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Layout of Compact Grade

Separated Junctions

Summary: This Standard sets out the layout requirements for compact grade separatedjunctions. It gives recommendations for the application of compact gradeseparation to existing at grade junctions and to new routes incorporatingjunctions of this type.

THE HIGHWAYS AGENCY TD 40/94

THE SCOTTISH OFFICE DEVELOPMENT DEPARTMENT

THE WELSH OFFICE

Y SWYDDFA GYMREIG

THE DEPARTMENT OF THE ENVIRONMENT FOR

NORTHERN IRELAND

Volume 6 Section 2Part 5 TD 40/94 Registration of Amendments

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DESIGN MANUAL FOR ROADS AND BRIDGES


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VOLUME 6 ROAD GEOMETRYSECTION 2 JUNCTIONS

PART 5

TD 40/94

LAYOUT OF COMPACT GRADESEPARATED JUNCTIONS

Contents

Chapter

1. Introduction

2. Design Procedure

3. Safety

4. Specific Road User Requirements

5. Flow Standards

6. Geometric Standards

7. Layout Options

8. References

9. Enquiries

Volume 6 Section 2 Chapter 1Part 5 TD 40/94 Introduction

1. INTRODUCTION

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General

1.1 This Standard sets out the design standards methodology for the geometric layout of compact graseparated junctions principally for use on rural andinter-urban roads. It is intended that the standard shbe used in those situations which are not appropriatethe full levels of provision covered in TD 22 (DMRB6.2.1).

1.2 It has been found that grade separation to thstandards contained in TD 22 (DMRB 6.2.1) and TA48 (DMRB 6.2.2) can be economically justified atdesign flows above 30,000 AADT on the main line,depending on turning traffic. Previously, usingstandards contained within TA 20 (DMRB 6.2), it hasnormally been possible to justify grade separation tolower design flows of 20,000 AADT on the main line,but again this depended on turning traffic.

1.3 Grade separation should be consideredwherever it can be economically justified andenvironmental constraints can be satisfied. Guidancon the process of choosing a junction type is given inTA 30 (DMRB 5.1). Using those standards compactgrade separation as set out here can be justified at flas low as 12,500 AADT on single lane dual mainlinedepending on turning traffic and more significantly,accident rates. Attention should be paid to ensuring there are no major differences in the level of junctionprovision along a route.

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1.4 This standard therefore sets out the level ofprovision to be considered for low traffic flowsituations on the minor road and effectively extendsdownwards the range of flows and conditions overwhich grade separation could well be justifiedeconomically to around 12,500 AADT on the throughroute. Further not readily apparent benefits may accrin accident reductions at other crossings and rights ofway, further reduction in severance and thecompensation needed to alleviate it, and a reduction ithe provision of accommodation bridges.

1.5 Full grade separation on a single carriagewayas a general rule is not recommended for reasons ofdriver perception and therefore reduced safety andshould generally be avoided in design. However TD 9(DMRB 6.1.1) makes recommendations about theprovision of grade separation at specific junctions onsingle carriageway roads. The provision of compactgrade separations on single carriageways is a permittalternative provided that the junction layout includes asection of single lane dualling on the mainline.

1.6 Comparisons of the overall size and layouts ograde separated junctions designed to TD 22 and thisstandard are indicated in Figure 1/1.

40m DesirableMinimum Radius

40m DesirableMinimum Radius

a. Grade Separation to TD 22 Standards

Note: Layouts drawn to scale

b. Example of Compact Grade Separation to TD 40 Standards

127m Radius(Relaxation of 3Design SpeedSteps)

225m Radius(Relaxation of 3Design Speed Steps)

Figure 1/1 Examples of the range of Grade Separations

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Chapter 1 Volume 6 Section 2Introduction Part 5 TD 40/94

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Principal Objectives

1.7 The objectives of compact grade separation cbe said to be to:-

a. improve safety by reducing the number andseverity of accidents;

b. provide a safer means of crossing high speedroutes for all road users;

c. reduce the environmental impact of gradeseparated junctions by providing a compactjunction layout;

d. regulate and maintain vehicle speed for minoroute traffic through the junction at a levelappropriate to the layout standards;

e. extend the use of grade separation;

f. remove right turn manoeuvres from high speeroads.

and specifically:-

g. minimise land take;

h. provide an operationally efficient junctionlayout;

i. provide an economic solution for themodification of existing junctions to gradeseparation standards.

Scope

1.8 This standard sets out the layout requiremenfor new compact grade separated junctions and for thupgrading of existing at grade junctions.

1.9 Recommendations are given on the siting of junctions, flow standards, layout alternatives, geometdesign and safety for all classes of road users. Someaspects of signs and road markings are included forcompleteness, though the full policy and detailedguidance on these matters are given in the Traffic SignsManual.

1.10 This standard also gives recommendations fothe identification of existing junctions to which compagrade separation may be applied.

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Mandatory Sections

1.15 Sections of this document are highlightedby being in boxes. These contain mandatorystandards which the Overseeing Organisationexpects in design. These are the sections withwhich the Design Organisation must comply ormust have agreed a suitable departure fromstandard with the relevant OverseeingOrganisation. The remainder of the documentcontains advice and enlargement which iscommended to designers for their consideration.

Implementation

an 1.11 This Standard should be used forthwith for thdesign of all schemes for the construction and

providing that, in the opinion of the OverseeingOrganisation, this would not result in significant

organisations should confirm its application toparticular schemes with the Overseeing Organisation.

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additional expense or delay progress. Design

Definitions

1.12 The terminology follows where possible thedefinitions contained in BS 6100: Subsection2.4.1:1990. and in TD 22 (DMRB 6.2.1).

1.13 The following additional terms have beendefined for use in this Standard.

Compact Grade Separated Junction: A gradeseparated junction designed in accordance withthis standard.

Compact Connector Road: In the context ofcompact grade separated junctions a two wayconnector road between the major and minorroads.

1.14 Where the term Full Grade Separation is usedin this standard it refers specifically to the standardscontained in TD 22 (DMRB 6.2.1). Grade Separationrefers to both full and compact types of gradeseparation.

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Volume 6 Section 2 Chapter 2Part 5 TD 40/94 Design Procedures

2. DESIGN PROCEDURES

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General Principles

2.1 The introduction of design standards forcompact grade separation is primarily aimed atimproving safety for all road users. At all stages inthe design and construction of the junction it is ofparamount importance that safety aspects are fullyinvestigated and considered.

2.2 This standard shall be applied in the designprocess to both the improvement of existing at gradejunctions on single and dual carriageways, and to thedesign of junctions on new single and dualcarriageway routes. Compact grade separationrequires in all cases the provision of either a sectionof single lane dualling or a full dual carriageway onthe major route.

2.3 The standards for compact grade separationhave in most instances been rigorously set in order tominimise the variation in designs to preventconfusion for the road user. Those items which dopermit a degree of variation at the discretion of thedesigner are:

a. Junction configurations;

b. Horizontal radii on compact connector road(desirable minimum radius with a relaxationof one design speed step)

c. Vertical Curvature;(desirable minimum radius with a relaxationof one design speed step)

d. Carriageway Width;(three options and a one step relaxation)

e. Curve Widening.(three options are permitted for each radius)

Junction Strategy

2.4 Junction design shall provide anunderstandable progression of standards for thedriver and therefore sequences of junctions shouldnot involve many different layout types. Theintroduction of a

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compact grade separation to replace an existing atgrade junction will have an effect within the route,beyond the immediate limit of the junction. It istherefore essential to assess the effect upon thenetwork and prepare a junction strategy. The siting ofthe compact grade separated junction will requirecareful consideration of the degree of access to beprovided.

2.7 Where a compact grade separated junction orjunctions are being considered the following mattersshall be taken into account:-

a. the closure of certain minor road junctionswhich have very low flows and for whichthere are alternative routes. The additionaljourney length, delay and inconvenienceresulting from the closure of a route must beconsidered in the context of the improvedsafety which will be achieved by the removalof all the right turn manoeuvres;

b. limiting remaining at grade junctions to left inleft out only;

c. where there is no alternative route it may benecessary to reconsider the location andnumber of junctions;

2.5 On existing single carriageways compact gradeseparation may be applied to a route which consists of anumber of priority junctions. The existing priorityjunctions may consist of a range of layouts from simpleT junctions to staggered single lane dualling with priorityjunctions.

2.6 On existing dual carriageways the provision ofcompact grade separation may be applied to a route whiconsists of a number of grade separated junctions androundabouts interspersed with major/minor at gradepriority junctions. Introduction of compact gradeseparation on a route such as this is intended to removeof the right turn manoeuvres associated with the mainlineresulting in a fully grade separated route. Some left-in/left-out junctions may be retained or introduced.

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d. improvements to the network to assist theclosure of certain junctions;

e. the provision of accommodation roads,cycleways, footpaths and bridleways;

f. the removal of agricultural accesses from themainline;

g. collecting a number of minor roads into asingle compact grade separated junction;

h. connecting minor roads or accommodationroads directly onto the compact connectorroad.

The Design Process

2.8 The design process for a compact gradeseparation shall be based upon the flow chart inFigure 2/1. The flow chart indicates the designprocess for the improvement of an existing priorityjunction to compact grade separation. The flow chartcan be adapted for comparison of alternative junctionoptions on a new route, and in this case the accidents,flows and turning proportions would be predictedrather than actual.

2.9 The design process may be applied to a junctionin isolation or to a series of junctions within a route.

2.10 Low and high growth design year traffic flowsmay be derived from the traffic model (The TrafficAppraisal Manual (TAM) refers in England, Wales andNorthern Ireland, and in Scotland the Scottish Trafficand Environmental Appraisal Manual (STEAM)refers).

2.11 In assessing the capacity and delays associatedwith a compact grade separated junction it is important toconsider each of the individual elements which comprisethe layout, particularly the additional priority junctionsand the compact connector roads.

2.12 The introduction of compact grade separation willresult in increased distances travelled for certain traffic

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movements due to the addition of the compact connecroads. The additional time taken should therefore beassessed throughout the day.

Junction Design

2.17 Junction entries and exits with the mainlineshall be based upon the recommendations containedwithin TA 20 (DMRB 6.2) modified to remove allright turn manoeuvres associated with the mainlinetraffic. Right turn manoeuvres will effectively berelocated to the priority junctions on the minor road.

2.18 Nearside diverging and merging tapers shouldbe provided on the mainline in accordance with TA20 (DMRB 6.2). Slip roads shall not be used atcompact grade separation since compact connectorroads are 2 way.

2.19 Alternative forms of junction layouts areprovided in Chapter 7.

.13 It can reasonably be assumed that delays at thenctions will only occur during the peak periods.

.14 In assessing the accident rates associated with aompact grade separated junction it is important to predie accident rates at each of the individual junction entry

nd exits within the compact grade separation.

.15 No statistical information is presently availablepecifically for accident rates at compact grade separatior priority junctions restricted to left turn manoeuvresnly. The removal of the right turn manoeuvre from theainline junction and the provision of a central reserve

hould result in a decrease in the number and severity occidents thereby achieving significant benefits. Guidan provided in Chapter 3 regarding the factors to beonsidered in the assessment of accident rates.

.16 Guidance is provided in Chapter 4 regardingpecific road user requirements which should beonsidered at this stage.


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ACCIDENTS1. Consider accident statistics for existing junction (or group of junctions) 2. Identify accidents associated with the right turns3. Calculate predicted reduction in accidents for a compact grade separation4. Calculate savings in accident costs for opening year

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DELAYS1. Consider present flows and turning movements for the existing junction2. Calculate delays during peak periods for existing junction3. Identify most suitable layout for grade separation bearing in mind likely costs4. Re-distribute traffic onto compact grade separation5. Calculate delays at compact grade separation for both peak and off peak periods6. Calculate saving/increase in delays for compact grade separation7. Calculate saving/increase in delay costs for opening year

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CAPITAL COSTS Calculate detail cost of scheme including:1. Construction Costs2. Cost of land required for scheme3. Cost of delays during construction4. Service diversions

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Growth up all costs for the design life of the scheme

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MAINTENANCE COSTS1. Calculate maintenance costs for existing junction over design life of scheme2. Calculate maintenance costs for compact grade separation over design life of scheme3. Calculate saving/increase in maintenance cost

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Does the Compact grade separation have a positive NPV overthe do minimum?NO YES

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Conversion to a compact ENVIRONMENTALgrade separation would Consider the advantages/disadvantages of a compact gradenot be viable/acceptable separation over the existing priority junction regarding;

1. Land take - is land required in a sensitive area, willbuildings need to be demolished

2. Visual Intrusion3. Noise and pollution4. Loss of existing landscape and vegetation5. Ecological effects

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SIGNIFICANTENVIRONMENTAL PROBLEMS

ENVIRONMENTAL PROBLEMS NOT 9

SIGNIFICANT

Compact Grade Separation Justified

Figure 2/1 Upgrading of existing Priority junction to Compact grade separation.


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Chapter 2 Volume 6 Section 2Design Procedures Part 5 TD 40/94

Drainage

2.20 All drainage within the compact gradeseparated junction should be of a positive nature tofacilitate the removal of surface water. Factors toconsider and which may influence the decision are:-

a. the geometric standards for the junction arelikely to result in steep gradients combined withsuccessive application and removal ofsuperelevation up to 5%;

b. surface water from the compact connector roadshould be prevented from flowing onto oracross the major and minor carriageways.

Economic Assessment

2.21 The economic assessment of compact gradeseparation is particularly sensitive to the effects oftraffic flows, traffic distribution and the junction layout:-

a. capital costs are significantly affected by thestructure, earthworks and pavement costs. It istherefore important that the opportunities fornarrower bridges are fully examined, usingnarrow carriageways and verges;

b. significant benefits can be achieved by thereduction in accident rates and severity by theremoval of right turn manoeuvres from themainline. The redistribution of traffic mayrequire additional priority junctions between thecompact connector road and the minor road. The accident rates for these junctions will needto be assessed and the benefits adjustedaccordingly;

c. the effect of providing grade separation willgenerally be a slight reduction in the delaysassociated with the at grade junctions. However the benefits will generally beoutweighed by the additional time taken bysome traffic through the compact gradeseparated junction, resulting from theredistribution of traffic onto the compactconnector road. It is therefore important tooptimise the compact grade separation layout tsuit the traffic flow conditions and minimise thedisbenefits associated with the delays;

d. the costs arising from maintenance need to betaken into account.

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.22 It is recognised that in certain circumstances these of COBA is not appropriate where theprovements are too fine in scale to be measured bye built-in traffic engineering formulae. For andividual compact grade separation it is recommendedat a manual economic assessment be carried out. Therocedures outlined here are based on those used formall Trunk Road scheme assessment as outlined on theepartment of Transport form ROADS 502. Incotland the use of NESA is recommended inreference to manual assessments.

ccident Costs

.23 To determine the accident benefits it isecessary to predict the number of accidents at each ofe proposed junction entries and exits within the

ompact grade separation and the compact connectorads based upon the redistributed traffic flows. These

re then compared with the recorded accident rates ate existing at grade junction taken over a minimum of 3

ears.

.24 In assessing the accident rates for left turn onlyestricted) entry and exits it can be assumed that theredicted accident rate will be 25% of that calculated for junction at which all movements are permitted. Thealculations should include redistributed traffic flowsom adjacent junctions whose movements have beenubsumed into the new layout. The accident rates fore un-restricted movements at the junction entries and

xits with the minor road must be calculated andcluded without adjustment.

.25 From Table 3/1 in Chapter 3 the severity ratiosr each of the individual junction entries and exits can

e determined for the road classification and speed limitnd applied to the predicted accident rates to calculatee number and severity of accidents.

.26 From Road Accidents in Great Britain (Theasualty Report) the average cost per accident byeverity can be obtained, and the severity ratios andverage costs per accident for the various classificationf road may be calculated at each of the junction entriesnd exits.


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Value of Time

2.27 To determine time savings (or increases)calculate the total vehicle hours saved (or lost) in thescheme opening year using both high and lowtraffic/economic growth:-

a. calculate average time savings (or increases)per vehicle for peak and off-peak periods. Thiswill necessitate the calculation of the averagetime taken through the various links of thegrade separated junction making allowances foracceleration and deceleration;

b. calculate the delays at the junction entries andexits during the peak periods;

c. factor the savings (or increases) per day inorder to arrive at an annual time saving (orincrease) by using the appropriate M Factorand value of time (VOT) obtained from COBA (NESA in Scotland).

2.28 If the local mix of traffic should differsignificantly from the trunk road average then the valueof time appropriate to the local traffic mix can besubstituted.

Vehicle Operating Costs

2.29 Compact grade separation can increasesignificantly the total vehicle kilometres travelled due tothe reassignment of the traffic through the compactgrade separation. The total additional distance for eachmovement and the annual vehicle operating costs canthen be determined.

Calculation of Present Value of Benefits

2.30 The accident, time and vehicle operating costscalculated for the scheme opening year (both high andlow growth) should each be capitalised over 30 yearsand discounted back to the base year to give the presenvalue of benefits (PVB).


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Volume 6 Section 2 Chapter 3Part 5 TD 40/94 Safety

3. SAFETY

General

3.1 One of the principal objectives of compactgrade separated junction design is to improve safety bythe elimination of right turn manoeuvres between themainline and the side road, by providing left turn onlypriority junctions and the closure of central reservegaps.

3.2 There is no definitive information providingaccident statistics for left turn only priority junctions. Investigation of accidents at 3 arm priority junctionsindicate that 68% of accidents are directly attributableto right turn manoeuvres, 19% are directly attributableto left turn manoeuvres and the remaining 13% areattributable to either both combined or other factors. Removal of the right turn and the provision of a centralreserve would most probably reduce the accident rateby 75% by the removal of certain conflictingmanoeuvres.

3.3 The layout of the compact grade separation wihowever lead to accidents at the junctions between thecompact connector road and the minor road which will

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ater for all turning manoeuvres. Taking into accountccident records, and the lower level of flows through e junctions with the minor road, it is possible to

stimate hat the accident distribution might be. The combinedffect suggests that the number of accidents at 3 armriority junctions can be reduced by approximately 50%nd by approximately 75% at 4 arm priority junctions,y the introduction of compact grade separation.

.4 Accident statistics for 3 arm priority junctionsdicate that the severity of accidents is dependent upone classification of the mainline and the mainline speed

mit and that generally accident severity decreases aspeed decreases. Compact grade separation willansfer the right turn manoeuvres from the higherpeed, higher classification road to the lower speed,wer classification road. The effect of this will be toduce the number of fatal and serious accidents with a

orresponding increase in slight accidents as indicated Table 3/1. This information can then be used as aasis to calculate the average cost/accident for theonditions applicable at each of the junction mouths.

Classification of Accident Speed Limit (mph)Road Severity 30 40 50 60 70

A

Fatal 1527 422 64 740 239(1.27) (2.28) (3.64) (3.03) (5.31)

Serious 22607 3994 384 5493 1220(18.82) (21.62) (21.87) (24.37) (27.11)

Slight 95962 14058 1308 17702 3042(79.91) (76.1) (74.49) (72.6) (67.58)

B

Fatal 515 56 4 154(1.22) (1.57) (1.79) (2.12)

Serious 7965 785 52 1707(18.85) (21.99) (23.32) (23.52)

Slight 33766 2728 167 5396(79.93) (76.44) (74.89) (74.36)

C

Fatal 326 30 0 80(1.07) (2.25) (0) (2.12)

Serious 5712 287 8 823(18.73) (21.56) (16.67) (21.77)

Slight 24461 1014 40 2877(80.20) (76.19) (83.33) (76.11)

Unclassified

Fatal 995 41 3 74(0.88) (1.63) (2.65) (1.61)

Serious 21127 566 28 1018(18.69) (22.43) (24.78) (22.14)

Slight 90912 1916 82 3505(80.43) (75.94) (72.57) (76.25)

Table 3/1 Numbers and Percentages (%) of Personal Injury Accidents for 3 Arm PriorityJunctions (extracted from STATS 19 Database, 1987 - 1991)

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Traffic Signs / Road Markings

3.8 The provision and layout of traffic signsand road markings is an integral part of thejunction design process and shall be consideredat an early stage. Advance direction and warningsigns shall be provided on both the major andminor roads. Positioning of signs within thejunction and the compact connector road shall becarefully considered so that they do not interferewith driver's visibility. It is essential that there isno over-provision of signing within the compactconnector road which may result in unnecessarydistractions to drivers negotiating the curves.

3.9 On the approach to the junction fromeither the major or minor road the junction mustbe clearly defined by means of appropriatesigning indicating the junction as a priority Tjunction and not as a slip road.

3.5 Taking into account the reduction in thepredicted number of accidents and the reduction inaccident severity ratios, evidence suggests that annuaccident costs for 3 arm priority junctions can bereduced by approximately 60% and for a 4 arm priorijunction by approximately 80%, by the introduction ocompact grade separation.

3.6 It is intended that compact grade separationshall be used to provide a safe means of crossing higspeed routes for all road users, and specificrequirements are detailed in Chapter 4. Thegeometrical standards for the compact connector roahave been established at a level of provision intendemaintain slow vehicular speeds through their lengththereby improving safety for other road users.

3.7 Other positive factors which improve safety aa result of compact grade separation are;

a. removing the possibility of large vehicleswhich cross central reserve gaps protrudinginto the offside lane;

b. removal of U turns on the mainline.


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3.10 Where full curve widening is notprovided, as identified by paragraphs 6.20, 6.21and Table 6/3, then regulatory signs shall beprovided in accordance with Chapter 3 of theTraffic Signs Manual on the compact connectorroad to advise motorists in one direction that theyshould give way to vehicles proceeding in theopposite direction. Statutory requirements forregulatory signs are contained in the Traffic SignsRegulations and General Directions and TrafficSigns Regulations (Northern Ireland).

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3.11 Within the compact connector road it isrecommended that signing be restricted to warningsigns indicating the nature of the alignment, and theproximity of junctions.

3.12 The application of special surface treatmentsmay be provided as an additional means of highlightingthe nature of the geometrical standards for the compactconnector road. This can be achieved by the applicationof a contrasting surface colour (red) within the hatchmarkings to emphasise the nature of the curve, thisshould reinforce the meaning and presence of themarkings to the driver.

Surfacing

3.13 Given the possibility of steep gradients on theapproach to the junctions consideration should be givento the properties of the surfacing particularly withregard to skid resistance and wear.


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Volume 6 Section 2 Chapter 4Part 5 TD 40/94 Specific Road User Requirements

4. SPECIFIC ROAD USER REQUIREMENTS

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4.1 Compact grade separated junctions areintended to be used by all classes of road users. Detailed attention to the needs of the following classeof road user is required throughout the junction.

Pedestrians

4.2 The compact grade separation should be useto remove conflict between pedestrians and vehicles.possible, separate pedestrian routes should be providaway from junction entry and exits although it isacknowledged that this is rarely practicable. Pedestrians should be encouraged to use the compagrade separation to cross the major road and excessdetours from their desired route should be avoided. Routes should therefore be provided through the gradseparation to link existing footpaths or footways whethe perceived level of usage by pedestrians justifiestheir inclusion. All footpaths should be clearly visibleto drivers on the mainline or minor road carriagewaysto ensure that pedestrians will feel secure in using thefacility.

Cyclists

4.3 Advice is available on the provision offacilities for cyclists in the following documents

- TA 57 (DMRB 6.3) - Roadside Features

- Local Transport Note (1/86) - Cyclists atRoad Crossings and Junctions

- Traffic Advisory Leaflet (1/88) - Provision forCyclists at Grade Separated Junctions

- Cycling Advice Note 1/89 - Providing for theCyclist - Cyclists at Road Crossings andJunctions - Shared Use by Cyclists andPedestrians

- Cycling Advice Note 1/90 - Making Way forCyclists - Planning, Design and Legal Aspectof Providing for Cyclists

4.4 The compact grade separation should be useto remove conflict between cyclists and vehicles. Cyclists should be encouraged to use the compact gr


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separation to cross the major road. To assist cyclistsgradients would ideally be limited to 5%, however theobjectives of compact grade separations will in manyinstances prevent this being achieved. Gradients shouldonly be increased above this figure to those set out inparagraph 6.15 with careful consideration of allfactors.

4.5 Where regular usage by both pedestrians andcyclists is identified then consideration should be givento combined facilities.

4.6 Identification of usage should be carried out atan early stage in the design.

Equestrians

4.7 Identification of tracks and bridlewaysregularly used by equestrians should be carried out anearly stage of the design process.

4.8 Uncontrolled crossing of high speedcarriageways by equestrians should be avoided andtracks and bridleways should be diverted onto thecompact grade separation wherever possible. Equestrians can be expected to divert for moderatedistances without undue inconvenience provided thatthe diversion takes no longer than a few extra minutes.

4.9 Where cattle grids are provided on the minorroad approaches to a compact grade separation,manually operated gates should be provided adjacent tothe grids for equestrians.

Use by Drivers of Agricultural Vehicles

4.10 It is intended that compact grade separationsshall be used to accommodate the requirements forlarge, slow-moving agricultural vehicles. Carefulconsideration must be given to the combined effect ofthe gradient and superelevation to ensure that high loadare not subject to sudden shifts caused by rapid changein crossfall.

4.11 Where farm or field accesses on the mainlineare situated close to the compact grade separatedjunction consideration should be given to joining themto the compact connector road.


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Use for Farm Animals

4.12 Where farms are adjacent to compact gradeseparated junctions it may be necessary to provide cgrids on the minor road approach or on theaccommodation road approach to the junction. Cattgrids should be sited in accordance with TA 57(DMRB 6.3).

Requirement for Hardened Verges

4.13 Where there is expected usage by acombination of equestrians, agricultural vehicles andfarm animals, hardened verges should be considerethe compact connector road to prevent damage to thverge and drainage and to encourage these users totravel more safely and comfortably off the maincarriageway.


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Volume 6 Section 2 Chapter 5Part 5 TD 40/94 Flow Standards

5. FLOW STANDARDS

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General

5.1 The introduction of compact grade separationintended to extend downwards the range of flows andconditions over which grade separation may beconsidered. It has been found that using these standimproving existing junctions to compact gradeseparation can be justified down to flows of around12,500 AADT on the mainline, depending upon turnintraffic.

5.2 Compact grade separation can be used withsingle carriageways and dual two lane all-purpose roawithin Categories 3A, 3B, 4, 5 & 6 of roads as definedin Table 4 of TD 9 (DMRB 6.1.1).

Flow Levels

5.3 Compact grade separated junctions appear tosuitable for use where mainline flows are betweenapproximately 12,500 AADT and 30,000 AADT andare normally associated with very low flows (generallybelow 10% of mainline flow) on the minor road.

Delays

5.4 Junction delay figures significantly in theeconomic assessment of alternative forms of junctionand is dependent upon the geometric standards and ttraffic conditions. In assessing the delays associatedwith compact grade separations it is important toinclude the additional travelling time required to passthrough the compact connector roads and to negotiatthe additional junction entry and exits. Delays at themajor/minor junctions may be significantly reducedduring the peak periods; travel time throughout the dawill increase due to the additional distance through thjunction.

5.5 Alternative layout options for 4 arm junctionsshould be assessed to determine the most suitable foto minimise the additional delays, which will dependupon the turning proportions. This should be carriedout at an early stage in the design when considering ttopographical and environmental constraints imposedupon the junction location and form.


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Capacity

is 5.6 Junction capacity is dependent upon theinteraction between the various traffic movementswithin a junction and the geometrical standards of t

ards

g

ds

prove viable where the existing junction already

be

she

junction. TRRL Report SR582 provides guidance forthe prediction of capacity based upon investigation ofexisting junctions. Analysis of capacities for compactgrade separations based on this report indicates that thremoval of the right turn manoeuvre can result insignificant capacity increases.

5.7 In the particular case of 3 arm (T) junctions,conversion to compact grade separation is only likely to

experiences delay or the junction is an essential elemenof the route, possibly providing a means of removingright turn manoeuvres at adjacent 3 arm junctions asindicated in Figure 7/8.

5.8 Analysis indicates that for the improvement ofan existing priority junction to compact gradeseparation standards an increase in capacity of 80% forsome movements can be achieved at the mainlinejunctions.

5.9 The layout of the compact grade separationshould be chosen to suit the traffic movements. Incertain conditions the redistribution of the turningtraffic can result in significant turning flows at theminor road junction entry and exits. It is importanttherefore to assess the capacity of these junctions whenconsidering the layout to be adopted.


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Volume 6 Section 2 Chapter 6Part 5 TD 40/94 Geometric Standards

f

6. GEOMETRIC STANDARDS

r

6.2 The geometric standards for the mainlineand for those minor roads which pass directlythrough a compact grade separation shall be providedin accordance with Standard TD 9 (DMRB 6.1.1).

Design Speed

6.3 Design speeds for the mainline and theminor road shall be determined from TD 9 (DMRB6.1.1).

6.4 As a general principle it is intended that thespeed of vehicles through the compact connectorroad shall be limited by its speed through the entryand exit junctions with the mainline and the minorroad. For this reason long straight sections within thecompact connector road shall be avoided.

General

6.1 This standard provides a geometric standard forcompact grade separation incorporating established desigstandards for major/minor junctions. The geometricstandards contained in TD 9 (DMRB 6.1.1) provide fordesign speeds down to a minimum of 50kph. Thegeometric standards for compact grade separation aregenerally based upon the principles contained in TD 9(DMRB 6.1.1) but are for a design speed of 30 kph whichis 3 design speed steps below the current minimum.

6an


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6.5 The design standards for compact gradeseparated junctions relate to a single design speed oapproximately 30kph. Junctions shall only bedesigned to this design speed, higher or lowerstandards shall not be used. A range of standardswould result in driver confusion and uncertainty andproblems of perception of the junction withconsequent safety implications.

Compact Connector Road

6.7 Compact connector roads shall be designedin accordance with Table 6/1 except for the specialconditions of paragraph 6.11 when the stoppingsight distance may be relaxed further as set out inparagraph 6.11. The stopping sight distances inTable 6/1 are not related to the design speed but arebased upon the requirements to provide adequatestopping sight distance through the compactconnector road to show clearly to the driver leavingthe mainline the shape of the junction.

n

.6 If speeds markedly in excess of this do occur ore anticipated then speed limits within the junction mayeed to be considered.

STOPPING SIGHT DISTANCESDesirable Minimum (m) 70Relaxation of One Design Speed Step (m) 50

HORIZONTAL CURVATUREDesirable Minimum Radius (m) with 5% Superelevation 40Relaxation of One Design Speed Step(m) with 5% Superelevation 32

VERTICAL CURVATUREDesirable Minimum Crest K 3.3Relaxation of One Design Speed Step Crest K 2.3Absolute Minimum Sag K with lighting 2.3Absolute Minimum Sag K without lighting 3.2

Table 6/1 Design Standards for Compact Connector Roads


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Chapter 6 Volume 6 Section 2Part 5 TD 40/94

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Geometric Standards

Horizontal and Vertical Alignment

6.8 Horizontal radii shall comply with Table6/1. In normal circumstances the DesirableMinimum Radius should be used, however indifficult circumstances a Relaxation of one designspeed step may be used at the discretion of theDesign Organisation.

6.9 Given the low design speed and the nature ofthe compact connector road, transition curves are notrequired within the compact grade separated junction.

Sight Distances

6.11 Where the minor road continues on a directalignment across the major road as indicated inFigures 7/1 & 7/2 the stopping sight distances shallbe in accordance with TD 9 (DMRB 6.1.1). Whereminor road through traffic is the principal user of thecompact connector road as indicated in Figure 7/3the stopping sight distance may be decreased to avalue based upon the design speed of 30kph asindicated in Table 6/2.

Desirable Minimum (m) 33

Relaxation of One Design Speed Step (m) 26

Table 6/2 Stopping Sight Distances based onthe design speed of 30kph forCompact Connector Roads usedprincipally by minor road throughtraffic

6.10 In the design of vertical curves consideration isgiven to stopping sight distance and driver discomfort,which requires the vertical rate of change of grade to bekept within tolerable limits. For the low design speedadopted for compact connector roads the stopping sightdistance criterion over the summit is not critical becauselarger changes of grade do not obstruct stopping sightdistance and the comfort criterion overrides. Whereverpossible vertical curvature on bridge decks should beavoided.

6.12 The stopping sight distances for compact gradeseparations have been set at a level intended to discourhigh speeds. The standard is however greater than thatrequired for the design speed adopted.

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6.13 For the connections to the major and minorroads, guidance on sight distance and visibilitystandards are given in TA 20 (DMRB 6.2). In thecase of compact grade separation these shall be takenas mandatory.

Gradients

6.15 The desirable maximum gradient forcompact connector roads shall be 8%, althoughrelaxation to 10% shall be permitted in difficultlocations. In selecting the gradient, the needs ofcyclists should be considered (paragraph 4.4) andthe risk of occurrence of icy conditions.

6.14 Where minor roads or accommodation roads areconnected to the compact connector road then thevisibility standards at the junction shall be in accordancewith TA 20 (DMRB 6.2) and Table 6/1 of this standard.

Superelevation

6.16 Normal standards for superelevation, set out inTD 9 (DMRB 6.1.1), would require excessivesuperelevation for all compact grade separations. Icyconditions can cause slow moving vehicles to slide to theinside of the curves with excessive superelevation, thiseffect can be exacerbated by steep gradients.

6.17 The geometric layout of the compactconnector road will require successive applicationand removal of the superelevation between theconnector road and the junction mouths. The need tomaintain consistent steering requirements through thecompact connector road is an important designconsideration which shall be taken into account in theapplication of superelevation.

6.18 Superelevation on compact connector roadsshall be limited to 5%.

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Standard Width of CentralHatch Marking (0.6m)

W

W

Curve Radius (R)

Width of Hatching atApex W

H

Cross Sections and Curve Widening

6.19 For the purpose of designing junctions andinterchanges, cross sections for the mainline and allconnector roads are given in the Manual of ContractDocuments for Highway Works, Volume 3 "HighwayConstruction Details" (MCHW 3.1). Different lanemarking details and widths of construction forcompact connector roads from those shown in"Highway Construction Details" are specified inthis standard.

6.20 Compact connector roads may be widened oncurves in accordance with Table 6/3, the wideningshall be applied in the form of central hatchedmarkings as illustrated in Figure 6/1. The width ofcurve widening shall be chosen to suit the anticipatedlevel of usage by large goods vehicles. Designersshall take into account the probability of largevehicles regularly meeting on the curve.

6.21 Where no curve widening is applied largegoods vehicles will cut across into the oncoming lane,where the minimum curve widening is applied largegoods vehicles will cut across into the whole of thehatched area. Where the normal curve widening isprovided there will be sufficient width for two largevehicles to pass in opposite directions. Regulatorysigns shall be provided when appropriate inaccordance with paragraph 3.10.

Figure 6/1 Application of CurveWidening


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InnerChannelRadius(m)

Width W of Hatching at Apex on Curve (m)H

NoCurveWidening

MinimumCurveWidening

Normal CurveWidening

40 0.6 1.90 3.40

32 0.6 2.54 3.74

Table 6/3 Curve Widening on CompactConnector Roads

6.22 Carriageway widths for the compactconnector road shall be such that they provide atransitional change in standards from the majorcarriageway width to the minor carriageway width inaccordance with Table 6/4.

6.23 The minimum width of carriageway for acompact connector road shall be 6.6m and themaximum shall be 7.9m, excluding curve widening. Where the traffic volumes are particular low and theproportion of large goods vehicles is correspondinglylow a relaxation to 6m may be permitted, by eitherreducing the carriageway width or omitting the centralhatched markings.


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Chapter 6 Volume 6 Section 2Geometric Standards Part 5 TD 40/94

MajorCarriageway

Compact ConnectorRoad

MinorCarriageway

Lane Width(m)

LaneWidth(W)(excludinghatching)(m)

StandardWidth ofCentralHatchMarkings(m)

Lane Width(m)

All Widths

3.0 0.3 per lane < 3.03.3 0.3 per lane 3.0 to 3.65

3.65 0.3 per lane 3.65

Table 6/4 Compact Connector Road Widths

6.24 A minimum width of 6m should also be providedadjacent to traffic deflection islands. In addition trafficdeflection islands should be set back 1 metre from thegive-way line at the junction with the mainline as shownin Figure 6/2.




Minor RoadWidth

x y z

< 3.03.0 to 3.65

3.65

3.03.33.65

0.60.60.6

4.74.74.7

NOTES:

1. All dimensions are in metres.2. All corner radii on islands are to be 0.7 - 1.0m radius.3. These details apply to both single lane dual and D2AP carriageways.4. For length of taper refer to TA 20 (DMRB 6.2).5. Where required, junctions may have both a merging and diverging taper.6. The minimum width adjacent to traffic islands is 6m (see paragraph 6.24)

x - y - x

20

28

6.2

hardstrip

see note 6

20

20

25

33

5.7

Start of Taper

c) MAINLINE JUNCTION WITH NEARSIDE MERGING TAPER

x - y - x

10

b) MAINLINE JUNCTION WITH NEARSIDE DIVERGE TAPER

x - y - x

20

28

6.2

hardstrip

see note 6

20

20

6.2

20

28

3.7

Start of taper

a) SIMPLE MAINLINE JUNCTION

x - y - x

20

28

6.2hardstrip

see note 6

20

20

6.2

20

28

z zy

Figure 6/2 Modified Major/Minor Junction for Left Turn Only Manoeuvres

(Dimensions are those set out in TA 20 (DMRB 6.2) but modified for left in/left out operation. As modified inparagraph 6.29, the conditions under which tapers may be provided are detailed in TA 20 (DMRB 6.2))



Chapter 6 Volume 6 Section 2Geometric Standards Part 5 TD 40/94

6.25 The length of solid island in the single lanedualling section shall be of the minimum lengthnecessary to prevent right turn manoeuvres. U turnsshall be banned at either end of the single lanedualling.

Hardstrips

6.26 The use of hardstrips is associated with highspeed roads, they shall not be used within thecompact connector road. Where hardstrips areincluded on the mainline they shall be terminatedwithin the junction as indicated in Figure 6/2.

Junctions

6.27 Junction entries and exits with the mainlineshall be based upon the standards contained withinTA 20 (DMRB 6.2) modified to remove all rightturn manoeuvres associated with the mainline traffic. Right turn manoeuvres will effectively be relocatedat the priority junctions on the minor road.

6.28 The modified junction layouts for the left in,left out junctions are indicated in Figure 6/2.

Merges and Diverges

6.29 Nearside diverging and merging tapers shallbe provided in accordance with the directions givenin TA 20 (DMRB 6.2). The provision of mergingand diverging tapers shall be carefully consideredgiving particular regard to the following factors:-

a. proportion of merging and diverging traffic,generally in in excess of 100 vehicles perday;

b. visibility standards at the junctions;

c. the gradient of the roads and their effect onspeed and acceleration;

d. the proportion of slow moving vehicles;

e. the cost of provision.

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Channelising Islands

6.30 All channelising islands associated with thejunction of the mainline and the compact connectorroad shall be physical islands to ensure separation oftraffic and to prevent vehicles attempting right turns.

ootpaths

.31 Where footpaths, footways or combined facilityor combined footpath/cycleways are to beprovided, in accordance with Chapter 4, aminimum width of 1.5m is permitted.


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Volume 6 Section 2 Chapter 7Part 5 TD 40/94 Layout Options

7. LAYOUT OPTIONS

General

7.1 The layout for the compact grade separationwill in many cases be dictated by the constraintsimposed by the existing junction to be modified, or stopography in the case of new routes. The formats othese junctions are numerous and it is not possible tprovide exhaustive examples. In some cases,underbridges could be considered as an alternative they may prove practical and justifiable economicallyThere may be situations, where, due to localtopography, this would be preferable in landscapeterms. In level and treeless landscapes, overbridgesbe visually intrusive and planting may be out ofcharacter. Environmental Design is addressed furthDMRB Volume 10, The Good Roads Guide.

7.2 Detailed examples of 3 alternative layouts foarm junctions are indicated in Figures 7/1 to 7/3 and anexample of a layout for a 3 arm junction is indicated Figure 7/4. Schematic representations of other layouare indicated in Figures 7/4 to 7/8.

7.3 Figure 7/9 indicates two method ofincorporating a compact grade separation into a secof carriageway which has a series of priority junctionin close proximity. The first option indicates thejunctions adjacent to the compact grade separationrestricted to left in left out requiring right turning traffifrom these junctions to divert to the compact gradeseparation to complete their manoeuvre. The seconoption indicates the closure of the adjacent priorityjunctions and the total diversion of these routes toconnect into the compact grade separated junction.

7.4 The preferred locations for the junction of thecompact connector road to the mainline are in the 1sand 3rd quadrants as indicated in Figures 7/1, 7/2 and7/4, as they facilitate the provision of requiredvisibilities without the need to widen under thestructure.

Typical Sketch Layouts

Figure 7/1 Detailed layout of 4 arm, D2APcarriageway with compact connectorroads in preferred location andincluding both merging and divergingtapers, and free flow on the minorcarriageway. For junctions where

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itef

o

and.

can

er in

r 4

ints

tions

c

d

t

minor road traffic movement isprimarily straight across.

Figure 7/2 Detailed layout of 4 arm, D2APcarriageway with compact connectorroads in preferred location withoutmerging and diverging tapers, and freeflow on the minor carriageway. Forjunctions where minor road trafficmovement is primarily straight across.

Figure 7/3 Detailed layout of 4 arm, D2APcarriageway with compact connectorroad forming a loop. For junctionswhere minor road traffic movement isprimarily turning onto and off themajor carriageway.

Figure 7/4 Detailed layout of 3 arm, D2APcarriageway with compact connectorroads in preferred location andincluding both merging and divergingtapers.

Figure 7/5 A selection of schematic layoutsindicating 4 arm compact gradeseparations on dual all-purposecarriageways.

Figure 7/6 A selection of schematic layoutsindicating 4 arm compact gradeseparations on single all-purposecarriageways.

Figure 7/7 A selection of schematic layoutsindicating 3 arm compact gradeseparations on dual all-purposecarriageways.

Figure 7/8 A selection of schematic layoutsindicating 3 arm compact gradeseparations on single all-purposecarriageways.

Figure 7/9 Schematic example of compact gradeseparation and restricted junctions.



Chapter 7 Volume 6 Section 2Layout Options Part 5 TD 40/94

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1st Quadrant 4th Quadrant

2nd Quadrant 3rd Quadrant

40m

40m 20m

20m R = 40m





40m

40m 20m

20mR = 40m

Figure 7/1 Example of Compact Grade Separation for 4 Arm Junction, Showing Merge and DiverTapers

Figure 7/2 Example of Compact Grade Separation for 4 Arm Junction, Without Mergeand Diverge Tapers



Volume 6 Section 2 Chapter 7P t Options

J

rge Tapers







40m

40m

20m R = 40m



art 5 TD 40/94 Layou

Figure 7/3 Compact Grade Separation for 4 Arm Junction, Showing Merge and Dive

Figure 7/4 Compact Grade Separation for 3 Arm Junction, Showing Merge andDiverge Tapers


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7/4

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Figure 7/5 Schematic Examples of 4 Arm Compact Grade Separation on Dual Carriagew

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Figure 7/6 Schematic Examples of 4 Arm Compact Grade Separation Single Carriageway




Figure 7/7 Schematic Examples of 3 Arm Compact Grade Separation on Dual Carriageways

Figure 7/8 Schematic Examples of 3 Arm Compact Grade Separation on Single Carriageways




July

Existing Layout indicating 3 arm junctions

Restricted Junction

Restricted Junction

Proposed Layout indicating Compact Grade Separation and Restricted Junctions

Proposed Layout indicating Compact Grade Separation and Total Diversion ofMinor Side Roads and/or Accommodation RoadsFigure 7/9 Schematic Example of Compact Grade Separation and Restricted Junctions


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

Volume 6 Section 2 Chapter 8Part 5 TD 40/94 References

8. REFERENCES

1. DESIGN MANUAL FOR ROADS ANDBRIDGES : HMSO

a. TD 22 (DMRB 6.2.1) - Layout of GradeSeparated Junctions

b. TA 20 (DMRB 6.2) - The Layout ofMajor/Minor Junctions

c. TA 30 (DMRB 5.1) - Choice between optionsfor Trunk Road Schemes

d. TD 44 (DMRB 6.2) - {Title to be insertedhere}

e. TD 9 (DMRB 6.1.1) - Highway Link Design

f. TA 57 (DMRB 6.3) - Roadside Features

g. Environmental Design (DMRB 10) - TheGood Roads Guides

2. MANUAL OF CONTRACT DOCUMENTSFOR HIGHWAY WORKS : HMSO

a. Highway Construction Details (MCHW 3.1)-Carriageway and Other Details

3. BRITISH STANDARDS

a. BS6100 Subsection 2.4.1 1990 - The BritishStandard Glossary of Building and CivilEngineering Section 2.4 Highway and RailwaEngineering; Subsection 2.4.1 HighwayEngineering: British Standards Institution 199(UDC 001.4:(624+69))

4. TRAFFIC APPRAISAL

a. Traffic Appraisal Manual - (TAM) 1: DoT:1982

b. Scottish Traffic Appraisal Manual -(STEAM): SDD:1986

c. TRRL Supplementary Report 582- TheTraffic Capacity of Major/Minor PriorityJunctions - Transport and Road ResearchLaboratory: 1980

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5. ACCIDENT ASSESSMENT

Casualty Report): HMSO: Annually

y

0

a. Road Accidents in Great Britain (The

b. STATS19 - DoT database of RoadAccidents: DoT: Annual

6. ECONOMIC ASSESSMENT

a. COBA - Cost Benefit Analysis - COBA 9Manual: DoT: 1981

b. NESA - Network Evaluation from Surveysand Assignments - NESA: SDD: 1986

c. ROADS 502 - Assessment of schemes: DoT:1989

7. MISCELLANEOUS

a. SI 1981 No 859 - The Traffic SignsRegulations and General directions 1981:HMSO 1981, SI 1982 No 1879 - The TrafficSigns [Amendment] Regulations 1982 and SI1982 No 1880 - The Traffic Signs General[Amendment] Direction 1982: HMSO : 1982 -SR 1979 No 386 Traffic Signs Regulations(Northern Ireland)

b. Local Transport Note (1/86) - Cyclists atRoad Crossings and Junctions - HMSO

c. Traffic Advisory Leaflet (1/88) - Provision forCyclists at Grade Separated Junctions - HMSO

d. Cycling Advice Note (1/89) - Providing for theCyclist - SDD

e. Cycling Advice Note (1/90) - Making Way forCyclists - SDD

f. The Traffic Signs Manual - Chapters 1, 3, 4, 5and 14 - HMSO



Volume 6 Section 2 Chapter 9Part 5 TD 40/94 Enquiries



9. 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 HouseSouthwark Street T. A. ROCHESTERLondon Civil Engineering SE1 0TE and Environmental Policy Director

The Deputy Chief EngineerThe Scottish Office Industry DepartmentRoads Directorate New St Andrew's HouseEdinburgh J. INNESEH1 3TG Deputy Chief Engineer

The Director of HighwaysWelsh OfficeY Swyddfa GymreigGovernment BuildingsTy Glas Road LlanishenCardiff K. J. THOMASCF4 5PL Director of Highways

Chief Engineer - Roads ServiceDepartment of the Environment for Northern IrelandRoad Service HeadquartersClarence Court10 - 18 Adelaide StreetBelfast W. J. McCOUBREYBT2 8GB Chief Engineer - Roads Service