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Worked Example For Interim Advice Note 149/11
Page 1 of 22 June 11
Worked Example including Sample Design Strategy Record Extracts This document provides additional advice to IAN 149/11. Existing Motorway Minimum Requirements
This Worked Example is based on a fictitious project and has been prepared solely for the benefit of illustrating the principles contained within Interim Advice Note (IAN) 149/11. It must not be used, or relied upon, in whole or in part, for any project, or by any party without the written authorisation of WSP UK Limited and Parsons Brinckerhoff Ltd. Users of the IAN must satisfy themselves on the applicability of the IAN to the particular project with which they are dealing. The DSR examples are not exhaustive and designers must determine the data to be included in the DSR. WSP UK Limited and Parsons Brinckerhoff Ltd accept no responsibility or liability for the consequences of this document if it is used for a purpose other than that for which is was commissioned. Persons wishing to rely upon this report for other purposes must seek written authority to do so from the owner of this report and/or WSP UK Limited/Parsons Brinckerhoff Ltd and agree to indemnify WSP UK Limited and Parsons Brinckerhoff Ltd for any and all loss or damage resulting therefrom.
Worked Example For Interim Advice Note 149/11
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1. Introduction
Purpose 1.1. This worked example provides guidance on the application of IAN 149/11 to an
improvement scheme for an existing motorway improvement. This example is not exhaustive and does not consider all aspects of the whole scheme. The example considers the following elements of IAN 149/11 in turn:
Highways Link Design Layout of Grade Separated Junctions Cross-Sections and Headrooms Motorway Signalling
The worked example also includes extracts from a draft Design Strategy Record (DSR) for each chapter of the IAN. These extracts and design decisions presented are illustrative only and are included to provide an indication of the details that should be included in a DSR. References to minutes are included where appropriate discussions should be considered, however reference is illustrative only and minutes are not included. The column headings of the DSR should be chosen to suit the scheme, the draft DSR columns headings vary to avoid implying a particular layout.
Existing Conditions 1.2. The scheme under consideration is the widening of a D3M rural motorway to D4M
between Junction J2 and Junction J5 as shown in Figure 1-1. Discussions with the Overseeing Organisation have been undertaken to identify the following:
i) Traffic flows ii) Accident record iii) Operational performance iv) Strategic scheme constraints
Figure 1-1 Worked Example Scheme Layout The existing conditions determined through these discussions are recorded in the DSR, Figure 1-2 Existing Conditions.
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Design Strategy Record
Ref. IAN Condition Condition Location Comments
1.0 Existing Conditions
1.1 Traffic Flows
1.1.1 Existing merge values 1400 vehicles per hour Junction2
1.1.2 Existing Traffic flow 5400 vehicles per hour J2-J3
1.1.3 Existing diverge/ merge values 900/ 1500 vehicles per hour Junction3
Extracted from ANPR and MIDAS data provided by Overseeing Organisation. See Figure 3-1.
1.2 Accident Record
1.2.1 5 year accident data 50 accidents over a 5 year period. No fatalities.
Junction2 Accident rate higher than average. Consideration to be given to extent of relaxations through junction. Discussion held with Overseeing Organisation and Emergency Service do not identify alignment or visibility as contributory factors. See minutes.
1.3 Operational Performance
1.3.1 Traffic Flow Peak hour congestion Junction4 Northbound, am peak Junction4 southbound pm peak
Discussions with Overseeing Organisation (see Minutes)
1.3.2 Drainage Standing water reported on carriageway
Ch 107+100 to 107+200
Discussions with Overseeing Organisation (see Minutes) Mitigation to be provided, see DSR section 2.4
1.4 Scheme Constraints
1.4.1 Central Reserve Width Proposed relaxation to Central Reserve Width
At localised pinch points Initial concern that reduced Central Reserve Width would interfere with traffic management of maintenance schemes. Relaxation of central reserve widths agreed with Overseeing Organisation, future TM arrangements to be modified to suit relaxed widths.
Rev 0 Design Organisation Sign Off Date
Figure 1-2 Existing Conditions
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2. Highway Link
2.1. A 3D geometric model of the existing motorway was generated by comprising best-
fit horizontal and vertical alignments through the existing ground model. This model was used to develop a design model of the proposed improvement. The following geometric parameters of the design model have been assessed:
Stopping Sight Distance Horizontal Alignment Vertical Alignment Superelevation
Values for these parameters, and the level of relaxation or departure required against TD9, have been logged in a schematic format, as shown in the sample at Figure 2-1. The relaxations required against IAN 149/11, are recorded in the DSR, as shown in Figure 2-2.
2.2. A review of the drainage flow path identified a number of locations where excessive
flow path lengths would be generated by the increased the paved width. These locations should be recorded in the DSR, along with proposed mitigation where appropriate.
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Design Strategy Record
Ref. IAN Condition Condition Location Comment
2.0 Highway Link Design
2.1 SSD
2.1.1 Relaxations on the approach to and through a junction.
1 step relaxation. Junction3 1 step relaxation is a result of the existing alignment. See Figure 2-1. Ref SSD 104
2.2 Horizontal Alignment
2.2.1 Horizontal alignment The horizontal alignment radii are at or above the Desirable Minimum TD9 Table 3, throughout the scheme.
Throughout scheme This high standard of horizontal alignment reduces the impact of other relaxations.
2.3 Vertical Alignment
2.3.1 Relaxations on the approach to and through a junction.
1 step relaxation. Junction3 1 step relaxation is a result of the existing alignment. See Figure 2-1.
2.4 Camber
2.4.1 Superelevation not appropriate for horizontal radius
Existing superelevation to be retained.
Ch99+500 to 100+800 & Ch101+150 to 102+150 Ch105+950 to 107+050 & Ch107+200 to 208+300
See Figure 2-1. Ref SUP 103 & 104, 107 & 108
2.4.2 Drainage Flow Paths Long drainage flow path in excess of TA80 Fig 2.4 criteria
Ch107+100 to 107+200 Change in superelevation results in long flow path. Rolling crown introduced. See Figure 2-1. Ref SUP 103 & 104, 107 & 108
Rev 0 Design Organisation Sign Off Date
Figure 2-2 Relaxations Against IAN 149/11
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3. Junction Layout 3.1. The basic layout, number of mainline lanes, merge, diverge layouts, can be
identified using the existing traffic flows and turning movements, derived from the ANPR data and MIDAS loops, plus an increase related to the likely achievable layout. In this example 33% has been added to the existing traffic flows as it would be equivalent to adding a 4th lane to a 3-lane motorway. In addition the layout should be tested against a higher flow and in this example +66% has been used, to test the decisions and understand the consequences if flows were greater the +33% flow. A brief explanation of the assumptions made should be recorded in the DSR.
3.2. The flows should be used to determine the requirements for each link, connector
road merge and diverge, individually, which are then modified to make a coherent layout in an iterative process. Layouts that are not permitted (eg 5 lanes) or clearly not achievable (cost, space) should not be included, however avoid being too restrictive at this stage, as possible layouts may be rejected without fully understanding the consequences.
3.3. Elements of the design process are set out below and Figure 3-1, Junction Layout
Selection indicates the iterative process. Alternatively a spreadsheet can be used as shown at Figure 3-2 and serve as a record of the decisions taken and form part of the DSR, Figure 3-3, together with other relevant data.
i. The existing Junction 2 northbound merge is a 2-lane slip Layout A taper,
which is not permitted (TD22, 2.28) so must be changed. A lane gain is required to suit the up and downstream flows. Inserting the merge and upstream flows into TD22 Figure 2/3 MW, indicates for the +33% flows it lies on the border of Layouts C, F and G. The +66% flows lie on the Layout F and G border, though at the edge of the chart. When the flows are moderated they clearly require Layout F. This layout significantly extends the merge length over which additional width is required in addition to the additional lane. This impacts on a significant physical constraint and therefore the relaxation allowed by IAN 149/11 Paragraph 3.3.3 a, to use the 120kph Rural All-Purpose values in TD 22 Table 4/3 has been utilised to reduce the taper length from 205m to 150m and the ghost island tail from 180m to 150m a total saving of 85m. This should be recorded in the DSR, Figure 3-3.
ii. For link Junction 2 to Junction 3, the +33% flow requires 4 lanes at 99% capacity (1,800 veh/hr/lane), therefore 3 lanes not appropriate. The +66% flow requires 5 lanes but this is not permitted (TD27/06, Paragraph 4.9.4) therefore use 4 lanes with flows moderated to 8,000 veh/hr. (Note 2,000veh/hr/lane regularly achieved away from junctions. For junctions, TD22 Figure 2/3 MW and TD22 Figure 2/5 MW assume 1,800 veh/hr/lane.)
iii. Junction 3 northbound diverge is currently a 2 lane Layout A taper. The through junction link may be 3 lane or 4 lane so consider both options initially. Inserting the diverge and downstream flows into TD22 Figure 2/5 MW, the +33% flows lie on the border between Layout A and Layout C (taper with lane drop) so either could be used without invoking a departure. The +66% traffic flows requires a Layout B (Option 1 Preferred) ghost island diverge. The moderated flows lie in the Layout C range, a lane drop taper diverge. Clearly a lane drop is preferable for the higher flows but would
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mean only 3 lanes through Junction 3, a capacity of 2,233 veh/hr/lane would be required. This is not achievable over a short length and therefore Layout B should be provided, though it will require a Departure. However, the choice has an impact on the downstream Junction 3 merge which therefore needs to be considered before a decision is made. The ‘decision process’ should be briefly recorded in the DSR.
iv. Providing 4 lanes through Junction 3 precludes the provision of a lane gain at the following merge because it would produce 5 lanes and is unachievable. Considering the wider impact, the Junction 3 northbound merge is the major contributor to links further north where capacity is an issue. Therefore it is preferable to restrict this merge to protect the motorway. All the flows require a Layout F, however IAN 149/11, Paragraph 3.3.4 allows this to be substituted by a Layout C as a relaxation. A Layout C will limit merge flows compared with the Layout F and could have ramp metering which would not be appropriate with a lane gain. This confirms the decision to provide a Layout B diverge northbound to Junction 3.
v. The next stage is to consider the width constraints in more detail to determine what widths of cross-sectional features might be achievable using IAN 149/11 Table 4-1. This may lead to further iterations of the identification of layouts that should be considered, for example the use of reduced lengths of diverge and merge components.
vi. Similarly, for example, Junction 2 northbound merge Layout F would require demolition and reconstruction of a retaining wall on a new line. This could be avoided by using the relaxation in IAN 149/11 Paragraph 3.3.3.a to use All-Purpose parameters for the taper and ghost island tail.
vii. At Junction 4, northbound merge a Layout F is required but this would provide 5 lanes, which is not permitted. Rather than substitute Layout F with Layout C or H, as IAN 149/11 Paragraph 3.3.4 allows, the lane gain could be used to form a dedicated lane to the next diverge, as the weave length has previously been identified as short. (See Figure 3-4.) This layout is not covered by standards and would require a departure.
viii. The weaving length (Lact) between Junctions 4 and 5, northbound is less than 2,000m. Therefore in accordance with the allowed relaxation given in IAN 149/11 Paragraph 3.4.10, the existing distance between the merge nose tip and the downstream diverge nose tip will be maintained without applying for a Departure from Standard. In addition, Lact must satisfy TD22 Figure 4/14, upper graph as required by IAN 149/11 Paragraph 3.4.11. The sketch in Figure 3-1 shows a Layout F merge, with the additional lane being dedicated to the next exit with a Layout D diverge downstream. Junction 5 northbound diverge requires 2 lane drop, one of which is the dedicated lane from the Junction 4 merge. This layout minimises the number of total number lane changes that must be carried out by drivers.
3.4. The layout produces two weave lengths to be checked; firstly between the merge taper and lane drop and secondly between the merge taper and the diverge taper. In the first case Lact 1 is 535m and the weave flows are 993 + 1,534 = 2,572; which, using TD 22 Figure 4/14 and D/V = 15 is acceptable. The second weaving length Lact 2 is 725m and the weave flows are 993 + 1,534 + 720 = 3,292; in this
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case TD22 Figure 4/14 is not satisfied by a small margin. Ideally this would be overcome by moving the merge and diverge noses to increase the weave length however this may not be achievable because of the extent of reconstruction required on the slip roads. If the noses cannot be moved further apart IAN 149/11 Paragraphs 3.4.5, 6 and 7, allows the geometrical parameters to be reduced to Rural All-Purpose 120 kph standard. Applying this to the merge taper and the diverge ghost island head and diverge taper would increase the two weave lengths from 725m to 810m and from 535m to 590m which satisfies TD 22 Figure 4/14.
3.5. Alternatively the two lanes of the on-slip road could be reduced to one lane which is
a dedicated lane to the Junction 5 off-slip see Figure 3-2. This would remove the weaving length issue entirely but is dependent on the capacity of Junction 5. The options considered and the decisions made should be recorded in the DSR.
3.6. For the Junction 4 merge, short weaving length and Junction 5 diverge to operate
successfully the layout must minimise lane changing. This can be achieved by providing gantry direction signing in advance of the Junction 4 merge to maximise the use of the lane drop at Junction 5 diverge. Gantry signing will also be required because of the ghost island diverge as TD 22 Paragraph 2.51 remains mandatory. As gantries are required, and this length will be congested, consideration should be given to providing variable mandatory speed limits which could be used to mitigate the short weaving length, if necessary in the longer term.
3.7. Once the layout and operational system is established, possibly with alternatives, a
traffic model and cost benefit analysis may be required to determine the return on investment and assist in the final choice of layouts and operational provision.
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Figure 3-2 Junction Choice
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Figure 3-3 (i) Junction Layout Selection
Design Strategy Record
Ref. Topic / Relaxation Location Discussion
3 Grade Separated Junctions
Traffic Flows
3.1 Assessment of traffic flows throughout scheme
Route character is heavily commuter therefore neutral month weekday AM and PM peaks used taken from ANPR & MIDAS loops to establish flows and turning movements
33% and 66% extra traffic examined to minimise risks of being wrong
Layout Selection
3.2 J3 n’bd diverge requires Layout C, but lane drop not appropriate for link through junction, therefore Layout B proposed.
J3 northbound IAN 149/11 Paragraph 3.3.10 & 11 do not permit changes without a Departure from Standard
Departure required for a coherent layout. 3 lanes through downstream junction would create bottleneck putting motorway flow at risk of congestion
3.3 J3 n’bd merge requires Layout F but would give 5 lanes which is not permitted.
J3 northbound Use IAN 149/11 Paragraph 3.3.4 to substitute Layout F with Layout C Relaxation required for coherent layout. Restricting this merge will protect motorway from congestion whilst still providing improvement for local raods.
Geometric Standard
3.4 J2 n’bd merge Layout F. Relaxation to use All-Purpose road dimensions for taper and ghost island tail to avoid adjusting existing retaining wall
J2 n’bd merge Slower traffic will be in merge lane 1 and will not be affected.
3.5 Near straights on J3 northbound merge and J3 southbound diverge are below standard, being 10m and 15m respectively.
J3 northbound merge & southbound diverge
These lengths will be maintained. The relaxation allowed in IAN 149/11 Paragraph 3.4.9, is used to avoid a Departure from Standard The operational assessment did not indicate any particular problem with the lack of near straight.
3.6
J4 Layout F merge, reduction to All Purpose 120kph standards to taper length proposed to improve weave length, see weaving lengths below.
J4 northbound merge
Not possible to move merge nose upstream because of impact on slip road which would require reconstruction over significant length.
3.7
J5 Layout D diverge, reduction to All Purpose 120kph standards proposed for ghost island head taper to improve weave length see weaving lengths below.
J5 northbound diverge
Not possible to move diverge nose downstream because of impact on slip road which would require reconstruction over significant length.
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Figure 3-3 (ii) Junction Layout Selection (continued)
Design Strategy Record
Ref. Topic / Relaxation Location Discussion
3 Grade Separated Junctions (cont.)
Weaving Length
3.8 J4 to J5 existing weaving length less than 2km. Lact2 not satisfy TD 22 Figure 4/14
J4 - 5 northbound
Distance between merge nose and downstream diverge nose will be maintained. IAN 149/11, Paragraph 2.4.10, as no possibility of increasing weave length because of impact on slip roads. Closure of either junction not an option. A ‘link road’ will be created using a ghost island to provide additional capacity for junction hopping traffic.
Upstream merge and downstream diverge reduced to Rural All-Purpose standard to maximise weave length, IAN 149/11, Paragraphs 2.4.4 to 7. Considered preferable to maximise weave length over merge / diverge parameters.
Following merge & diverge reductions, weave lengths satisfy TD22 Figure 4/14 at D/V = 15.
Mitigation for the relaxations include; gantry mounted direction signing designed to minimise total lane changes required and control signals, including option for VMSL.
Rev 0 Design Team Sign Off Date
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Figure 3-4
Figure 3-5
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4. Cross-Section 4.1. There are 17 overbridges and underbridges on this section of the network. The
schematic provided in Figure 4-1 illustrates the extent of departures from a TD27 compliant design. The widths available at each structure have been assessed and a suitable level of relaxation to IAN 149/11 Table 4-1 has been determined to avoid the need for demolition or widening of the constraining structure.
4.2. Full widening to TD 27 would be possible for some lengths between structures, but
each structure constrains the width available. This would result in repeated narrowing and widening of lanes, with the potential for full lane widths provision only available over relatively short lengths, between 1 and 1.5 kilometres.
4.3. This proposal has been rejected in favour of providing continuous lane widths,
relaxed in accordance with IAN 149/11 Table 4-1 Priority 4, with reductions to hardshoulder and central reserve widths where required at the structures. The schematic provided in Figure 4-2 illustrates the extent of relaxations to IAN 149/11 Table 4-1.
4.4. Further reductions to the overall width have been considered by narrowing central
reserve throughout, relaxing the cross section in accordance with IAN 149/11 Table 4-1 Priority 5 or 6. However the savings in earthworks costs do not justify the costs of the additional central reserve work required in this case, and this proposal has also been rejected. These decisions and other similar ones should be recorded in the DSR as shown in Figure 4-3.
4.5. Having established the basic layout and the options at specific constraints, the
operational system can be considered, such as Hard Shoulder Running versus 4-lane and uncontrolled versus controlled motorway. For example, the length through Junction 3, where there was considerable discussion above as whether 3 or 4 lanes were required, would suit 4 lanes without a hardshoulder. This is because the flow is unlikely to reach capacity and the slip roads can provide an alternative route for emergency vehicles.
4.6. Similarly, between Junction 4 and Junction 5, northbound, where 4 lanes plus a
dedicated lane is proposed, a hardshoulder is not essential. It is a short length with a considerable total paved width, including a ghost island which, coupled with the relatively lightly trafficked dedicated lane, would allow drivers to create room for emergency vehicles.
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Design Strategy Record
Ref. IAN Condition Condition Location Solution
4 Cross-Sections and Headroom
4.1 Cross Sections
4.1.1 Cross Section Priority Order 4 Throughout Full width only available over short lengths, relaxed lane widths adopted throughout scheme. Provides consistent width for road users, and reduced impact on earthworks costs. See Figure 4-2.
4.1.2 Priority Order 6 OB S11
OB S14
OB S19
Reduced width used through these structures to avoid reconstruction.
Hard shoulder and central reserve widths relaxed. Cross section to Priority Order 6. See Figure 4-2.
4.1.3 Priority Order 9 UB S3 – S5
OB S6
UB S7
UB S12 – S15
Reduced width required to avoid reconstruction of OB S6.
Also significant difficulties to widening underbridges because of headroom to roads under and time/expense of working over rail line.
Reduced width available through structures. Hardshoulder relaxed to EAR. Cross section to Priority Order 9. See Figure 4-2.
EAR provision discussed with Emergency Services. See Minutes.
Rev 0 Design Organisation Sign Off Date
Figure 4-3 Cross-Section Choice
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5. Motorway Signalling 5.1 The relaxations permitted by IAN 149/11 removes the requirement of 11 gantries,
by not providing lane control on some links, the increased spacing allowed and change in signal visibility requirements. Figure 5-1 Motorway Signalling to TD46/05 indicates the requirements to TD46/05 and Figure 5-2 Motorway Signalling to IAN 149/11, the requirements following relaxations permitted by IAN 149/11 relaxations. An example DSR, recording the relaxations, appears in Figure 5-3.
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Design Strategy Record
Ref. IAN Condition Location Comment
5 Motorway Signalling
Direction Signing
5.1 TD46 requirement for AHEAD signing Throughout scheme
AHEAD signing limited to historic provision with AHEAD signing only shown on FINAL ADS except at J4-J5 where weave length is short and clear signing will be required.. Elsewhere, no perceived need for additional AHEAD signing, as does not exist at present.
Control Signalling
5.2 Omission of Lane Control, although 4 lanes Link J2-J3 IAN 149/11 supersedes TD46/05 Paragraph 3.11, which required Lane Control on all 4 lane carriageways. Not justified in environmental or economic terms and impacts on scheme affordability.
5.3 Spacing and signal visibility distance relaxations
Throughout scheme
Relaxations allowed by IAN 149/11, have been fully utilised to reduce signals, including VMS, reducing cost and visual impact. The operational review of the existing motorway does not show any particular concerns that would be mitigated by further signals.
Rev 0 Design Organisation Sign Off Date
Figure 5-3 Motorway Signalling Relaxations