheme.

The SHUM do minimum network formed the basis on which the five previous development scenario

networks were developed, each with their own additional network infrastructure specific to the housing

development components. Out of those networks, scenario one was selected as the most suitable future

year network upon which the latest developments demand is to be assessed. Because of this, there are

some infrastructure elements included which are not necessarily required in relation to the new

Technical Note




development scenario. Given their location it is not expected to have an effect on the assessment of

these sites.

4 Demand Development

An important part of forecasting the likely traffic conditions on the highway network includes

understanding changes to travel demand. Demand changes are a reflection of changes in income,

transport prices, demographics and land use changes. The methodology employed for developing the

forecast demand matrices for the 2031 future year is discussed below.

The methodology can be broken down into four stages. Stage 1 of the process is the same, irrespective

of the level of future year development. However, details of Stages 2 to 4 differ depending on the

development scenario being tested.

• Stage 1 – Development of background growth factors for internal to internal (within the modelled

area) and internal to external trips. These factors were derived using the National Trip End

Model (NTEM) forecasts and TEMPRO. The latest version of the dataset, NTEM 6.2 was used

in conjunction with the current version of TEMPRO 6.2. This ensured the forecasts benefit from

nationally and locally derived growth projections in accordance with government guidance. For

external to external movements, the East of England Regional Model (EERM) version 1.3

forecasts were used to provide growth factors. The growth factors were applied to the 2008

calibrated SHUM base year demand through a Furnessing process.

• Stage 2 – Collection and assessment of development information in the area is undertaken to

calculate the number of trips that specific developments can be expected to generate. These

trips are then allocated to a relevant model year (2031 in this instance), development scenarios

and trip demand purposes.

• Stage 3 – The trip distribution for the development trips is determined using a gravity model.

• Stage 4 – The future background growth (Stage 1) and proposed development trips (Stage 2

and 3) are added to the base year demand to produce final future year trip demand matrices.

Trips were not constrained back to TEMPRO growth.

Two separate sets of matrices have been created for the assessment; a reference case and

development scenario 6 (DS), both built from the future year demand defined in scenario one. The

reference case demand takes the scenario one demand but removes the specific housing development

components and leaving in the 4,700 baseline dwellings which are consistent across all the

development scenarios.

For development Scenario 6 the demand is the sum of the reference case demand plus the new

housing allocations:

a. 3,000 dwellings east of Luton;

b. 3,000 dwellings north of Baldock; and

c. 400 between Little Wymondley/Todds Green and Stevenage.

On account of the east of Luton development being located outside of the SHUM modelled network

area, the decision was taken to assume that 50% (1,500) of the trip associated with housing allocation

would enter the SHUM modelled network, based on the assumption that not all the traffic generated by

the development will travel through the model area. Also, it was assumed that the routes that the

development traffic would use meant the dwellings were split across two external model zones which

represent traffic travelling along the A505, and B565 London Road via Preston Road. Dwellings were

proportioned out 90%/10% to A505 and B565 respectively.

A similar approach has been applied to the development quantum for North of Baldock as it primarily

falls outside the model area. In this instance, 70% of the trips associated with the dwellings (2,100) have

been assumed to enter the SHUM modelled network.

The 400 dwellings specified for Little Wymondley, Todds Green and Stevenage was initially evenly split

across the three areas, then the Stevenage proportion (133 dwellings) further disaggregated across 48

Technical Note




previously identified development sites within the core area of Stevenage. This equates to 2.77

additional dwellings per development site. Table 4.1 summarises, by development site, the housing

quantities applied and the zones it was allocated to. Appendix A contains the full list of development

sites included as future year demand.

Table 4.1 Housing quantum and zone allocations

Development Sites Dwellings Zone

East of Luton – along A505 1,350 4310

East of Luton – along B565 150 4309

North of Baldock 2,100 4315

Little Wymondley 133 3302

Todds Green 133 3303

Stevenage 133 Multiple

Total Dwellings 4,000 -

The outcome of the anticipated levels of growth between the Base Year of 2008 and the future year of

2031, for the reference case and development scenario, are summarised in Table 4.2. This shows that

there is a growth in traffic levels across the model area of around 40% in 2031, and that the proposed

quantum of housing yields a 3% increase in trips. Table 4.3 provides a breakdown of demand by user

class.

Table 4.2 Summary of growth in demand in the model area for each Scenario

Peak Base Year (2008)

Matrix Total

Ref. Case

2031

DS

2031

Additional

Trips

AM 40,001 53,868 55,447 1,580 (3%)

% growth over base year - 35% 39% -

PM 38,624 53,099 54,758 1,660 (3%)

% growth over base year - 37% 42% -

Note: DS 2031 represents the new housing scenario with sites east of Luton, north of Baldock and Little

Wymondley and Todds Green.

Table 4.3 2031 Peak Demand Matrices (Vehicles)

AM Peak PM Peak

Journey Purpose Ref Case Development

Scenario

Ref Case Development

Scenario

Home Based Work 30568 31512 21757 22581

Home Based Education 2616 2717 1418 1498

Home Based Other 8266 8672 15808 16406

Non Home Based 3091 3190 6477 6613

Employers Business 2040 2069 1747 1769

LGV 4786 4786 4430 4430

HGV 2501 2501 1460 1460

Technical Note




Total 53,868 55,447 53,099 54,758

5 Scenario Testing

This section discusses the transport impact of the new housing development scenario on the highway

network along routes in the vicinity of the sites and at particular network pinch points.

The new demand forecasts were assigned to the SHUM model 2031 network for the AM peak (08:00-

09:00) and PM peak (16:00-17:00) hour. Outputs generated for use in analysing the impacts include a

number of plots displaying changes in assigned traffic volumes, changes in junction delays and out-turn

volume over capacity ratios (V/C%) for key routes in the vicinity of the sites.

The analysis has shown in the vicinity of the sites that the additional development trips do not have an

overly significant impact on the performance of the network beyond its condition without them. Sizable

increases in vehicle flows have been seen along the main routes from the sites as shown in Figures 5.1

to 5.3 below for the AM peak, it’s a similar picture in the PM Peak (Appendix B.1-B.3). Flows across

the rest of the network do not show many significant changes, typically changing by less than 50 PCU’s

(car equivalents). This demonstrates that the developments are not causing large re-routing of trips

beyond the immediate vicinity of the sites.

Technical Note




Figure 5.1 AM Peak East of Luton Area difference in flows

Figure 5.2 AM Peak North of Baldock Area difference in flows

Key

Increase in link flow from Ref. Case

Decrease in link flow from Ref. Case

Key



East of Luton Site

North of Baldock Site

A505

B655

A602

B656

Preston Rd

A505

A507

Stotfold Rd

B656 Royston Rd

A1(M)

Hatch La

Technical Note




Figure 5.3 AM Peak Little Wymondley & Todds Green Area difference in flows

The increases in flow have subsequently led to an increase in junction delays and road links operating

over capacity across the model area. Appendix B contains diagrams for the AM and PM peak where

there is significant junction delay around the development sites in the ‘with developments’ scenario (DS).

There are no significant junction delays around the Little Wymondley and Todds Green sites or in the

Baldock area. In the Hitchin area where the East of Luton traffic enters the model there is however some

junctions with a noticeably higher delay. In the majority of these cases, the apparent decline in network

performance can be attributed more to previously identified pinch points than these new development

sites, although it is clear that their contribution of trips to the network does in places exacerbate the

situation.

The only notable exception to the location of pinch points, where junction performance has deteriorated

due to the new developments is at the A1(M) Junction 9, southbound on-slip. Without development there

are high delays for merging traffic (approx 50s), but with the additional development traffic from the

North Baldock site, traffic blocks back to the top of the slip road. This only occurs in the AM peak and

results in inbound traffic re-routing along local roads as shown in Figure 5.4.

Key



Todds Green

Little Wymondley

A602 Chantry La

Stevenage Rd

Technical Note




Figure 5.4 AM Peak – A1(M) junction 9 Flow difference between with development (DS) and

without (Ref. Case)

Other junctions where delays, and volume over capacity ratios, were significantly higher are Grace

Way/A1155 Fairlands Way junction and A602 Six Hills Way/Monkswood Way junction and the two

junctions either side of it, shown in Figure 5.5. The congestion at both junctions is greater in the AM

peak. Although these junctions were not originally identified as a pinch point, the junctions have a very

similar delay situation to the without development traffic scenario and therefore are not directly

attributable to the developments impact. It’s a similar situation for the junctions close to A1(M) junction 7,

that also appear in Figure 5.5, and at J8 northbound onslip in the PM peak.

Any future detailed development testing is likely to require consideration of the need for mitigation

infrastructure at these junctions mentioned.

Key



A1M Junction 9

Technical Note

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27 Doc. F8/10 Revised: April 2009



Figure 5.5 AM Peak Reference Case significant junction delays.

5.1 Pinch Points

Particular pinch points on the network act as constraints to effective operation of the network.

The diagram Figure 5.6 identifies where the model analysis has indicated that in 2031 there is a problem with network operation which corresponds with those junctions identified as pinch points in LDF

Housing Assessment undertaken in November 2012. The junctions have been identified through

modelling indicators which show where there are still more than 100 vehicles queuing at the end of the

AM or PM peak hour. There are a range of indicators that could be used, but queuing traffic at particular

locations at the end of the modelled peak hour enables us to focus on the pinch points on the network

and identify in more detail what the issues are.

Grace Way

Six Hills Way

A602 Broadhall Way A1(M) Junction 7

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Figure 5.6 Problem locations in 2031 in the Local Plan Scenarios

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This information is also shown in tabular form in Table 5.1 and 5.2. The tables show when the problems

occur (i.e. morning peak, evening peak or both) for each of the identified problems.

Table 5.1 Identified Problem Locations in 2031

Ref. Problem Location Ref DS

HM2 A505 / B655 Pirton Road � �

HM3 Payne's Park

�

HM4 A602 / B656 London Road (Hitchin Hill) � �

HM5.1 Hitchin Industrial Area / Cadwell Lane � �

HM5.2 A505 Cambridge Road / Woolgrove Road / Willian Road � �

HM6 Arch Road / Hitchin Road (Great Wymondley) � �

HM7 Fishers Green Road � �

HM9.1 A1(M) Junction 7 Northbound Onslip � �

HM9.2 A1(M) Junction 7 Roundabout (southbound offslip)

�

HM9.3 A1(M) Junction 7 Northbound mainline (from Junction 6) � �

HM10.1 A602 Hitchin Road / A1072 Gunnels Wood Road Roundabout (southbound approach)

� �

HM10.2 A602 Hitchin Road / A1072 Gunnels Wood Road Roundabout (westbound approach)

� �

� Problem location in the morning peak only

� Problem location in the evening peak only

� Problem location in both morning and evening peak

Table 5.2 Locations where worsening conditions at Pinch Points is having an effect in 2031

Ref. Problem Location

Related Pinch Point Ref.

Ref DS

A A505 / Grays Lane HM2 � �

B A505 / Willow Lane HM2 � �

C Willow Lane / Hawthorn Close HM2 � �

D B656 Hitchin Hill / St John's Road HM4 � �

E Cadwell Lane / Hillfield Avenue HM5.1 � �

F Cadwell Lane / Wallace Way HM5.1 � �

G A602 Broadhall Way / A1072 Gunnels Wood Rd HM9.2 � �

� Problem location in the morning peak only

� Problem location in the evening peak only

� Problem location in both morning and evening peak

The tables show there are operational issues at the same locations across the network with and without

the new development trips. Table 5.1 displays the junctions which are known pinch points from the

previous housing assessment (Nov. 2012). For sites HM3 and HM9.2 the problem only occurs in the

‘with development’ scenario. This demonstrates the additional developments in this proposal result in a

Technical Note

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further exacerbation of the network problems. The mitigation options proposed for those pinch point

junctions from the previous assessment will be even more critical under the current development

proposals.

Table 5.2 highlights additional junctions found to have queues over 100 vehicles as a knock on effect of

the pinch point junctions. In these instances, pinch points HM2 and HM5.1 are the contributors to the

deterioration of these additional junctions. The excessive delays at the pinch points and their

neighbouring junctions are shown by the blue circles in Figures 5.7 below. The size of the circles

represents the size of delay.

A602 Broadhall Way / A602 Monkswood Way (Eastbound approach) is the only junction with more than

100 vehicles queuing at the end of the period but not obviously related to any pinch point. It was

however identified to be a problem in the Stevenage Core Strategy Testing work conducted in April 2013

and mitigation in the form of a traffic signal upgrade was recommended.

Figure 5.7 DS AM Peak Junction Delay in East Luton Area

6.0 Summary

The additional development assessment for 2031, highlights that there will be a number of highway

impacts across the network when the developments are in place, and not all of them concentrated in the

vicinity of the development sites. Despite this, no new junction problems were identified in this

assessment. Rather, the development traffic only seems to exacerbate pre-existing problems in the

network. This demonstrates that the highway networks performance does not significantly decline as a

direct impact of the additional development traffic. It should be noted that some coarse assumptions

have been made about the level of traffic that will enter the SHUM network from these development

sites, and variations in these assumptions may alter this conclusion,

It is important to note, this assessment is not able to specifically identify the highway impacts that occur

immediately surrounding the development sites for East of Luton and North of Baldock because of their

location outside of the model network. Instead this assessment provides a broad overview of potential

problem locations identified in the future year of 2031, when specific housing development scenarios are

in place. Any further assessment, relating to a particular scenario would require further testing to fully

Cadwell Lane

B655

A505

A602

Hitchin Hill

Technical Note

Page: 14 of




understand the potential of any transport intervention(s) relating to a land-use scenario or individual

development components.

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