The Design of Major Interchanges - Standards for Highways · The Design of Major Interchanges...

The Design of Major Interchanges

Summary: This standard augments TD 22/92. It sets out the Design and AssessmentProcedures to be used for Major Interchange Design. It also sets standards forthose features which are likely to be required in major interchanges and forwhich no current standards are available.

THE HIGHWAYS AGENCY TD 39/94

THE SCOTTISH OFFICE DEVELOPMENT DEPARTMENT

THE WELSH OFFICE

Y SWYDDFA GYMREIG

THE DEPARTMENT OF THE ENVIRONMENT FOR

NORTHERN IRELAND

Volume 6 Section 2Part 4 TD 39/94 Registration of Amendments

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DESIGN MANUAL FOR ROADS AND BRIDGES


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VOLUME 6 ROAD GEOMETRYSECTION 2 JUNCTIONS

PART 4

TD 39/94

THE DESIGN OF MAJORINTERCHANGES

Contents

Chapter

1. Introduction

2. Design Procedure

3. General Layout Advice

4. Major Merges and Major Diverges

5. Operational Assessment

6. Examples of Upgrade Paths forMajor Interchanges

7. References

8. Enquiries

Volume 6 Section 2 Chapter 1Part 4 TD 39/94 Introduction

1. INTRODUCTION

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1.1 Standard TD 22 (DMRB 6.2.1) sets out theOverseeing Organisation's design standards andmethodology for the geometric layout of gradeseparated junctions on trunk roads.

1.2 Advice Note TA 48 (DMRB 6.2.2)provides guidance on the principles for safety andtraffic operation on which the Standard TD 22(DMRB 6.2.1) is based.

1.3 This document provides guidance on thedesign of major interchanges including theexpansion and improvement of existing interchangesand junctions beyond the scope of Standard TD 22(DMRB 6.2.1) and Advice Note TA 48(DMRB 6.2.2). It follows research into majorinterchanges and high capacity highway corridoroptions, including use of link roads.

1.6 Solutions for major interchanges will oftencontain elements designed in accordance withStandard TD 22 (DMRB 6.2.1) and Advice Note TA48 (DMRB 6.2.2) and it is therefore intended thatthis document be read in conjunction with them.

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General

1.4 Major interchanges will generally be requiredat the intersection of motorways and major trunk roadswhere capacity is increased by carriageway widening. Major interchanges will, in many cases, be larger andmore complex than the original junction.

1.5 Choice of location will often be severelyrestricted, compared with completely new construction. Layout options may also be restricted by adjacentdevelopment and other constraints. Management oftraffic on existing roads will often play a significant partin the assessment of options and the planning ofconstruction.

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.7 Due to the general larger size of majorterchanges, consequential greater environmentalpact and the number of factors to be considered, aore formalised assessment of options is required.

1.10 Operational and driveability assessments on the decisions and manoeuvres required to nealternative interchange layouts are included.

1.11 Examples of possible layouts, for the

existing interchanges are illustrated, including consideration of connections to motorway link roa

ope

8 Recommendations are given on a designethodology and appraisal process for the compariso alternative solutions.

9 New layouts are provided for major merges anajor diverges to allow for three lanes joining oraving the mainline. Merges and diverges larger thanese are unlikely to be required due to the practical aerational limits on the maximum width of thembined carriageway. For dealing with very largerning movements, connection of interchange links tootorway link roads is likely to be required. Thereforee choice of motorway carriageway provision andajor interchange layout will be an iterative process.

pansion, rearrangement or replacement of typical

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Chapter 1 Volume 6 Section 2Introduction Part 4 TD 39/94

Implementation

1.12 This document should be used forthwith onall schemes for the construction and improvement ofmotorways currently being prepared, provided that, inthe opinion of the Overseeing Organisation, thiswould not result in significant additional expense ordelay progress. Design Organisations should confirmits application to particular schemes with theOverseeing Organisation.

Definitions

1.13 The terminology follows where possible thedefinitions contained in BS 6100: Subsection 2.4.1and Standard TD 22 (DMRB 6.2.1).

1.14 The following additional terms have beendefined for use in this document:

1.15 Major merge: A merge where three lanesjoin the mainline carriageway.

1.16 Major diverge: A diverge where three lanesleave the mainline carriageway.

Mandatory Sections

1.17 Sections of this document which form part ofthe standards the Overseeing Organisation expects indesign are highlighted by being contained in boxes. These are the sections with which the designer mustcomply. The remainder of the document containsadvice and enlargement which is commended todesigners for their consideration.


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Volume 6 Section 2 Chapter 2Part 4 TD 39/94 Design Procedure

2. DESIGN PROCEDURE

2.1 This document relates primarily to theexpansion or redevelopment of existing junctions andinterchanges. Consideration of traffic managementduring construction is a fundamental requirement inproviding a safe and economical construction. Thescope for design will be constrained by the existingjunction layout and also by development adjacent tothe junction in many cases.

2.2 Each scheme shall be considered within theoverall network strategy. This strategy shall includeall network considerations including the degree ofaccess to be provided and the omission of existingconnections for the benefit of longer distance trafficor the environment.

2.3 Flow standards and the methodology ofdetermining traffic flow levels is as specified in TD22 (DMRB 6.2.1) and TA 48 (DMRB 6.2.2). Consideration shall be given to network constraintsand the use of constrained traffic models to avoidforecast traffic demand far above that which can beprovided by the local network as it may be developedover the life of the proposed scheme.

General Principles

2.4 A number of options should be considered anddeveloped. The development of each option will be aniterative process. While all options should cater for thepredicted design flows, the options for considerationshould range from alternatives which have a minimaleffect on existing traffic movements to those whichprovide the optimum final design. A decisionframework including all the alternatives should beproduced.

2.5 Consideration of all the major contributingfactors included in the design methodology should beundertaken in the assessment to produce a solutionwhich most closely meets the requirements whileremaining value for money.

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2.6 The requirements for incident management andmaintenance operations are important considerations andinclude the provision of access routes and u-turn facilitiesfor maintenance and emergency vehicles as well asadditional widths of carriageway.

2.7 Network consistency and the use of standardfeatures including signs and road markings over a routeor within an area is also important. Drivers should not besurprised by rare features as they may carry an increasedaccident risk. Interchange simplicity and driverunderstanding are also relevant. The layouts must beappraised for the adequate provision of signing.

.8 Through route continuity should be maintainedhere possible. The principal signed or through routehould have priority and the secondary route should divergen the left.

.9 A program of sensitivity testing of the predictedrning flows should be undertaken to examine the

onsequences and costs of the flows in the design yeareing different from the predictions. Examples ofensitivity testing of flows are given in Appendix 3 of TA3 (DMRB 6.2).


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Chapter 2 Volume 6 Section 2Design Procedure Part 4 TD 39/94

RURAL STANDARDS URBAN STANDARDS

120 100 85 70 100 85 70 60

Motorway/ +++ +++AP Road

Link Road +++++++ +++++++

AP Road +++ +++

Link Road +++++++ +++++++

Interchange Link +++ +++

Slip +++ +++

Table 2/1: Interchange and Junction Element Design Speed Steps - kph

(This table is an interpretation of Table 4/2 in TD 22 (DMRB 6.2.1)

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2.10 There is a requirement in TD 22 (DMRB 6.2.1)for link road and interchange link design speeds to begenerally lower than the mainline. This reduction is oneor two steps down from that provided for the mainline. Where constraints require interchange link design to bebased on further reductions and the links are thereforesignificantly lower than driver expectation, advisory ormandatory speed limits need to be used. Table 2/1 showdesign speed steps on interchange and junction elementfor both the rural and urban situations as set down in TD22 (DMRB 6.2.1).

2.11 There may be situations where a majorinterchange is particularly complex or constrained inavailable area. In these circumstances it may be desirabto apply an overall speed limit to the interchange, belowthat which applies to the adjacent sections of the mainlinThis would allow more compact elements of lower designspeeds to be provided. The application of such a speedlimit is a matter for discussion with the OverseeingOrganisation at an early stage in the design. Thecomplete interchange needs to be presented to drivers aan entity and would normally be the subject of uniformtraffic control treatment which may require specialfacilities to vary it during the day.

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.12 The most efficient form of interchange layout ishat which presents drivers with both the minimumumber of clear unambiguous decision points and withdequate time/distance between decisions to ensure th

he path through the interchange is easily understood. his means that the siting and size of advance directionigns must be appropriate for the speed and reading tim

nvolved. An acceptable interchange layout will mostuccessfully balance the impact of the interchange on tnvironment with cost and the operational requirements

or traffic.

esign Methodology

.13 A flow chart illustrating the design methodologys shown in Figure 2/1.

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A

1. Determine Strategic Network Extension or Improvement Plan

2. Determine Design Year

3. Establish Urban or Rural Standards

4. Determine Constraints

5. Develop Local Network and Interchange Strategy

6. Select Options for Appraisal

7. Select Appraisal Criteria

8. Develop Traffic Flows

9. Determine Mainline and Connector Road Lane Requirements

10. Check Merge, Diverge and Weaving Layouts

11. Check that remaining geometric standards can be achieved

12. Check that an effective and economic signing system can be provided

13. Carry options forward to appraisal stage

Satisfactory

Satisfactory

Satisfactory

Satisfactory

Not Satisfactory

Not Satisfactory

Not Satisfactory

Not Satisfactory


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Figure 2/1 : Design Methodology


Determine Strategic Network orImprovement Plan

2.14 It is important that interchanges and lengths omotorway between interchanges are considered togefrom the outset. Choice of mainline carriagewayprovision involving wide carriageways and/ormotorway link roads will influence interchange layoutConversely problems in achieving adequate interchandesigns including merge, diverge and weaving areasmay influence the choice of mainline provision.

Determine Design Year

2.15 Careful consideration of the design year will brequired, bearing in mind the design year strategyadopted for the routes connected by the interchange.will often be easier to add capacity to a motorway routhan to reconstruct a major interchange and thereforehigh possible design year traffic flows should beadequately covered.

Establish Urban or Rural Standards

2.16 Major interchanges will normally be located ointer-urban routes designed to rural standards. Howerestricted space available around existing interchangmay require consideration of speed restrictions andpossibly lower urban design standards, especially inperi-urban areas. A clear and definite change betweerural and urban standards will be required so thatdrivers are made aware of the changed drivingenvironment. This can be made by the introduction oposted speed limit either for the whole complex or forthose elements linking directly to the local urbannetwork.

Determine Constraints

2.17 Choice of location for major interchanges onexisting routes will be limited, compared with newroutes. In many instances development, attracted byeasy access to the motorway system, may haveextended up to the existing highway boundary.


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Constraints may include the following:

Environmental Constraints:

Land take Effect on property Effect on landscape Effect on ecology Effect on rights of way Effect on heritage Noise and air quality Visual impact

Engineering Constraints:

Condition of existing structures Topography Geology Existing traffic flows Existing interchange layout Ability to manage traffic during construction Ability to manage traffic during maintenance.

Develop Local Network and Interchange Strategy

2.18 This stage follows initial consideration of thebroad network strategy and constraints. It will includeassessment of the need to maintain provision for allexisting traffic movements at the interchange or theredirection of traffic to adjacent junctions orinterchanges via motorway link roads or other routes.

2.19 The aim is to identify a satisfactory minimumcost solution. A comparison of at least two solutionsshould be made, even for relatively straightforwardproblems. For more complex problems severalsolutions should be prepared for analysis. Optionsshould include those with minimum effect on existingtraffic together with options that may cause greaterdisturbance during construction but would provide apotentially more efficient and/or compact layout forfuture use. The incremental cost of each should becompared with the quantified benefits/costs of thealternative solutions.


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Select Appraisal Criteria

2.20 Appraisal criteria should be based on thefactors normally considered in major schemeassessments and should include the operationalassessment described in Chapter 5. It may beappropriate to apply different weighting to differentcriteria, depending on local factors but this should beagreed with the Overseeing Organisation. Whereidentified, minimum values to be achieved should beincluded.

Develop Traffic Flows

2.21 Derive low and high growth design year traffiflows for each section of mainline and connector roadaccordance with the Traffic Appraisal Manual (TAM)or the Scottish Traffic and Environmental AppraisalManual (STEAM) as required by the OverseeingOrganisation.

Lane Requirements

2.22 Determine mainline and connector road lanerequirements for each option as described in Chapterof TD 22 (DMRB 6.2.1).

Merge, Diverge and Weaving Requirements

2.23 Check merge, diverge and weaving layoutsincluding lane balance. These layouts are to beprovided in accordance with TD 22 (DMRB 6.2.1) andChapter 5 of this design document. If the route isparticularly constrained by the proximity ofinterchanges or by high weaving flows, controlledspeed environments may need to be considered, eithall day or for part of the day.

Geometric Standards

2.24 Check that remaining geometric standardsspecified in TD 9 (DMRB 6.1.1) can be achieved.


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2.28 The development of major interchanges shallbe treated in the same way as other major highwayprojects.

2.29 Relevant documents which shall beconsulted in conjunction with the appraisal processare contained in:

TA 30(DMRB 5.1) - Choice between Options forTrunk Road Schemes.

Vol 5 DMRB - Assessment and Preparationof Road Schemes.

Vol 11 DMRB - Environmental Assessment

Signing

2.25 Check that an effective and economic signisystem incorporating both advance direction signing

and subsequent route confirmation can be provide

2.26 Steps 9 to 12 (on Figure 2/1) may lead to thamendment or rejection of a particular option. Severe

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problems may lead to a reassessment of the localnetwork strategy.

2.27 If arrangements are satisfactory they can thenbe carried forward to the appraisal stage.

The Appraisal Process

2.30 In many cases the scale and effect of the worksrequired will necessitate preparation of a fullenvironmental appraisal either for the interchangeworks alone or in conjunction with adjacent motorwaywidening or construction proposals.


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2.31 The Public Consultation type framework forthe comparison of several options provides a suitablebasis for the assessment. This will therefore ensure thconsideration is given to:

The effects on travellersThe effects on occupiers of propertyThe effects on users of facilitiesConservation policiesDevelopment and transport policiesCosts n

2.32 The effects on travellers will include anappraisal of the complexity and safety of the proposedinterchange layouts. Where there are significantdifferences between the times and/or distances involvein negotiating the interchange, economic assessmentsoperating costs and time savings or delays should becarried out.

2.33 Driver stress and driver comprehension of thelayout will depend on the number and timing ofdecisions and manoeuvres required. These will beaffected by the speed of traffic and its density whichmay mean short gaps for manoeuvres and increasedstress when weaving. This might suggest to drivers ththere is an increased accident risk. Furtherconsideration of these factors is included in theoperational assessment described in Chapter 5.

2.34 Travellers will also be affected by delaysduring construction and the economic assessment mustake account of these costs. Solutions that result in thebest final arrangement may cause the greatestdisturbance to traffic during construction. It is thereforeimportant that consideration is given to the provision oftemporary works as part of the designers solution. Sucmeasures

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while increasing construction costs can significantlyreduce the cost of delays. It is also important that th

at cost of future maintenance, including traffic delay cis taken into account.

2.35 Safety of both motorway users and constructiopersonnel is of prime importance in the design of majointerchange improvement schemes. It is essential tha

designers consider the safety implications of theconstruction methods and traffic management measur

ecessary for execution of the work.

2.36 It will be necessary to establish the importancgiven to the feasibility of providing additional capacitat a future date for each option.

d of 2.37 Environmental factors are likely to be very

significant. There will often be limitations on the lanavailable for new highway works and amelioration

some parts of the motorway boundaries. The use oflong lengths of elevated carriageway or the provision

additional levels over existing interchanges is likely be environmentally intrusive.

easures due to the presence of development along



Volume 6 Section 2 Chapter 3Part 4 TD 39/94 General Layout Advice

3. GENERAL LAYOUT ADVICE

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Existing Layouts

3.1 Most existing motorway to motorwayconnections are true interchanges providinguninterrupted movement for all turning traffic by theuse of direct and semi direct interchange links andloops.

3.2 Interchanges providing the most direct linkssuch as the 4 way 4 level diamond layout tend to be thmost environmentally intrusive (see Figure 3/1(a)). Many interchanges have therefore been designed withcombination of interchange links and loops to takeaccount of site specific constraints and traffic flows.

3.3 The 4 way 2 or 3 level cyclic layout provides agood compromise between operational andenvironmental considerations (see Figure 3/1(b)).

3.4 3 way interchanges may be considered as Y oT interchanges. Y interchanges are formed where twomotorways approach at an acute angle and provision fmovements between the acute angled legs is notincluded. This arrangement normally allows high speeconnections between the respective motorways (seeFigure 3/1(c)). Many 3 way T interchanges alsoprovide relatively high speed interchange links,although loops are used in some cases (see Figure3/1(d)).

3.5 Some motorway to motorway connectionsutilise the three level intermediate roundabout layout(see Figure 3/2(a)). While not being a true interchangein that traffic has to give way at the roundabout it hasthe advantage of minimising land take. As additionalmotorway capacity is provided, upgrading of theroundabout junctions can be particularly difficult due torestricted space. Roundabouts often provide additionaconnections to the local road system, which are alsodifficult to include in a full interchange layout.

3.6 Chapter 2 of TA 48 (DMRB 6.2.2) describesalternative layouts for grade separated junctions andinterchanges and should be consulted for furtherinformation.

Improvement of Three Level Roundabout Junctions

3.7 Roundabouts do not provide a free flowsolution for major motorway to motorway interchangesand to this extent they are a relaxation from thedesirable standard. However, they can provide a

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asonable solution where interchanges are particularlonstrained. Operation can be enhanced byignalisation but this is further out of character with aee flowing system.

.8 When motorways require widening, disruption through traffic can be minimised by adoption ofyouts that put all merging and diverging at junctionsn motorway link roads (see Figure 3/2(b)). For ease aintenance link roads should be continued through thnction where possible. This will generally be easierr the low level motorway than for the high levelotorway. However, regardless of traffic leaving orining the main line or a link road, problems will

ontinue at the roundabout due to the limited turningapacity. The capacity can be assessed by using theomputer program ARCADY/3.

.9 An initial solution to providing increasedapacity is offered by the adoption of dedicated left turnes. Such provision should be either by marking oridening of the original roundabout carriageway or bye construction of a fully separate interchange linkad. This will ensure that the left turn is taken at low

peed or high speed and that the designers intention islear to drivers. TD 16 (DMRB 6.2.3) should beonsulted for advice on the provision of segregated lefrn lanes at roundabouts.

.10 Enhanced right turn capacity can be providedy the addition of one or more dedicated interchangenks.

.11 Illustrative examples of possible layouts fornhancement of junction capacity up to conversion toll free flow links are given in Chapter 6.

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Chapter 3 Volume 6 Section 2General Layout Advice Part 4 TD 39/94

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(a) 4 Way 4 Level Diamond Layout (b) 4 Way 2 Level Cyclic Layout

(c) 3 Way Free Flow 'Y' Layout (d) 3 Way Free Flow 'T' Layout

Figure 3/1 : Major Interchange Layouts




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(a) 3 Level Roundabout

(b) 3 Level Roundabout with Link Roads

Figure 3/2 : 3 Level Roundabout Interchange Layouts


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Improvement of 3 Leg `Y' Interchanges

3.12 Improvement of 3 leg `Y' interchanges willnormally be carried out in accordance with Chapter 4 ofthis document.

3.13 However, where motorways leading to a Yinterchange require widening to dual four lanes theycannot be simply joined together due to the resultingvery wide carriageway. In these situations mainlinemotorways with link road layouts are more likely (seeFigure 3.3(a)).

3.14 One problem associated with Y interchangesand major merges and diverges generally, is theweaving, particularly of large vehicles, after themerging of two carriageways and prior to the diverge oftwo carriageways.

3.15 A possible solution would be the use ofcrossover lanes to allow all large and slow vehicles tobe grouped together prior to merges and diverges (seeFigures 3.3(b) and (c)).

3.16 However, they would require the use ofadditional structures and extensive special signing. With the increased adoption of link roads, interpretationof traffic signs for correct route finding will becomemore important. There is a danger that crossover laneswould complicate road layouts and confuse drivers. Their use is therefore generally not recommended butthere may be situations with high volumes of LargeGoods Vehicles where such designs should be includedamong the options to be considered.

Improvement of Major 3 and 4 Leg Interchanges

3.17 Many existing 3 leg `T' and 4 leg majorinterchanges have been designed with two laneinterchange links which were necessitated by theirlength and gradient rather than traffic flow (para. 4.3TD 22 (DMRB 6.2.1)). This provision has proved tobe fortunate as in many cases forecast turning trafficflows for schemes involving the widening of theoriginal mainline motorways can be accommodated onthe existing interchange links. It is particularlysignificant where they involve a large structural content,as it would be difficult to widen or replace high levelstructures spanning existing motorway carriageways.

3.18 This illustrates the benefit of building incapacity for future requirements, if it can beeconomically justified, in situations where adding suchcapacity at a later date would be very difficult. Additional capacity may be required where two

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hange links join prior to merging with thene. A similar situation would exist at the divergehe mainline prior to the splitting into separatehange links.

These interchange links are eventually likely toe three lanes and so result in major merges and diverges. The size of major merges and diverges

ately limited by the maximum acceptable widthinline carriageway upstream and downstream ofterchange. Auxiliary lanes provided for lanece at merges and diverges will add to the width ofcarriageway at the interchange.

The need for major merges and major divergese avoided by:

Providing separate merges and diverges foreach interchange link road and turningmovement.

The separation of merges, especially if theyinvolve lane additions, may be beneficial. However the signing of successive closelyspaced diverges can be difficult with signingmostly from gantries, particularly wheremainline carriageways are 4 lane.

Connection of interchange links to motorwaylink roads.

This would direct traffic from one or a pair ofinterchange link roads onto a motorway linkroad. Traffic could reach the mainlinemotorway from the motorway link road via asubsequent motorway transfer road.

Illustrative examples of possible layouts arein Chapter 6.

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(a) 'Y' Interchange of 2 Motorways with Mainline Motorway and Link Roads

(b) 'Y' Interchange with 'Crossover' Lanes (Merge)

(c) 'Y' Interchange with 'Crossover' Lanes (Diverge)

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Figure 3/3 : 3 Leg `Y' Interchange


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Provision for Managing Traffic in the Event ofIncidents or Maintenance

3.22 In addition to the requirements of para 2.6 it isalso important that all designs for major interchangesare checked for the implications of operation duringincidents or planned maintenance. Adequate provisionshould be made for redirecting traffic and for the use ofcontra flow as well as making suitable provision for thequick access of emergency vehicles. This may beachieved by short links or u-turn facilities easilyavailable to emergency vehicles.



Volume 6 Section 2 Chapter 4Part 4 TD 39/94 Major Merges and Major Diverges

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4. MAJOR MERGES AND MAJOR DIVERGES

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General

4.1 This chapter sets out the layout requirementfor major merges and diverges where three lanesenter or exit from the mainline carriageway.

4.2 Consideration is given to the possiblerequirement for redesign of merges and divergesbetween interchange links where the interchangelinks are operating close to their two lane capacity.

4.3 Hourly flows, as predicted from para. 3.1 TD22 (DMRB 6.2.1) shall be used for calculating laneprovision for the merging, diverging and mainlinecarriageways.

4.4 Maximum lane capacities shall be taken as1600 vehicles per hour for all-purpose roads and1800 vph for motorways as specified in para. 3.2 TD22 (DMRB 6.2.1), subject to the proviso containedtherein.

General Principles

4.5 Lane balance should be provided for trafficmerging to or diverging from the mainline. Whethertraffic is joining or leaving the mainline all drivers whoapproach a part of the junction either from the mainlinor a connector road should be able to leave that part equivalent comfort.

4.6 Lane drops and lane gains will be provided atmajor diverges and major merges to limit the width ofcarriageways through the interchange. However,consideration needs to be given to additional widthrequirements for maintenance activities, incidents, anfor future contra-flow systems during majormaintenance where such provision is cost effective. This can be assessed using the computer programQUADRO. It should be made clear to drivers where



lane drops and lane gains occur by the consistent usigns and road markings.

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4.7 The minimum spacing between the tips ofthe noses of successive merges, successive divergesor a diverge followed by a merge shall be 3.75V m,where V is the design speed in kph for the mainline,but may be increased to the minimum requirementsfor effective signing and motorway signalling.

Major Merges - General Principles

4.8 The nearside lane (lane 1) of the left-handjoining carriageway should always be provided witha lane gain. Lane gain warning signs will indicate todrivers the pattern of the junction ahead. These signare large and their siting will need to be consideredfrom the outset.

4.9 The individual merging area, within amerge, for each joining lane not provided with a lanegain, should be separated from other merging trafficand there should be space between them for mainlinetraffic to adjust.

4.10 In situations where the left hand flow isgreater than the mainline flow, priority should stillbe given to mainline traffic and the junction set outso that traffic entering from the left gives way exceptwhere lane gains are provided.

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Chapter 4 Volume 6 Section 2Major Merges and Major Diverges Part 4 TD 39/94

Designing Major Merges

4.11 Major merges where three merging lanes joinwith a three lane mainline should be set out as Layou(a), Figure 4/1. This provides a balanced merge layoincorporating a ghost island taper merge for the fastemore manoeuvrable vehicles travelling in lane 3 andlane gains for the other two merging lanes. This is seto be a safe option. The layout will also appear todrivers to be similar to existing TD 22 (DMRB 6.2.1)merge layouts. Large Goods Vehicles travelling in la1 of the mainline prior to the merge will only have tomove one lane to the left to attain a normal LargeGoods Vehicle carriageway position on a five lanecarriageway.

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4.12 Where the mainline traffic flow is light with alow proportion of Large Goods Vehicles and themerging left-hand flow is near capacity; there is scopefor the dropping of the offside lane of the mainline andproviding lane gains for all the merging lanes. Thislayout provides increased merging capacity for the lefthand carriageway while also maintaining through routecontinuity for the mainline. The mainline offside laneshould be maintained at full width for 3.75V m beyondthe nose, where V is the design speed in kph, and thenreduced in accordance with Drawing No. D6 of theHighway Construction Details (MCHW3). This isillustrated as Layout (b), Figure 4/1.

Nose Taper Ghost Island

(2)&(3) (1) (6)(slip roadright hand lane) lane widths 3.7m

Ghost Island width 2mat widest point

(a) Mainline Double Lane Gain at Ghost Island Merge Layout

3.75Vm 200m 300m

(2)&(3)

Nose

(b) Offside Lane Drop

Notes:- V is the design speed in kph.All offside lanes are reduced according to Drawing D6 in MCHW3

Figure 4/1: Major Merge LayoutsN.B. Figures in brackets refer to columns in Table 4/4 TD 22 (DMRB 6.2.1)

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4.13 There may also be extreme situations whereonly four lanes can be provided downstream of a majomerge. Indeed there are existing examples of 3 leg `Yinterchanges where 3 lanes join a 3 lane mainline withonly three lanes provided downstream of the merge. Atraffic flows increase on the joining carriageways,additional capacity will be required downstream of themerge. Where it is only possible to provide four lanesdownstream of the merge, this may be achieved byreducing from 5 lanes to 4 lanes 450m (3.75 Vm for120 kph design speed) downstream of Layout (a),Figure 4/1, in a similar fashion to Layout (b), Figure 4/1or by the use of Layout (a), Figure 4/2.

L6b

4bocmp



r'

s

ane balance as set out in para. 2.12 TD 22 (DMRB.2.1) is not achieved with this layout and it should onle used as a last resort.

.14 Alternatively, where the merging flow is lowerut a 3 lane carriageway has been provided for reasof continuity, the fast lane (lane 3) on the left-handarriageway could be "hatched-out" and a two laneerge layout designed to TD 22 (DMRB 6.2.1)rovided. This is shown as Layout (b), Figure 4/2.

Nose


(6)(1)(2)&(3)

(6)(1)(2)&(3)

Taper Ghost Island

(6)(1)(2)&(3)(slip road

(slip road

right hand lane)

right hand lane)

lane widths 3.7m

lane widths 3.7m

Ghost Island width 2m min.

Ghost Island widths 2m min.

of widest point

at widest points

(slip roadmiddle lane)

200m

3.75Vm

(a) Mainline Lane Gain at Double Ghost Island Merge Layout

(b) Lane Drop on Merging Carriageway

Figure 4/2: Alternative Major Merge LayoutsN.B. Figures in brackets refer to columns in Table 4/4 TD 22 (DMRB 6.2.1)




ads. pply,

as

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nd

lowslm of

4.15 Where the traffic flows on each carriagewayare balanced, consideration can be given to "hatchingout" one lane on each carriageway and providing adouble lane gain for the left-hand carriageway as showin Figure 4/3. This would remove the need for traffic tmake all adjustments at the merge and thus provide amore balanced layout, ensuring that there is adequateprovision for traffic merging from the left-handcarriageway. Adequate distance to allow traffic toadjust is required between the dropping of the lane anthe merge which should be undertaken in accordancewith Drawing No. D6 of the Highway Construction iDetails (MCHW3).

4.16 Resulting from the increased capacity nowbeing provided on mainlines a corresponding increasein the turning flows at major interchanges will occur,such that two-lane interchange links may approach thdesign capacity in some cases. Hence, the existingpractice of providing two lanes downstream of themerge of two, 2-lane interchange links will beinadequate. Where interchange links do approach thedesign capacities, at least three lanes capacity will berequired downstream of the merge.

4.17 This can be achieved by a number of layouts illustrated in Figure 4/4. Which is the more suitabledepends on the relative traffic loadings. The geometrparameters shown in Table 4/4 TD 22 (DMRB 6.2.1)are used for the

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ti



merges of interchange links and other connector roOn urban motorways, urban standards of course ain which case, in the context of major urban

n interchanges, the ghost island length may be takeno 120m. The layouts assume that 3.75V metres is

provided between the successive merge nose tips

d

nterchange link and then reduced to three lanes by the

eirprovided. This is a conventional TD 22 (DMRB 6.2.1)

ghost island merge with a single lane gain. This laymay be more appropriate where there is a large

ir percentage of Large Goods Vehicles on the left-hainterchange link and little variation in the relative

turning flows on the interchange links.

as 4.19 There may be situations where the traffic fon the two interchange links are unbalanced but stil

ic have a requirement for 3 lanes capacity downstrea

equired by para. 4.19, TD 22 (DMRB 6.2.1) and para..7 of this design document.

.18 In Layout (a), Figure 4/4 four lanes capacity isrovided immediately downstream of the nose on the

ropping of the nearside lane (lane 1) prior to the mergeith the mainline. This layout provides a significantegree of flexibility to accommodate relative changes in

he turning flows on the joining interchange links whichay occur during the day or as a result of seasonal

ariations. Alternatively Layout (b), Figure 4/4 may be

he nose. In this case the fast lane (lane 2) of thenterchange link with the lower volume should be"hatched-out" as shown in Layout (c), Figure 4/4. Alane gain is provided for the remaining lane.

Nose

(2)&(3)

200m

3.75Vm

200m

3.75Vm

Lane Drop on Both Carriageways

300m

Figure 4/3: Alternative Major Merge LayoutN.B. Figures in brackets refer to columns in Table 4/4 TD 22 (DMRB 6.2.1)




April 199

(6)

Nose Taper Ghost Island(6)(1)(2)&(3)Nose

Taper

Auxiliary Lane

(2)&(3)

(4)(5)


(1)(2)&(3)Ghost Island

Taper

Nose

(1)

(2)&(3)

(6)


(6)(1)

Ghost Island

TaperNose

(6)

(2)&(3)(2)&(3)

(200m)

(a) Lane Drop After The Merge Nose

(b) TD22 (DMRB 6.2.1) Ghost Island Merge

(c) Lane Drop Prior To The Merge Nose

Figure 4/4 : Merge Between Two Interchange LinksN.B. Figures in brackets refer to columns in Table 4/4 TD 22 (DMRB 6.2.1).

The layouts assume 3.75 Vm between nose tips.


4 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 4/5


Major Diverges - General Principles

4.20 Major diverge layouts should be set out sothat traffic wishing to leave the mainline may do soas easily and quickly as possible. The intention is tocreate the diverging route alongside and allowsufficient time for the full volume of manoeuvresrequired for drivers crossing over and taking uppositions in the diverging lanes, to take place. Traffic travelling straight ahead should therefore beinhibited as little as possible. It will be veryimportant for drivers to be able to identify the lanethey require from the overhead gantry signs early onas they approach the diverge.

4.21 At major diverges where it is anticipatedthat the connector road and the mainline willfrequently be carrying traffic flows approaching theirdesign capacities, prolonged opportunity to leave themainline should be given by the provision ofextended auxiliary lanes as recommended in para.4.17 TD 22 (DMRB 6.2.1).

Designing Major Diverges

4.22 Major diverges where three diverging lanesleave a five lane mainline should be set out as Layout(a), Figure 4/5. This provides a balanced layout fordrivers wishing either to continue on the mainline orleave on the diverging route.

4.23 Where only four lanes of capacity is providedupstream of the major diverge, Layout (b), Figure 4/5may be used. This layout provides a degree offlexibility but where the proportion of the mainline flowleaving the through route fluctuates widely, a singleauxiliary lane could be extended upstream of the start ofthe diverge taper.

4.24 In situations where two-lane interchange linkflows approach their design capacity there will be arequirement to provide an additional lane on theinterchange link, resulting in a four-lane width prior tothe diverge into two separate two-lane interchangelinks. This auxiliary lane should be generatedaccording to Layout (c), Figure 4/5. The geometricparameters shown in Table 4/5 TD 22 (DMRB 6.2.1)are used for the diverges of interchange links and otherconnector roads. On urban motorways, urban standardsof course apply. The layout assumes that 3.75V metresis provided between the successive diverge nose tips asrequired by Para. 4.19, TD 22 (DMRB 6.2.1) and Para.4.7 of this design document.

4.25turnturnmakdiscoccsepnor



It is preferable that diverges for left and righting traffic at interchanges be combined so that alling traffic leaves the mainline at one point and thenes a subsequent decision to go left or right. This isussed further in Chapter 5. There may be rareasions when it may be desirable to considerarating the diverges on the mainline but this is notmally recommended.




April 1

Taper Auxiliary Lane Nose

Auxiliary Lane Nose

NoseAuxiliary Lane

Auxiliary LaneNose

Taper

Taper

(6)(5)

(3)&(4)

(3)&(4)

(5)(6)

(5) (3)&(4)

(3)&(4)(5)(6)

Taper across 2 lanes

(6)+(6)

For full detailsof the treatmentof this area, see

"Highway ConstructionDetails"

(a) Mainline Double Lane Drop at Parallel Major Diverge

(b) Mainline Lane Drop at Parallel Major Diverge

(c) Diverge Between Two Interchange Links

Figure 4/5 : Major Diverge LayoutsN.B. Figures in brackets refer to columns in Table 4/5 TD 22 (DMRB 6.2.1).

The layouts assume 3.75 Vm between nose tips


994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 4/7

Volume 6 Section 2 Chapter 5Part 4 TD 39/94 Operational Assessment

5. OPERATIONAL ASSESSMENT

entht-.

5.1 The operational assessment is a criterion,among many others, which shall be included in theappraisal process in Chapter 2. The concept of adriver decision point and manoeuvre analysis isuseful in assessing the complexity of an interchangeand provides useful information on the operationalaspects of different interchange layouts. This aidsthe determination of the most appropriate layout forindividual situations. Taking into account theDriveability Appraisal (para. 5.18) also indicateslayout characteristics regarding driver stress andcomfort.

General

5.2 Decision points are required in all situationswhere motorists are presented with a choice of routedirections and a corresponding manoeuvre may resuwhere it is necessary to achieve the selected route. Manoeuvres can also occur in the absence of decisiopoints such as at merges and entrances to roundabo

5.3 In general the most desirable interchangelayout will be that option which has the minimumnumber of decisions and manoeuvres. Also it isdesirable that decisions and manoeuvres occur wherthey can most readily be made without causingcongestion or turbulence. This is ideally on motorwalink roads or interchange links rather than on themainline but this consideration alone should not rule completely the adoption of separate merges anddiverges. The provision of a double diverge at closespacing has specific signing difficulties associated wiit and may cause particular stress on drivers travellinthrough the interchange.

5.4 The minimum number of decision points andmanoeuvres is not the only consideration however, athe alternative layouts also need to be appraised fortheir practical driveability and driver workload. Alldriving tasks which may increase driver stress anddiscomfort or lead to safety problems on theinterchange must be listed and evaluated whencomparing options.



lt 5.7 The numerical analysis involves the assessmof the alternative interchange layouts for left-turn, rig

n turn and straight on decision points and manoeuvresuts. The decision points and manoeuvres should be

is described further in the illustrative example.

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5.5 In particular the distance between decisions andmanoeuvres is important with greater distancesproviding more time for drivers to select their routeand/or carry out the desired manoeuvre. Somedecisions and manoeuvres are of greater difficulty andmore important than others. These differ from driver todriver and the approach adopted is therefore toundertake a numerical analysis of the decision pointsand manoeuvres augmented by a driveability appraisal.

5.6 By comparing both the numerical analysis anddriveability appraisal of different options the designercan determine the most appropriate interchange layout. However the need to provide for alternative links tocope with situations of incidents or maintenance mustnot be overlooked.

Numerical Analysis

indicated for each alternative on a layout drawing. This

Decision Points

5.8 A decision point occurs where there is adiverge from the mainline. The motorist must decideeither to diverge from the mainline or continue straighton. The decision may need to be taken in varyingtraffic conditions and at speeds from slow saturationspeed to motorway speed.

5.9 A decision point occurs where a diverge existson an interchange link. The decision may be taken at ornear motorway speed.



Chapter 5 Volume 6 Section 2Operational Assessment Part 4 TD 39/94

ics

5.10 A decision point exists at an exit from aroundabout. A driver must decide to take the exit orcontinue on the roundabout.

5.11 A merge is not considered a decision point,although the manoeuvre may require a degree ofapplication and skill.

Manoeuvres

5.12 Manoeuvres are made by traffic merging anddiverging to and from the mainline. Minor manoeuvreare also undertaken by traffic carrying through on themainline due to disturbance in the traffic streamresulting from the merging and diverging vehicles.

5.13 Manoeuvres also occur at merges and divergeon interchange links.

5.14 A double manoeuvre is required at aroundabout, i.e. one entering manoeuvre and oneexiting manoeuvre.

Traffic Flows

5.15 The decision point and manoeuvre layouts mube considered in conjunction with a traffic flow diagramwhich indicates the numbers of vehicles wishing tomake each turning movement.

5.16 The number of vehicles making eachmovement is a very significant factor as a reducedprovision can be acceptable for low flows whereas forturning movements with heavy flows a high standard oprovision will be required.

5.17 By allocating the traffic flows to the differentdirections of travel, the number of decisions andmanoeuvres made for each direction can be found anthe total number for the interchange layout calculated.Distinguishing between decisions and manoeuvreswhich occur on the mainline or on the interchange linkis also important as their location is a significant factor

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5.18 The purpose of the driveability appraisal isto allow the consideration of factors which influenceinterchange operational characteristics and drivercomfort but which cannot be readily incorporated ina numerical analysis of the interchange.

the practical driveability of a layout and any drivers behaviour difficulties which may impede the operation

of the interchange or lead to safety problems.

5.20 A number of factors, other than decisions and

s considered in assessing the operational characteristof a major interchange. These include:

WeavingGeometric effects

Gradient

st

f

d

s.

riveability Appraisal

.19 A totally numerical analysis might not cover

anoeuvres at merges and diverges, need to be

Degree of curvatureType and configuration of the merge anddivergeThe number of lanes on the mainline, onmerges and on divergesVisibility, including at nightNetwork consistencyThe proportion of Large Goods Vehicles

.21 Consideration should be given to theime/distance where a second decision occurs after arevious one. Drivers may still be recovering from, oreeking confirmation of, the first decision. As the

ength between decision points shortens the intensity ofhe lane changing and turbulence increases as driversttempt to access lanes appropriate to their desired exitoints.

.22 Some decisions may be relatively trivial whenompared with others and therefore the designer mightonsider them of lesser importance i.e. continuingtraight ahead on the mainline through an interchange. his still may involve examination of signs, comparisonf junction or route numbers with maps or memory anday require a lane change. The driver may also be

affected by lane changing undertaken by other traffic. This decision could therefore involve some stress whichmay not be eased until a confirmatory sign was reachedand the traffic stream became less turbulent.




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5.23 More complicated decisions involve a largenumber of factors including:

Previous map examinationSigningRoad markingsKnowledge of destination or route and junctinumbersPressure from other driversFollowing the vehicle in front

These decisions involve possible high levels of stres

5.24 When assessing the driveability of the differinterchange layouts the designer should concentratethe unfamiliar driver, new to the layout. Consideratishould be given to the necessary manoeuvres to attdestination and what cognitive difficulties and stressthey may present to the unfamiliar driver. Complexinterchanges with lane drops can present particulardifficulties for drivers of larger, slower vehicles.

5.25 For regular drivers decisions will become mtrivial as they become familiar with a pattern oftravelling through the interchange and will only beconcerned with planning their most beneficialmanoeuvres to attain their position ahead. Thereforsituations where the percentage of familiar (i.e.commuter) drivers is high, the number of decisionsbecomes less important but the significance ofmanoeuvres remains unchanged.

5.26 Major interchanges present older drivers witparticular problems. There is evidence to suggest thsome older drivers may avoid the more complexinterchanges due to their complicated nature. Olderdriver problems are reduced by increased lengthbetween decision points or slower speeds but this isgenerally not possible at major interchanges. Indeeother traffic may force a higher speed on older drivethereby increasing their difficulties. In general olderdrivers require simple information and more time toselect and respond, and this need for simplicity indesign should be considered in the assessment.



5.27 Having carried out both a numerical analysthe decision points and manoeuvres as well as a

obtained considerable useful information to aid theselection of the most appropriate alternative. Also areawhere the selected alternative can be altered to provid

n an improved interchange for drivers may have beenidentified.

driveability analysis of the alternatives, the designer ha




h

Illustrative Example

5.28 In order to illustrate the operational assessmeprocedure an example comparing two four-leginterchange layouts is presented. The two alternativewhich are considered are a single exit/single entrydiamond layout and a double exit/double entry cycliclayout.

Example of Numerical Assessment

5.29 The two alternatives are shown in Figure 5/1and the decision points and manoeuvres appropriate each are identified. Sample traffic flow figures havebeen assumed as shown in Figure 5/1(a).

5.30 Consider traffic travelling from south to eastthrough the interchanges. For the single exit/singleentry layout, drivers must decide to depart from themainline and then make a decision to go right at thediverge on the interchange link. They also have tomake manoeuvres at these points, as well as at themerge of the interchange links and the merge with themainline. These decisions and manoeuvres are dividinto those which occur on the mainline or oninterchange links and multiplied by the appropriatetraffic flow from the traffic flow diagram. Likewisethis can be repeated for the other movements throughthe interchange and the totals for the interchangecalculated. It should be noted that drivers continuingstraight through the interchange must make a decisioto do so at diverges and a manoeuvre is also allocatemerges and diverges to allow for the disturbance ofmainline traffic due to the merging and divergingtraffic.

5.31 For the same movement through the doubleexit/double entry interchange, drivers must decide tocontinue straight ahead at the first diverge and thenmake another decision to depart at the second diverg



Likewise manoeuvres are made at these points andwhere the driver merges with the mainline and at thsecond merge due to the disturbance caused by mtraffic.

Alternative Item Mainline Inter-change

Total

Single exit/Single entry

Decisions 33570 13012 48582

Manoeuvres 67140 26024 93164

Double exit/Double entry

Decisions 60634 0 60634

Manoeuvres 121268 0 121268

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e.

5.32 The results of the numerical assessment forboth alternatives are presented in Table 5/1.

5.33 From Table 5/1 it can readily be seen that thedouble exit/double entry layout requires approximately30% more decisions and manoeuvres than the singleexit/single entry option. Also all the decisions andmanoeuvres occur on the mainline for the doubleexit/double entry alternative. For the single exit/singleentry alternative, approximately 70% of decisions andmanoeuvres take place on the mainline with theremainder on the interchange links.

Example of Driveability Assessment

5.34 The assessment of the topics in para 5.20 whiccan be made on the driveability of either layout in thisillustrative example is less than would be possible in areal situation as many of the factors relating todriveability are scheme dependent rather thandependent on the layout type.

5.35 The assessment can be presented as adescriptive analysis of the driveability of theinterchange as this will allow the designer scope toconsider all factors relevant to a particular layout, aswell as those points raised in this design document.



alsoeerging


A

560

7819

2540254029602868

2868

7819

1048

56029601048

(a) Traffic Flow Diagram

ManoeuvresDecision Points

Decision Points Manoeuvres

(b) Single Exit / Single Entry

(c) Double Exit / Double Entry

Figure 5/1 : Decision Point/Manoeuvres Alternative Layouts


pril 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 5/5


f the

a

he

e

t

Single Exit/Single Entry Layout

5.36 The single exit/single entry interchange layoutis consistent with the majority of the motorway networand therefore will be very familiar to drivers. Also thesigning of one exit to one other route presents driverswith an uncomplicated choice between routes. However a second decision without significant advancsigning is required quickly after the driver has decidedto depart from the mainline.

The layout of a major parallel diverge, likely at singleexits, is easier to access than more minor diverges. However the large number of vehicles exiting at thesediverges is likely to reduce this benefit. Also largetraffic numbers at merges will reduce the driveability of these elements.

Traffic travelling straight ahead at the interchange willexperience the minimum of disturbance from trafficmerging and diverging to and from the mainline.

The four-level single exit/single entry interchange hashigh geometric standard and provides drivers makingturning movements with an easy route to steer througthe interchange.

Layouts with only one exit and entry provide themaximum length available for weaving on the mainline

Double Exit/Double Entry Layout

5.37 The double exit/double entry interchangelayout is not consistent with the majority of themotorway network and is therefore less familiar todrivers. The signing of the same route number both tothe left and straight ahead increases the possibility ofdrivers making the incorrect choice of direction. However the provision of double diverges, particularlywhere lanes are dropped, may lend itself to the signinto a particular destination of a specific lane, whichautomatically leads via a fixed path, to the requiredroute and direction.

Reduced traffic levels on diverges and merges willimprove their driveability although this may be offsetby reduced provision at these locations.

Tedtltpos

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LaTdc

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a5.38 For ease of presentation and comparison o

h driveability assessments of alternative layouts thedescriptive analysis can be summarised in a table and

identified. The marking system ranges from 0, for a. very good standard, to 5 for a very poor standard. T

g

raffic travelling straight ahead at the interchange willxperience greater disturbance from traffic merging andiverging to and from the mainline and this will reduce

he driveability of the interchange for these drivers. Aarge proportion of the decision point and manoeuvreotals for this layout result from straight ahead trafficassing the diverges and merges. For a high percentagf this traffic these decisions and manoeuvres might beaid to be trivial.

he two-level double exit/double entry interchange hasurved alignments through the interchange and includeseverse curves on right-turn interchange links.

ayouts with double exits and entries shorten thevailable mainline weaving length between junctions. his makes it more difficult for drivers to attain theiresired position on the approaches to junctions and alsoauses greater disturbance to straight through traffic.

xample Summary of Driveability Assessment

ating allocated to each alternative for the factors

lternative with the lowest total represents the bestayout with regard to driveability. The summary andomparison table for the example is shown on Table/2.

.39 In this illustrative example the singlexit/single entry four-level interchange has the lowestumber of decisions and manoeuvres as well as the besriveability assessment. It therefore represents theore desirable layout from an operational aspect. The

hoice of option must however take into considerationhe environmental impact assessment and incrementalost analysis.




Single Exit/Single Entry

Double Exit/Double Entry

Route Finding 1 2

Merges 2 1

Diverges 1 2

Curvature 1 3

Geometry 2 2

Weaving 1 2

No of Lanes 2 1

Network Consistency 0 2

Totals 10 15

Table 5/2: Summary and Comparison of

Alternative Driveability AppraisalFramework

5.40 Alternatively the descriptive analysis could beomitted and a more extensive framework, including abetter defined valuation system, could be adopted. Thframework assesses each direction through theinterchange under a series of factors and a ranking isallocated according to the standard provided.

5.41 The ranking system ranges from 0 (very good)to 5 (very poor) as before but is better defined. Forexample, in the case of interchange links, covering radand sight distance etc, the ranking system is defined a

0 - very good1 - good2 - desirable minimum3 - one design speed step below desirable4 - two design speed steps below desirable5 - departures involved

F

5mwalod

5fr



Direction N-S N-E

Movement Through Turning

Traffic Volume Q(NS) Q(NE)

Factors:

Diverges 0 2

Geometry of Links 1 3

Merges 0 1

Ease of Route Finding 1 3

Ease of CorrectingWrong Manoeuvre

4 4

Sub-total 6 13

TOTAL 6 x Q(NS) 13 xQ(NE)

Table 5/3: Typical Example of DriveabilityAppraisal Framework

is

iis:

or merges/diverges the ranking system is defined as:

0 - none

1 - lane drop or lane gain2 - simple merge/diverge3 - ghost island or 2 lane exit4 - congested entry/exit with

parallel auxiliary lanes5 - long auxiliary lanes

.42 The total ranking for each direction isultiplied by the traffic flow for the movement. Thiseights the appraisal for movements with heavier flowss they are of more importance than movements withw flows. The minimum total indicates the mostriveable option.

.43 A typical example of a driveability appraisalamework is illustrated in Table 5/3.



Volume 6 Section 2 Chapter 6Part 4 TD 39/94 Examples of Upgrade Paths for Major Interchanges

6. EXAMPLES OF UPGRADE PATHS FORMAJOR INTERCHANGES

thanernedtion

s ausualo the

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hanr.

General

6.1 This chapter presents examples of somecommon interchange layouts and illustrates a range ofpossible improvements for consideration. Relevanthighway schemes from different DOT Regional Officeswere reviewed in the development of this designdocument and account has been taken of this research determining possible improvements to the examples. Layouts are shown as line diagrams in order to includea reasonable number of alternatives. The examplesshown are not intended to represent the complete rangeof solutions and other options or combination of optionscould be considered. Individual major interchangeschemes will have to take account of their particularcircumstances and constraints.

6.2 The three layouts considered for improvementare:

(i) 3 level roundabout

(ii) 3 leg "T" interchange

(iii) 4 leg interchange

Improvement of 3 Level Roundabout Interchange

6.3 Varying degrees of improvement can beundertaken to 3 level roundabouts depending on theindividual situation, and slightly different appraisalswould be undertaken for the different levels ofimprovement.

6.4 One situation is where a range of possibleimprovements and capacity enhancements is possiblewith corresponding ranges of costs and environmentaleffects. Another is where severe constraints limit theimprovements that can be made.

6.5coApforwicoimmecohigfurof intexoth


April 1994 PAPER COPIES OF THIS ELECTRON

in

This will require a "value for money"mparison between the range of improvements. praisals for these improvements will be simpler more extensive alterations and mainly be concth the additional capacity provided, the construcsts and the environmental effects of eachprovement. Where there are severe constraintdium term solution with a design life less than

uld be adopted pending further developments thway network local to the interchange, when ather increase in capacity may be required. In tcomparisons, the shorter "design life" implies therchange would be subjected to traffic flows incess of its capacity for more hours in the year ter designs prepared for the normal design yea


IC DOCUMENT ARE UNCONTROLLED 6/1

Chapter 6 Volume 6 Section 2Examples of Upgrade Paths for Major Interchanges Part 4 TD 39/94

d

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nk

uldkling

6.6 Typical improvements include:

(i) Dedicated left turn lanes close to theroundabout (see Figure 6/1(a)).

These lanes may be provided for largelyinsignificant landtake and cost while providinga significant improvement to the roundaboutoperation. However the improvement in theoperation of the interchange will be restricted,as these dedicated lanes would use the samediverges and slip roads as traffic for otherroutes.

(ii) Selected dedicated left turns on separate linkswhere possible. Where separate links are notpossible left turns on dedicated lanes areprovided close to the roundabout (see Figure6/1(a)).

The provision of separate left turn links willprovide additional operational improvementcompared with dedicated left turn lanes as


PAPER COPIES OF THIS ELECTRONI6/2

they will have individual diverges, merges an

additional landtake and cost and require theminimum 3.75V m spacing between the

(iii) Selected new right and left turn interchangelinks to separate the heaviest movement frothe roundabout (See Figure 6/1(b)).

The selected new right turn interchange liwill result in significant cost due to the

requirement for two overbridges but will have a

dominates. Possible sterilisation of land cooccur between the right turn interchange linand the existing roundabout. Greater visuaintrusion of the interchange on the surround

area would also result from the height of the

connector roads. They will however result in

successive merges and diverges.

very significant improvement on the operationof the interchange where one movement

right turn connector road.

) Dedicated Left Turn Lanes andelected Separate Interchange Links

(b) Selected Right and Left Turn

Figure 6/1: Possible 3 Level Roundabout Improvements


C DOCUMENT ARE UNCONTROLLED April 1994


t

6.7 The other situation is where two or more typesof extensive improvements are possible. These willrequire a framework type appraisal as indicated inChapter 2 - Design Procedure.

6.8 The range of possible improvements include:

(iv) Selected new right and left turn interchangelinks in the least intrusive quadrant(s) (seeFigure 6/1(b)). i

(v) Connection of the 3 most heavily traffickedlegs with free flow semi-direct interchangelinks (see Figure 6/2(a)).

(vi) Connection of the 3 most heavily traffickedlegs with free flow cyclic interchange links (seeFigure 6/2(b)).

le

tu

co



with Free Flow

6.9 Layouts (v) and (vi) have advantages whereone leg of the original junction has lower traffic flowsthan the other three legs. The original roundabout is

retained to provide turning movements to and from the

connections from the original junction and provide U

6.10 Layout (vi) is likely to provide a more compacnterchange layout but with additional structural

given to providing the remaining set of cyclicinterchange links at a later date, completely replacing

the roundabout with a free flow interchange.

ss busy leg. It can also maintain additional

rn facilities for maintenance vehicles.

ntent. Also where space allows, consideration can be

a) Connection with Semi-direct Interchange Links

(b) Connection with Cyclic Interchange Links

Figure 6/2: 3 Level Roundabout connections between Three Legs



Interchange Links


e

ge

Improvement of Three Leg "T" Interchange

6.11 A typical motorway "T" interchange with legsA, B and C and conventional left side merges anddiverges is shown in Figure 6/3(a). The major route isconsidered to be A-B although the layout can beorientated so that any movement has priority. They areless complicated than four leg interchanges and, excepon loops, traffic can expect to travel through theinterchange at relatively high speed.

6.12 On existing dual three lane motorways it wouldbe normal to introduce two lane diverges with a lanedrop so that all interchange links were two lanes wide. If necessary three through lanes could be maintained othe major route and all merges and diverges can bedesigned in accordance with TD 22 (DMRB 6.2.1).

6foain

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t

n

.13 Widening of the mainline motorways to dualur lanes would not greatly change the situation as th

pproaching mainlines could simply diverge intodividual two lane interchange links.

.14 However, widening of the mainlines to dual 5nes would require some interchange links to be atast three lanes wide, introducing the need for majorerges and diverges.

.15 Figure 6/3(b) shows a basic layout designedith major merges and diverges. This allowsterchange links to be direct, rather than semi direct. owever, depending on construction and trafficanagement problems it may be necessary to retainxisting semi direct link layouts even when existingerge and diverge areas are improved and interchan

nks widened.

(c) Interchange with Large Goods Vehicle Crossover Lanes

(a) Typical layout with Semi-direct Interchange Links (b) Major Merge/Diverge Layout with Direct Links

C

BAB

A

C

A B

C

Figure 6/3: 3 Leg `T' Interchange Layouts




d fortes problemssfactoryink roadarly this might interchangehrough the of the

r link roadhange. Inrchangeant valuen of thed

ning

6.16 Figure 6/3(c) shows the possible addition ofcrossover lanes which reduce the problems associatewith Large Goods Vehicles and other slow movingvehicles at major merges and diverges. Crossover laare generally considered undesirable because ofproblems associated with signing two routes for thesame destination and also as the additional structureswill cause greater environmental intrusion, landtake acost. However the provision of crossover lanes may bappropriate on motorways with high volumes of LargeGoods Vehicles.

6.17 Figure 6/4(a) shows an alternative layout usinmotorway link roads. Left turn movements are madefrom the link roads and right turn movements madefrom the original mainlines. Movements between themainline and link roads take place via transfer roads aappropriate distances from the main interchange. Thlength of



carriageway shown hatched would be retained maintenance purposes. This layout elimina

with major merges and diverges although satines designs must be provided for the mainline/l

transfer roads. Signing would need to be cleassociated with the relevant carriageway and

involve complex gantries. The use of directnd links allows the adoption of a design speed te interchange which is close to that of the rest

route.

g movements start and finish on the mainline odepending on the layout of the original intercthis layout the structures from the original inteare retained. This approach is also of signific

t at four leg interchanges where reconstructioe original multi-level interchange structures aninterchange links would be very difficult.

6.18 Figure 6/4(b) shows a variation where tur

(a) Direct Interchange Links

(b) Semi-direct Interchange Links

C

A B A B

C

Figure 6/4 : 3 Leg `T' Interchange Layouts with Link Roads




g

the

y

re

ffic

er org

es.

k

Improvement of Four Leg Interchange

6.19 This example relates equally to cyclic, diamoor other layouts containing existing two-laneinterchange links which are generally adequate forforecast traffic flows.

6.20 The following layouts shown in Figure 6/5indicate possible improvements which should beconsidered. For simplicity only one area of theinterchange is being considered in details as shown the key plan in Figure 6/5.

6.21 Expansion of existing layout leading to majomerges and diverges on the mainline (Figure 6/5(a))

This layout will ultimately be restricted by themaximum acceptable mainline width upstream anddownstream of the interchange. It will probably needmainline lane drops and lane gains in order to minimthe carriageway width at the major merges anddiverges.

6.22 Separate Merges and Diverges for eachinterchange link (Figure 6/5(b)).

Merges and diverges would remain within TD 22(DMRB 6.2.1) Standards. This arrangement can behelpful with merges, especially if lane additions areprovided and can reduce problems with Large GoodVehicles having to move to the left. However it will bdifficult to sign closely spaced diverges on 4 lanemainline carriageways with gantry signing. Separatemerges and diverges are likely to cause more turbulon the mainline compared with single merges/diverg

6.23 The adoption of layouts incorporating linkroads are likely where forecast traffic flows are greatthan can be accommodated by a widened motorwaywhere the spacing between junctions is short resultinin reducing weaving provision. The selection of themost appropriate layout of interchange links willdepend on mainline and turning traffic flows, trafficdestinations and preferred mainline and link roadcarriageway widths. Link road layouts generallyremove the requirement for major merges and diverg

6.24 One Interchange link connecting to mainlineand one interchange link connecting to motorway linroad (Figure 6/5(c)).

ELECTRONIC COPY -

PAPER COPIES OF THIS ELE6/6

nd This layout is appropriate where the majority of turnintraffic is travelling a number of junctions downstreamof the interchange and there is sufficient capacity on

mainline. Link road flows are likely to be low.

6.25 Both interchange links connected to motorwalink road (Figure 6/5(d)).

onThe connecting of both interchange links to the link

r keeps disturbance to the mainline to a minimum. The. is a possibility that three-lane link roads may result

6.26 As previous layout but with provision for oneise interchange link to connect to mainline motorway and

link road (Figure 6/5(e)).

desirable balance between mainline and link road traflows.

se

encees.

road is desirable where a large proportion of traffictravels only a few junctions along the motorway. This

where traffic flows are high.

This layout combines the connections provided inlayout (c) and (d) where their provision results in a

6.27 Braiding which allows both interchange linksto connect to the mainline or link road (Figure 6/5(f)).

Link road layouts incorporating braiding are appropriatewhere turning traffic flows are high and other solutionswould result in undesirably wide link roads orcongestion on the mainline. They may also beappropriate where another junction is very close to themajor interchange and adequate weaving provision,required for the other solutions, cannot be provided onthe link road or mainline.

NOT FOR USE OUTSIDE THE AGENCY

CTRONIC DOCUMENT ARE UNCONTROLLED April 1994


(c) One Interchange Link to Mainline and

one to Link Road

(b) Separate Merges and Diverges

(a) Major Merges and Major Diverges (d) Both Interchange Links to Link Road

(e) One Interchange Link connects toMainline and Link Road

(f) Braiding

Area Considered inExamples Below

Major merge

Figure 6/5 : Possible Improvements to 4 Leg Interchanges


April 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 6/7

Volume 6 Section 2 Chapter 7Part 4 TD 39/94 References

7. REFERENCES

7Introduction 9G

(a) TD 22 (DMRB 6.2.1) - Layout of Grade (Separated Junctions

(b) TA 48 (DMRB 6.2.2) - Layout of GradeSeparated Junctions

(c) BS 6100 Subsection 2.4.1 - The BritishStandard Glossary of Building and CivilEngineering Terms - Part 2 Civil Engineering:Section 2.4 Highway and Railway Engineering:British Standards Institution.

8Design Procedure

(a) TD 22 (DMRB 6.2.1) as Chapter 1

(b) TA 48 (DMRB 6.2.1) as Chapter 1

(c) TA 23 (DMRB 6.2) - Determination of Size ofRoundabouts and Major/Minor Junctions

(d) Traffic Appraisal Manual - (TAM) 1: DoT:1982

(e) Scottish Traffic and EnvironmentalAppraisal Manual - (STEAM) : SDD : 1986

(f) TD 9 (DMRB 6.1.1) - Road Layout andGeometry : Highway Link Design

(g) TA 30 (DMRB 5.1) - Choice Between Optionsfor Trunk Road Schemes

(h) Vol 5 DMRB - Assessment and Preparation ofRoad Schemes

(i) Vol 11 DMRB - Environmental Assessment.

(b

(c

10

(a

(b

(c

6

(a



eneral Layout Advice

a) TA 48 (DMRB 6.2.2) as Chapter 1

of Roundabouts) TD 16 (DMRB 6.2.3) - The Geometric Design

) TD 22 (DMRB 6.2.1) as Chapter 1

Design Standard


) QUADRO 2 Manual : DoT: 1982

) Highway Construction Details - (MCHW3)

Examples of Upgrade Paths for Major Interchanges




Volume 6 Section 2 Chapter 8Part 4 TD 39/94 Enquiries


April 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 8/1

8. ENQUIRIES

All technical enquiries or comments on this document should be sent in writing as appropriate to:-

The Civil Engineering and Environmental Policy DirectorHighways AgencySt Christopher House T A ROCHESTERSouthwark Street Civil Engineering and London SE1 0TE Environmental Policy Director

The Deputy Chief EngineerThe Scottish Office Industry DepartmentRoads DirectorateNew St Andrew's House J INNESEdinburgh EH1 3TG Deputy Chief Engineer

The Director of HighwaysWelsh OfficeY Swyddfa GymreigGovernment BuildingsTy Glas RoadLlanishen K J THOMASCardiff CF4 5PL Director of Highways

Chief Engineer - Roads ServiceDepartment of the Environment for Northern IrelandRoads Service HeadquartersClarence Court10-18 Adelaide Street W J McCOUBREYBelfast BT2 8GB Chief Engineer - Roads Service

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