Layout of Grade Separated Junctions

DN-GEO-03035 January 2009

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DN Design Standards TRANSPORT INFRASTRUCTURE IRELAND (TII) PUBLICATIONS

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TII Publication Title Layout of Grade Separated Junctions TII Publication DN-GEO-03035 Number

Activity Design (DN) Document Set Standards Stream Geometry (GEO) Publication Date January 2009 Document 03035 Historical TD 22 Number Reference

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National Roads Authority Volume 6 Section 2 Design Manual for Roads and Bridges Part 1 TD 22/06 Addendum

NRA ADDENDUM TO

TD 22/06

LAYOUT OF GRADE SEPARATED JUNCTIONS

This Addendum supersedes the NRA Addendum dated January 2005 to Standard TD 22/92. The revisions have arisen with the publication of Standard TD 22/06 and this Addendum amends the scope of TD 22/06 for use in Ireland.

Standard TD 22/06 – Layout of Grade Separated Junctions – is applicable in Ireland with the following amendments:

GENERAL At several locations:

For: “LGV” Read: “LCV” For: “” Read: “road” For: “TD 9 (DMRB 6.1.1)” Read: “NRA TD 9”

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SPECIFIC

Chapter 1, Introduction 1. Page 1/1, Paragraph 1.1, first sentence: Delete “on rural and urban trunk roads and motorways” and replace with: “on Motorways and Type 1 National Roads” 2. Page 1/1, Paragraph 1.4: For: “v. Introduction of the ghost island diverge layout (“tiger tail”).” Read: “v. Not used.” 3. Page 1/1, Paragraph 1.4: For: “viii. Revised requirements for determining hourly flows for design.” Read: “viii. Not used.” 4. Page 1/1, Paragraph 1.4, at end of sub-paragraphs add: xiv. Defines the term dumb-bell . xv. Removes the option for a two taper diverge. xvi. New merging and diverging diagrams. xvii. Introduces the use of a Single Lane Parallel Diverge with associated details. xviii. New rules for crossfall on merges and diverges. xix. Revised guidance on the lighting policy of grade separated junctions. 5. Page 1/1 Paragraph 1.5, line 4: For : “trunk roads and motorways” Read: “Motorways and Type 1 Dual Carriageway National Roads” 6. Page 1/1 Paragraph 1.5, lines 17 and 18, Delete “Traffic Signs Regulations and General Directions (TSGRD), the Traffic Signs Manual, DMRB Volumes 8 and 9 and Local Transport Note 1/94.” and replace with: “Traffic Signs Manual and DMRB Volumes 8 and 9.” 7. Page 1/1: Delete Paragraph 1.6 and replace with:

“1.6 This Standard must be used forthwith for the design of all schemes for the construction and/or improvement of Motorways or Type 1 Dual Carriageway National Roads. This

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Standard should be applied to the design of schemes already being prepared unless, in the opinion of the National Roads Authority, application would result in significant additional expense or delay progress. In such cases, the Design Organisation should confirm the application of this Standard to particular schemes with the National Roads Authority.”

8. Page 1/2, after Paragraph 1.12, add paragraph; “1.12A Dumb-bell Link Road: A short link road connecting two either side of a dual carriageway or motorway at an .” 9. Page 1/2, Paragraph 1.14, line 5: Delete “At a diverge it is to separate the points of exit to a slip road.” 10. Page 1/2, Paragraph 1.14, line 7: For: “See Figures 2/4.4F, 2/6.1B Option 1 and 2/6.3D Option 1.” Read: “See Figure 2/4.4F. 11. Page 1/2, Paragraph 1.19 delete “Large Goods Vehicle (LGV”) and replace with: “Large Commercial Vehicle (LCV)” 12. Page 1/2, delete paragraph 1.26. 13. Page 1/3, delete paragraph 1.29 and replace with: “1.29 Roads: Urban and Rural: an Urban Road is a road which is in a built up area and has either a with a of 60 km/h or less, or has a dual carriageway (excluding motorways) with a speed limit of 80 km/h or less, or a motorway with a speed limit of 100 km/h or less. All other roads are Rural Roads.” 14. Page 1/3, paragraph 1.30, line 5: For: “give way” Read: “yield” 15. Page 1/3, delete paragraph 1.32. 16. Page 1/3, delete paragraph 1.34. 17. Page 1/3, delete paragraph 1.35. 18. Page 1/3, Paragraph 1.37, Line 4 delete “Overseeing Organisation” and replace with: “National Roads Authority”

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19. Page 1/3, Paragraph 1.38 first line, lines 8 and 11, delete “Overseeing Organisation” and replace with: “National Roads Authority” 20. Page 1/3, Paragraph 1.39 add at end: “Design aspects with Relaxations are considered to conform to standards.”

Chapter 2, Design Procedure 21. Page 2/1, Paragraph 2.8, line 4: For: “all-purpose” Read: “a Type 1 Dual Carriageway National Road” 22. Page 2/2, Figure 2/1: For: “All-Purpose or Motorway”. Read: “Motorway or Type 1 Dual Carriageway National Road” 23. Page 2/3, Paragraph 2.10, sub-paragraph (ii), delete “Traffic Signs Manual, Chapter 4” and replace with: “(Traffic Signs Manual)” 24. Page 2/3, Paragraph 2.14, Line 6: For: “Motorway or All-Purpose”. Read: “Motorway or Type 1 Dual Carriageway National Road” 25. Page 2/4, Paragraph 2.19, line 4: For: “TSGRD” Read “Traffic Signs Manual” 26. Page 2/4, Paragraph 2.23, Second line, delete “large goods vehicles” and replace with: “large commercial vehicles” 27. Page 2/5, Figure 2/2, sixth box down, delete “AP or MW (Tables 3/1a and 3/1b. TD27 (DMRB 6.1.2) and Paragraph 3.5” and replace with: “Type 1 Dual Carriageway National Road or Motorway (see Table 3/1 and NRA TD 27 (DMRB 6.1.2) Tables 2 to 5” 28. Page 2/5, Figure 2/2, Section of flow chart dealing with diverges: Delete box containing text: “Check widths of any ghost islands (see Paragraph 2.53)”

29. Page 2/6, Paragraph 2.30, delete first part of the paragraph: “Where, for reasons of route continuity, the mainline capacity provided is in excess of the design flows and a merging design Jan 2009 4

National Roads Authority Volume 6 Section 2 Design Manual for Roads and Bridges Part 1 TD 22/06 Addendum

flow of over one lane capacity is expected, then layout C of Figure 2/4.2 may be substituted for layout F of Figure 2/4.4, but normally, with such a large flow expected to merge, a lane would be added to the mainline. For layout C the meaning of „where design flows on the mainline are light‟ (see Figure 2/4.2) is that there is sufficient capacity on the mainline to accept the flow from the slip road.” 30. Page 2/6, Paragraph 2.30, at end of Paragraph 2.30 insert: “The Single Lane Taper Merge (Layout A) shall not be used.” 31. Page 2/6, Paragraph 2.31, line 2: For: “TSGRD diagram 1042.1” Read “the Traffic Signs Manual” 32. Page 2/6, Paragraph 2.32, line 3: For: “(TSRGD diagram 1042.1)” Read “(see Traffic Signs Manual)” 33. Page 2/6, delete paragraph 2.35. 34. Page 2/7, Figure 2/3 AP and Page 2/8, Figure 2/3 MW to be replaced with Figure 2/3 reproduced below:

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Figure 2/3: Merging Diagram

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35. Page 2/10, Figure 2/4.2 delete diagram C – Ghost Island Merge. 36. Page 2/14, delete paragraphs 2.37, 2.38 and 2.39 and associated title. 37. Page 2/14, delete paragraph 2.42. For: “Motorway Incident Detection and Automatic Signalling (MIDAS) should also be considered in such circumstances.” Read: “The use of Intelligent Transport Systems and automatic signalling should be considered in such circumstances.” 38. Page 2/14, Paragraph 2.44, last sentence delete “Overseeing Organisation” and replace with: “National Roads Authority” 39. Page 2/14, at end of Paragraph 2.46 add additional paragraph: “2.46A The minimum length of a diverge connector road from a motorway shall be Desirable Minimum Stopping Sight Distance (SSD) for the mainline from the tip of the diverge nose to the stop or yield line at the end of the connector road.” 40. Page 2/15, delete entire page and replace with: “Single Lane Parallel Diverge

2.48A The single lane parallel diverge is applicable to all Road Classes in Table 4/4 with the exception of urban roads with speed limits of 80km/h or less, for which layout A - Taper Diverge (as shown in Figure 2/6.1) shall be used in cases where Figure 2/5 indicates “Bs”.

2.48B The single lane parallel diverge is only applicable to diverging lane cross-sections of 1x4.0m or 2x3.0m , as shown in Table 4/4.

2.48C The layout of the single lane parallel diverge is shown in Figure 2/6.1A. This layout is a combination of layout A – Taper Diverge and layout B (Option 2) – Parallel Diverge, by incorporating a single auxiliary lane. Point A is the between the single lane parallel diverge and the taper diverge. Upstream of point A, the layout will be that of a layout B (Option 2) – Parallel Diverge, but incorporating a single auxiliary lane and downstream of point A it will be that of the taper diverge. The shaded area on the Figure illustrates the resulting single lane parallel diverge.

2.48D The method of construction for the single lane parallel diverge (as described above and shown in Figure 2/6.1A) will be the same for all applicable Road Classes.

General points to note are:

Point A for the 1x4.0m diverging lane is generally located close to the tip of the nose. Point A for the 2x3.0m diverging lane generally occurs 2/3 of the way along the auxiliary lane.

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The extent of the single lane parallel diverge carriageway will always encompass the full extent of a layout A – Taper Diverge.”

41. Page 2/16, Figure 2/5 AP and Page 2/17 Figure 2/5 MW, delete pages and replace with Figure 2/5 below:

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Figure 2/5: Diverging Diagram

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42. Page 2/18, Figure 2/6.1, diagram A – Taper Diverge: For: “1 or 2 lanes” Read: “1 lane” 43. Page 2/18, Figure 2/6.1, diagram B (Option 1 Preferred) - Ghost Island, to be deleted and replaced with Figure 2/6.1A: Bs - Single Lane Parallel Diverge as shown on the next page.

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44. Page 2/18, Figure 2/6.1, diagram B (Option 2 Not Preferred) - Parallel Diverge, delete “Not Preferred” 45. Page 2/18, Figure 2/6.1, Notes 1 and 2 referring to Ghost Islands to be deleted. 46. Page 2/20, Figure 2/6.3, diagram D (Option 1 Preferred) - Ghost Island diverge for Lane drop including for conversion of existing Lane Drop at Taper Diverge to be deleted. 47. Page 2/20, Figure 2/6.3, diagram D (Option 2 Not Preferred) - Lane Drop at Parallel Diverge, delete “Not Preferred”. 48. Page 2/20, Figure 2/6.3, Notes 1 and 2 referring to Ghost Islands to be deleted. 49. Page 2/22, delete Figure 2/7 50. Page 2/23, delete Figure 2/8 51. Page 2/24, Paragraph 2.57, delete second bullet point and replace with:

“• The minimum length of the diverge slip road shall be in accordance with Paragraph 2.46A.” 52. Page 2.24, Paragraph 2.57, delete fourth bullet point and replace with:

“• The advice given in NRA TA 70 shall be followed in respect of lighting.” 53. Page 2.24, Paragraph 2.58, line 2: For: “gateway be erected” Read: “ be provided - see NRA TA 70. Where it is not practical to provide a roundabout, a gateway shall be erected.”

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Chapter 3, Traffic Flows 54. Delete Chapter 3 in its entirety and replace with: 3. Traffic Flows Hourly Design Flow 3.1 Hourly Design Flows shall be calculated using an appraisal method agreed with the National Roads Authority. For roads of the Main Urban Type, and weaving area design shall be based on the 30th highest hourly flow. For Inter-Urban and Recreational road types, the 50th and 200th highest hourly flows respectively shall be used. The highest value of the total design flow, corrected as in Paragraphs 3.4 and 3.5 for LCVs and gradient, projected to the 15th year after opening, shall be taken as the basis of design for merges, diverges, and weaving sections. Mainline Traffic Capacity 3.2 For the purpose of designing junctions and interchanges, the maximum lane capacity for Type 1 Dual Carriageway National Roads should be taken as 1600 vehicles per hour (vph) and for Motorways as 1800 vph. These values have been used in Figures 2/3 and 2/5 in this Standard. If higher values have been used in the design of the mainline carriageways, then the equivalent number of lanes should be used, instead of design flows, for that part of Figures 2/3 and 2/5 that relate to the mainline. Design Flows and Connector Road Cross Sections 3.3 Connector road cross sections corresponding to design traffic flow ranges are given in Table 3/1. Further details of the cross sections are given in Paragraph 4.1.

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For Mainline Road Class Connector Road Cross Section for: Motorway Type 1 Dual Slip Road Interchange Carriageway Link/Loop National Merge Diverge Merge/Diverge Road Peak 0-900 0-800 Single Lane - 4.00m Single Lane - Corrected Carriageway 4.00m Design Flow Carriageway 900-1350 800-1200 Single Lane - Two Lane - on Connector 4.00m 6m Road: Carriageway Carriageway Vehicles per 1350-2700 1200-2400 Two Lane - Two Lane - hour 7.3m 7.3m 2700-3600 2400-3200 Two Lane - carriageway carriageway 7.3m carriageway Table 3/1 Connector Road Cross Sections for Design Flows Notes: 1. Cross-sectional details for Connector Road verges and hard strips are given in NRA TD27/07 Tables 2 to 5

Flow Corrections for Uphill Gradients and for LCVs 3.4 Corrections for uphill gradients and for the presence of LCVs as set out in Table 3/2 shall be made to the predicted hourly flows before corresponding values are read off from Table 3/1 and from Figures 2/3, 2/5 and 4/12. 3.5 To establish the mainline gradient a one kilometre section shall be used, 0.5 km either side of the merge or diverge nose tip, and the average gradient determined. The merge connector road gradient shall be based on the average of the 0.5 km before the nose tip.

%LCV Mainline Gradient Merge Connector Gradient <2% >2% <2% 2%-4% >4% 5 - +10 - +15 +30 10 - +15 - +20 +35 15 - +20 +5 +25 +40 20 +5 +25 +10 +30 +45 Table 3/2 Percentage Correction Factors for Gradients and the presence of Large Commercial Vehicles

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Chapter 4, Geometric Standards 55. Page 4/1, delete Paragraph 4.1, and replace with: “4.1 For the purpose of designing junctions and interchanges, Table 3/1 gives the required number of lanes and widths for the running carriageway for a range of design flows and road types. Details of other components such as hard shoulders, hard strips and verges that make up the cross section for slip roads and interchange links, are detailed in Chapter 3 of NRA TD 27. Values for the appropriate widths of such components can be found in Tables 2 to 5 of NRA TD27.”

56. Page 4/1, Delete Table 4/1 and replace with:

Rural Type 1 Rural Type 1 Urban Type 1 Urban Type 1 Rural Urban Dual Dual Dual Dual Mainline Design Speed Motorway Motorway Carriageway Carriageway Carriageway Carriageway 120km/h 100km/h 120km/h 100km/h 85km/h 70km/h Connector Interchange 85 70 85 85 70 60 Road Design Link Speed km/h Slip Road 70 60 70 70 60 60 Link Road 120 or 100A 100 or 85 120 or 100A 100A or 85 85 or 70 70 or 60 Dumb-bell 70 70 70 70 70 60 Link Road Table 4/1 Connector Road Design Speed

57. Page 4/2, at end of Paragraph 4.8, add paragraph: “Vertical Alignment for Merges and Diverges 4.8A Vertical design of merges and diverges (see Figure 4/0) shall provide: At the start of nose, a constant crossfall shall be maintained across the main carriageway, the tip of the nose and the slip road carriageway (points X between A-A on Figure 4/0). At the back of the nose either: i. a constant crossfall across the main carriageway, the back of the nose and the slip road carriageway (points X between B-B on Figure 4/0) or; ii. a separate crossfall for the mainline and the slip road with a single change in crossfall by a maximum of 3%, located on either edge of the nose or within the nose (points Y between B- B on Figure 4/0).

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From the back of the nose to the point where both slip road and mainline verge widths have been fully developed (shown as point V on Figure 4/0), the crossfall of the un-paved verge shall be a maximum of 5% (points X between C-C on Figure 4/0).”

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58. Page 4/3, insert new Figure 4/0 before Figure 4/1:

Figure 4/0: Vertical Alignment for Merges and Diverges

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59. Page 4/6, Paragraph 4.17, line 4: For: “… This will apply along the length of the connector road until the driver‟s eye is square with the back of the merge nose.” Read: “… This will apply until the driver reaches the Stopping Sight Distance from the back of the merge nose.” 60. Page 4/6, Paragraph 4.17, line 6: after “From that point forward, the Stopping Sight Distance must be that for the mainline design speed.” Insert: “As illustrated in Figure 4/3A.” 61. Page 4/6, delete Paragraph 4.18 and replace with:

“4.18 For diverges, the Desirable Minimum Stopping Sight Distance for the mainline design speed shall be maintained until the driver reaches the tip of the diverge nose. The stopping sight distance can then be reduced to the Desirable Minimum for one design speed step below the mainline design speed (but not below 70km/h). When the driver reaches the back of the diverge nose, the stopping sight distance can then be reduced to the Desirable Minimum for the design speed of the connector road as illustrated in Figure 4/3B. On lengths where two sight distances overlap, the requirements of the longer Stopping Sight Distance shall be met. If the length of the connector road between the back of the nose and the stop or yield line of the at-grade junction at the end of the connector road is less than the mainline Stopping Sight Distance, then a 0.26m object at the stop or yield line must be visible from a distance equal to the mainline Stopping Sight Distance. See Figure 4/3C.”

62. Page 4/6, delete Paragraphs 4.20 and 4.21 and replace with: “4.20 At connections to the local road system and where at- grade junctions form part of the grade separated junction, the sight distance requirements of TD 16, TD 50 and TD 42 shall be adopted for roundabouts, traffic signals and major/minor priority junctions respectively. Approaching drivers shall have unobstructed visibility of the at-grade junction from a distance corresponding to the relevant Stopping Sight Distance on the approach road. Hard Strip and Hard 4.21 Where the hard shoulder has to taper into a connector road hard strip or vice versa, this shall be done over the length of the diverge or merge taper.”

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63. Page 4/7: Delete Figure 4/3A and replace with new figure below: 64. After Page 4/7 insert Page 4/7A with Figure 4/3B reproduced below:

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65. Page 4/8: Delete Figure 4/3B and insert Figure 4/3C reproduced below:

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66. Page 4/9: Delete Table 4/3 and replace with new table below:

Road Class Length of Nose Nose Minimum Length of Length Reduction entry taper Ratio length auxiliary auxiliary of taper (see (m) (m) lane length lane taper Ghost (See Note 2) Note 1) (m) (m) Island (1) (7) (2) (3) (4) (5) Tail

4m 3.65m (m) lane lane (6) Min Preferred Absolute

Motorways Rural Motorway Mainline 220 205 1:40 115 230 75 180 n/a Within 150 130 1:25 75 160 55 150 n/a Interchange Urban Motorway Design Speed 105 95 1:15 50 125 40 n/a 1:40 1:30 100A Type 1 Dual Carriageway National Roads 120 km/h 160 150 1:30 85 190 55 150 n/a 100A km/h 140 130 1:25 75 160 55 150 n/a Urban 80 km/h 80 75 1:12 40 100 40 n/a see 1:40 1:30 speed limit Note 3 Urban 60 km/h 70 65 1:12 35 80 40 n/a see 1:40 1:30 speed limit or Note 3 less Note 1 Nose ratio is the ratio of nose back width to nose length for minimum angle at nose. The maximum angle will be limited by the ability of vehicles to negotiate the change in direction. Note 2 Table to be read in conjunction with Figures 2/4.1 to 2/4.5 Note 3 Ghost islands for merges on urban roads are not permitted (see paragraph 4.22). See paragraph 2.28 for the use of Figure 2/4.2, Layout D. For reduction taper, (7) on Figure 2/4.2D when the taper is less than the preferred minimum it is a Departure from Standards. Table 4/3 Geometric Design Parameters for Merging Lanes

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67. Page 4/10: Delete Table 4/4 and replace with new table below: Road Class Nose Nose Minimum Length of Length of exit taper (m) Ratio (See length (m) auxiliary aux lane 4.00m 2x3.65m 2x3.00m Note 1) lane taper (m) lane lane lane length (m) (1) (2) (2) (3) (4) (5) (6) Motorways Rural Motorway Mainline 180 185 150 1:15 80 200 75 Within 130 130 110 1:15 70 150 55 Interchange Urban Motorway Design 130 130 110 1:15 70 150 55 Speed 100A Type 1 Dual Carriageway National Roads 120 km/h 150 150 120 1:15 70 170 55 100A km/h 130 130 110 1:15 70 150 55 Urban 80 80 90 75 1:12 40 100 40 km/h speed limit Urban 60 70 80 65 1:12 35 80 40 km/h speed limit or less Note 1 Nose ratio is the ratio of nose back width to nose length for minimum angle at nose. The maximum angle will be limited by the ability of vehicles to negotiate the change in direction. Note 2 Table to be read in conjunction with Figures 2/6.1 to 2/6.4 Table 4/4 Geometric Design Parameters for Diverging Lanes

68. Page 4/10, Delete paragraph 4.24 69. Page 4/11, Figure 4/4, merge diagram: For: “Lane width 3.7m” Read: “Lane width 3.65m” 70. Page 4/12, Paragraph 4.26, line 3: For: “to TSRGD diagram 872.1 (reversed mirror image) with a diagram 876 distance plate „200yds‟,” Read: “showing the number of lanes ahead, also that traffic in the slip road must merge into the mainline nearside lane with a distance plate „200yds‟ ”

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71. Page 4/12, Paragraph 4.26, line 7: Delete: “diagram 872.1 ” 72. Page 4/12, Paragraph 4.26, line 8: At end of sentence add: “see Traffic Signs Manual.” 73. Page 4/12: Paragraph 4.28, first indented paragraph, last line delete “Overseeing Organisation” and replace with: “National Roads Authority” 74. Page 4/12: Paragraph 4.28: Delete last sub-paragraph, text reads “At the time of publishing ………..in the meantime.” 75. Page 4/12: Paragraph 4.29, first sentence: For: “Parallel merges and diverges (Figure 2/4.1 (Layout B) and Figure 2/6.1 (Layout B Option 2))” Read: “Parallel merges and diverges 4m wide, similar to Figure 2/4.1 (Layout B) and 2/6.1 (Layout B)” 76. Page 4/13, Paragraph 4.31, line 10: For: “to TSGRD diagram 1040.3” Read “in the Traffic Signs Manual” 77. Page 4/19, delete Paragraph 4.35 and replace with:

“For Rural Motorways and Type 1 Dual Carriageway National Roads, the Desirable Minimum weaving length shall be 2 kilometres. However, when the design flow on the main line is not greater than 50% of the capacity (AADT) for level of service D as given in NRA TD 9 for rural motorways, an Absolute Minimum weaving length of 1 kilometre can be considered as a Relaxation. The weaving formula must not be used for weaving lengths above 3 kilometres. The requirements for MSAs on rural motorways are as for rural motorway junctions.”

78. Page 4/19, delete paragraph 4.36. 79. Page 4/19, Paragraph 4.38, first sentence: For: “All-purpose Roads” Read: “Type 1 Dual Carriageway National Roads” 80. Page 4/21, Figure 4/13: Delete Note 3. “For diverges, the mirror image shall apply.”

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81. Page 4/22, Figure 4/14: For: “D is the hourly flow from para 3.3” Read: ”D is the lane capacity in vehicles/hour/lane from Paragraph 3.2”

Chapter 5, Layout Options 82. Page 5/1, Paragraph 5.2, line 5: For: “all-purpose road” Read “Type 1 Dual Carriageway National Road” 83. Page 5/1, Paragraph 5.5, line 2: For: “the current Requirements for Road Restraint Systems.” Read “NRA TD19 “Safety Barriers” (DMRB 2.2.8A)” 84. Page 5/4, After paragraph 5.7, add: “5.7A Due to the identified difficulties associated with diamond junctions with un-signalised crossroads or staggered cross-roads (as shown in Figure 5/1.1 a), the use of such layouts shall be regarded as a Departure from Standard.” 85. Page 5/5, Paragraph 5.11, line 5: For: “give way” Read: “yield” 86. Page 5/13, Paragraph 5.28, line 8: Delete from “Advice on the provision of emergency telephones…….” to the end of the paragraph. 87. Page 5/15, Paragraph 5.35, first sentence: For: “It is normal practice” Read: “It is the requirement on all rural motorways and dual carriageways” 88. Page 5/15, delete Paragraph 5.37, and replace with: “5.37 For grade separated junctions three advance direction signs must be provided. These are to be located 1km and 0.5km in advance of the start of the diverging lane, followed by an exit taper gantry sign at the start of the diverging lane. A confirmatory „Exit‟ sign located at the back of the nose must be provided.” 89. Page 5/15, Paragraph 5.38, first sentence: For: “(TSRGD diagram 823, 824 and 825)” Read: “(see Traffic Signs Manual)” 90. Page 5/15, Paragraph 5.39, line 2:

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Delete: “the TSRGD,” 91. Page 5/15, Paragraph 5.39, last sentence: Delete: “and Local Transport Note 1/94”

Chapter 6, Facilities for Non-motorised Users 92. Page 6/1, Paragraph 6.3: For: “will be identified by the NMU Audits see HD 42 (DMRB 5.2.5).” Read: “will be identified by the Design Organisation and considered at the various stages of Road Safety Audit.” 93. Page 6/1, delete paragraphs 6.9 and 6.10 and replace with: “6.9 The design of facilities for cyclists should take into account their vulnerability as road users. The provision of facilities for cyclists should therefore be considered during the planning stages of new or improved National Road schemes, further guidance is given in TA 57. 6.10 Cyclists can face serious problems at slip roads as they are in conflict with relatively fast moving traffic. There have been a number of fatalities and serious injuries at such sites. Ensuring that the cyclists cross the road at right angles, at a point where there is good visibility, and then rejoining the main carriageway on the nearside past the junction can help reduce the risk of accidents. 6.11 When the usage is over 20 cycles/day, the design of a route for cyclists should be as follows: a) For diverges – it is assumed that the cyclists will be using the hard shoulder and will be encouraged to use the diverge by means of road markings. At a point 5m from the intersection of the back of the verges (Point V on Figure 4/0) and at least 20m from the back of the nose, a crossing point is constructed. A cycle track will depart from the slip road then turn through 90° to meet the nearside edge of the slip road at the crossing point. This track will be dropped or raised to the level of the slip road over the width of the cycle track (2m minimum). At this point the cyclists are directed to a „stop line‟ located at the back of the hard strip before crossing the slip road at right angles. Once the slip road has been crossed the cyclists will yield before joining the hard shoulder along the mainline carriageway (see Figure 6/1 and 6/3). b) For merges – it is assumed cyclists will be using the hard shoulder. A cycle track is constructed to take cyclists off the hard shoulder. At a point 5m from the intersection of the back of the verges (Point V on Figure 4/0) and at least 20m

Jan 2009 27

National Roads Authority Volume 6 Section 2 Design Manual for Roads and Bridges Part 1 TD 22/06 Addendum

from the back of the nose this track turns through 90° to meet the edge of the slip road and it will be dropped or raised to carriageway level over the width of the cycle track (2m minimum). At this point the cyclists are directed to a „stop line‟ before crossing the slip road at right angles, they will then join the hard strip along the edge of the slip road carriageway and use the mainline hard strip/hard shoulder whilst merging with the mainline carriageway (see Figure 6/1 and 6/2). Signing 6.12 To encourage cyclists to use the cycle route to rejoin the mainline at merges and diverges appropriate road markings and signs should be used (see the Traffic Signs Manual). 6.13 For merges a cycle symbol marking should be located at the beginning of the cycle track and close to the crossing to inform the cyclists that there is a crossing. The cycle symbol markings should be accompanied with the to let the cyclist know that they are continuing on the right track. Opposite the merge crossing, a cycle route sign with a directional arrow is located within the verge together with a cycle symbol and arrow painted on the surface of the hard strip (see Figure 6/2). 6.14 For diverges a cycle symbol marking with route number and arrow should be painted on the hard shoulder both 10m before and 20m after the tip of the mainline diverge taper directing cyclists to use the hard shoulder/hard strip of the slip road. Prior to the cycle track diverge the same markings shall apply to direct the cyclist to use the cycle crossing facility (see Figure 6/3). 6.15 The suggested road markings layout is shown in Figures 6/1 to 6/3.”

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National Roads Authority Volume 6 Section 2 Design Manual for Roads and Bridges Part 1 TD 22/06 Addendum

Figure 6/1 to 6/3: Provision for Cyclists at Grade Separated Junctions

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Chapter 7, References 94. Page 7/1, Chapter 7 delete text and replace with the following: 7. REFERENCES BS6100 Subsection 2.4.1 1990, Glossary of Building and Terms, Part 2 Civil Engineering, Section 2.4 Highway and Railway Engineering, Subsection 2.4.1 . British Standards Institution, 1990. Design Manual for Roads and Bridges: Volume 6, Road Geometry: NRA TD 9 – Road Link Design (NRA DMRB 6.1.1); NRA TD 27 – Cross Sections and Headroom (NRA DMRB 6.1.2); TD 16 – Geometric Design of Roundabouts (DMRB 6.2.3); TD 50 – The Geometric Layout of Signal-Controlled Junctions and Signalised Roundabouts (DMRB 6.2.3); TD 39 – The Design of Major Interchanges (DMRB 6.2.4); TD 40 – Layout of Compact Grade Separated Junctions (DMRB 6.2.5); TD 42 – Geometric Design of Major/Minor Priority Junctions (DMRB 6.2.6). Traffic Signs Manual. Department of the Environment and Local Government. DfT Traffic Advisory Leaflet: 1/88 Provision for Cyclists at Grade Separated Junctions.

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Chapter 8, Enquiries 95. Page 8/1, delete text and replace with the following:

“8. ENQUIRIES

All technical enquiries or comments on this Standard should be sent in writing to:

Head of Project Management and Engineering National Roads Authority St Martin‟s House Waterloo Road Dublin 4

......

E O‟CONNOR Head of Engineering Operations”

Jan 2009 31

DESIGN MANUAL FOR ROADS AND BRIDGES

VOLUME 6 ROAD GEOMETRY SECTION 2 JUNCTIONS

PART 1

TD 22/06

LAYOUT OF GRADE SEPARATED JUNCTIONS

SUMMARY

This standard sets out the design requirements and methodology for the geometric design and layout of grade separated junctions on trunk roads and motorways. It revises and combines the previous standard (TD 22/92) and advice note (TA 48/92). It takes into account the amendments included in the interim revision (TD 22/05).

INSTRUCTIONS FOR USE

1. Remove Contents pages from Volume 6 and insert new Contents pages for Volume 6 dated February 2006.

2. Remove TD 22/05 from Volume 6, Section 2 which is superseded by this Standard and archive as appropriate.

3. Insert TD 22/06 into Volume 6, Section 2.

4. Please archive this sheet as appropriate.

Note: A quarterly index with a full set of Volume Contents Pages is available separately from The Stationery Office Ltd.

February 2006 DESIGN MANUAL FOR ROADS AND BRIDGES TD 22/06

THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE

WELSH ASSEMBLY GOVERNMENT LLYWODRAETH CYNULLIAD CYMRU

THE DEPARTMENT FOR REGIONAL DEVELOPMENT NORTHERN IRELAND

Layout of Grade Separated Junctions

Summary: This standard sets out the design requirements and methodology for the geometric design and layout of grade separated junctions on trunk roads and motorways. It revises and combines the previous standard (TD 22/92) and advice note (TA 48/92). It takes into account the amendments included in the interim revision (TD 22/05). Volume 6 Section 2 Part 1 TD 22/06 Registration of Amendments

REGISTRATION OF AMENDMENTS

Amend Page No Signature & Date of Amend Page No Signature & Date of No incorporation of No incorporation of amendments amendments

February 2006 Volume 6 Section 2 Registration of Amendments Part 1 TD 22/06

REGISTRATION OF AMENDMENTS

Amend Page No Signature & Date of Amend Page No Signature & Date of No incorporation of No incorporation of amendments amendments

February 2006 DESIGN MANUAL FOR ROADS AND BRIDGES

VOLUME 6 ROAD GEOMETRY SECTION 2 JUNCTIONS

PART 1

TD 22/06

LAYOUT OF GRADE SEPARATED JUNCTIONS

Contents

Chapter

1. Introduction

2. Design Procedure

3. Traffic Flows

4. Geometric Standards

5. Layout Options

6. Facilities for Non-Motorised Users

7. References

8. Enquiries

February 2006 Volume 6 Section 2 Chapter 1 Part 1 TD 22/06 Introduction

1. INTRODUCTION

General viii. Revised requirements for determining hourly traffic flows for design. 1.1 This standard sets out the layout and size requirements for new and improved grade separated ix. Use of absolute maximum gradient on motorway junctions and interchanges on rural and urban trunk connector roads of 6% is permitted. roads and motorways. It sets out requirements for the provision of weaving sections for traffic between x. Introduction of three types of loops. junctions. It gives guidance on access to and egress xi. Clarification of the requirements for sight from service areas. distance for diverge and merge slip roads. 1.2 This standard does not cover the design xii. Auxiliary lanes on sections to be requirements and methodology for the geometric layout extended beyond crest to enable visibility of the of either major interchanges (including the expansion end of the lane. and improvement of existing interchanges and junctions) or compact grade separated junctions xiii. More guidance on facilities for non-motorised (principally for use on rural and inter-urban roads). users. These are set out in TD 39 (DMRB 6.2.4) and TD 40 (DMRB 6.2.5). Scope 1.3 Guidance on the structured process of choosing between junction types is given in advice note 1.5 This standard sets out the design requirements TA 30 (DMRB 5.1). and methodology for the geometric design of and choice between different grade separated junction 1.4 The main changes from the previous standard layouts on trunk roads and motorways. It draws on the and advice note are summarised as follows: experience gained since the publication of the previous standard (TD 22/92) and advice note (TA 48/92) and i This standard combines and supersedes the revises and combines the two documents. It takes into previous standard TD 22/92 and the advice note account the amendments included in the interim TA 48/92. It includes the revisions included in revision (TD 22/05). It provides guidance on the the interim revision TD 22/05 (DMRB 6.2.1). principles for safety and traffic operation and on the choice between different grade separated junction ii. Junctions onto connector roads are prohibited. layouts. Recommendations are given on the siting of grade separated junctions in urban and rural areas, iii. New design requirements for diverge and merge geometric design and the provision for non-motorised slip roads to ensure adequate length for users. Some aspects of signs and road marking are deceleration and acceleration. included for completeness, although policy and detailed guidance on these matters are given in the Traffic Signs iv. Existing two lane merges which are subject to Regulations and General Directions (TSRGD), the improvement must be altered to a one lane Traffic Signs Manual, DMRB Volumes 8 and 9 and parallel merge or two lane ghost island merge as Local Transport Note 1/94. appropriate. v. Introduction of the ghost island diverge layout Implementation (“tiger tail”). 1.6 This standard must be used forthwith for the vi. Principle of averaging for weaving lengths at design of all schemes for the construction and ghost island layouts. improvement of all-purpose and motorway trunk roads currently being prepared, provided that in the vii. Comprehensive requirements for merge and opinion of the Overseeing Organisation, this would diverge layouts for Motorway Service Areas. not result in any significant additional expense or

February 2006 1/1 Chapter 1 Volume 6 Section 2 Introduction Part 1 TD 22/06

1.17 Lane Gain: A layout where a merging connector delay. The Design Organisation must confirm its road becomes a lane or lanes of the downstream main application to particular schemes with the carriageway. See Figures 2/4.3E, 2/4.4F and 2/4.5G. Overseeing Organisation. 1.18 Lane Drop: A layout where a lane or lanes of the Definitions upstream carriageway becomes the diverging connector road. See Figures 2/6.2C, 2/6.3D and 2/6.4E. 1.7 The terminology used in this standard follows, 1.19 Large Goods Vehicle (LGV): A goods vehicle, where possible, the definitions contained in BS 6100: the permissible maximum weight of which exceeds 7.5 Subsection 2.4.1: 1992. tonnes. 1.8 The following additional terms have been 1.20 Link Road: In the context of junctions, a one defined for use in this Standard (see also Figure 1/1). way connector road adjacent to but separate from the 1.9 Auxiliary Lane: An additional lane at the side of mainline carriageway carrying traffic in the same the mainline carriageway to provide increased merge or direction, which is used to connect the mainline diverge opportunity or additional space for weaving carriageway to the local highway network where traffic. See Figure 2/4.1B and Figure 2/6.3D Option 2. successive direct connections cannot be provided to an adequate standard because the junction spacing is too 1.10 Connector Road: A collective term for close. See Figure 5/6. interchange links, link roads, slip roads and loops. 1.21 Loop: A connector road, one or two way, which 1.11 Design Organisation: The organisation is made up of the elements of the loops shown in commissioned to undertake the various phases of Figure 4/1 and which passes through an angle in the scheme preparation. range of approximately 180 to 270 degrees. The loop is considered to extend to the end of the near straight 1.12 Downstream: That part of the carriageway(s) length of road contiguous with the back of the diverge where the traffic is flowing away from the section in or merge nose. question. 1.22 Low Radius: A radius between the minimum 1.13 Fork: An at-grade junction of two roads, usually loop radius in Table 4/2 and the Two Steps below within an interchange, which diverge from the approach Desirable Minimum Radius with Superelevation of 7% road at similar angles. Usually both diverging roads as required by TD 9 (DMRB 6.1.1) for the slip road or have equal status. (For a fork junction, as defined in interchange link design speed. BS 6100: Subsection 2.4.1, the minor road deviates from the straight major road.) See Figure 4/6. 1.23 Mainline: The carriageway carrying the main flow of traffic; generally traffic passing straight through 1.14 Ghost Island: An area of the carriageway the junction or interchange. suitably marked to separate lanes of traffic travelling in the same direction on both merge and diverge layouts. 1.24 Near Straight: A length of road with a radius no The purpose of the ghost island at a merge is to separate less than the Desirable Minimum Radius with the points of entry of two slip road traffic lanes. At a Superelevation of 5% as required by TD 9 (DMRB diverge it is to separate the points of exit to a slip road. 6.1.1) for the mainline design speed. See Figures 2/4.4F, 2/6.1B Option 1 and 2/6.3D Option 1. 1.25 Nose: A paved area, approximately triangular in shape, between a connector road and the mainline at a 1.15 Interchange: A grade separated junction that merge or diverge, suitably marked to discourage drivers provides free flow from one mainline to another. from crossing it.

1.16 Interchange Link: A connector road, one or two 1.26 Overseeing Organisation: The highway or road way, carrying free flowing traffic within an interchange authority for the road construction or improvement between one level and/or direction and another. See scheme. Paragraphs 4.2 and 4.3.

1/2 February 2006 Volume 6 Section 2 Chapter 1 Part 1 TD 22/06 Introduction

1.27 Parallel Merge/Diverge: A layout where an Mandatory Sections auxiliary lane is provided alongside the mainline carriageway. See Figures 2/4.1B, 2/4.5H, 2/6.1B 1.37 Mandatory sections of this document are Option 2 and 2/6.3D Option 2. contained in boxes. The Design Organisation must comply with these sections or obtain agreement to 1.28 Reserved Lane: A lane carrying traffic that is a Departure from Standard from the Overseeing segregated from weaving traffic. Organisation. The remainder of the document 1.29 Rural Road: As defined in TA 46 (DMRB contains advice and explanation, which is 5.1.3), namely all-purpose roads and motorways that are commended to users for consideration. generally not subject to a local speed limit. Departures from Standards 1.30 Slip Road: A connector road within a junction between a mainline carriageway and the local highway 1.38 In exceptional situations, the Overseeing network, or vice versa, which meets the local highway Organisation may be prepared to agree to a network at-grade. Traffic using a slip road usually has Departure from Standard where the standard, to give way to traffic already on the mainline or on the local highway network. See Paragraph 4.2. including permitted Relaxations, is not realistically achievable. Design Organisations faced by such 1.31 Taper Merge/Diverge: A layout where merging situations and wishing to consider pursuing this or diverging traffic joins or leaves the mainline course must discuss any such option at an early carriageway through an area forming a funnel to or flare stage in design with the Overseeing Organisation. from the mainline carriageway. See Figures 2/4.1A and Proposals to adopt Departures from Standard must 2/6.1A. be submitted by the Design Organisation to the Overseeing Organisation and formal approval 1.32 “Tiger Tail”: A ghost island layout at a diverge received before incorporation into a design layout. utilising TSRGD diagram 1042.1 to separate the points of exit to a slip road. So called because the vertical sign Relaxations used to inform drivers of the layout incorporates an illustration that resembles a tiger’s tail. See Figures 2/6.1B Option 1 and 2/6.3D Option 1. 1.39 In difficult circumstances Relaxations may be introduced at the discretion of the Design 1.33 Upstream: That part of the carriageway(s) where Organisation, having regard to all relevant local traffic is flowing towards the section in question. factors, but only where specifically permitted by this standard. Careful consideration must be given 1.34 Urban All-Purpose Road (UAP): An all- to layout options incorporating Relaxations, having purpose road within a built up area, either a single weighed the benefits and any potential disbenefits. carriageway with a speed limit of 40 mph or less or a Particular attention must be given to the safety dual carriageway with a speed limit of 60 mph or less. aspects (including operation, maintenance, 1.35 Urban Motorway: A motorway with a speed construction and demolition) and the limit of 60 mph or less within a built up area. environmental and monetary benefits/disbenefits that would result from the use of Relaxations. The 1.36 Weaving Section: The length of the carriageway consideration process must be recorded. The between a successive merge or lane gain and diverge or preferred option must be compared against options lane drop, where vehicles leaving the mainline at the that would meet full standards. diverge or lane drop have to cross the paths of vehicles that have joined the mainline at the merge or lane gain. See Figure 2/9 and Figures 4/9 to 4/14.

February 2006 1/3 Chapter 1 Volume 6 Section 2 Introduction Part 1 TD 22/06

Taper Merge/Diverge and Parallel Merge/Diverge

Design details are given in Chapter 2 of this document.

Figure 1/1 Definition of Main Terms

1/4 February 2006 Volume 6 Section 2 Chapter 2 Part 1 TD 22/06 Design Procedure

2. DESIGN PROCEDURE

General Principles 2.5 It is important to ensure that adequate forward visibility is provided in accordance with TD 9 (DMRB 2.1 Junction and Interchange design is an iterative 6.1.1). The possible adverse effects on forward process which is a key part of the overall design process visibility of features such as mature vegetation, lighting for schemes. Figure 2/1 is a flowchart for junction and columns, signs and vehicle restraint systems should be interchange design. Figure 2/2 outlines the connector considered at an early stage in design. Drivers will road design process. more readily understand the use of standard features than unusual features and if it is necessary to use 2.2 The design of junctions is affected by decisions unusual features these should be well signed or be taken on the degree of access to be provided on the readily understood. scheme. It is important to consider from the outset how much access should be allowed. It may not be possible 2.6 Earthworks and landscaping should be an integral to cater for the full predicted demand. The fact that part of junction design rather than an afterthought. other roads are crossed, does not imply that a junction should be provided, or that if one is provided, it should 2.7 Design Organisations should consider the be omni-directional. potential for dazzle and silhouetting of signs when the sun is low in the sky. The designer should also attempt 2.3 There may be occasions when the design should to avoid the need for drivers to approach a manoeuvre not provide for certain movements to prevent use of the or a decision point looking into the rising or setting sun. by local commuters for the benefit of longer distance traffic and for environmental reasons. The Urban/Rural process of choosing between options is covered more fully in TA 30 (DMRB 5.1). 2.8 The design of grade separated junctions is based on the design hourly flow which usually varies 2.4 A junction layout should give drivers and other according to road type and according to whether the road users a clear understanding of what is required of road is motorway or all-purpose or rural or urban. them. Poor layouts lead to driver confusion, Urban standards for most elements of road design are, indecisiveness and rash decisions that could contribute however, lower than those applicable to rural design, to accidents. since lower driver expectation accompanied by higher perception offset the increased risks caused by The design should provide: reductions in standards. For example, the presence of kerbs, frequent lack of hardstrips, narrow central • advance notification of the layout on the reserve, lighting and speed limits would all indicate the approach to a junction; urban nature of a road. The lower urban standards are • conspicuous junction locations and layouts; shown within the hierarchy of geometric standards, ranging from rural motorways down to urban all- • understanding of permitted changes to the purpose roads, related to Design Speed (see Table 4/3 direction of travel; and Table 4/4).

• understanding of other traffic movements; Location

• avoidance of potential hazards. 2.9 The location of a grade separated junction can have a significant effect on both its operational Thus, in assembling the design components, designers performance and environmental impact. Therefore, should ensure that as drivers approach a junction they consideration of the major contributing issues should be are able to easily perceive the junction form and layout undertaken at the initial design stage to produce the so that they can select their path through the junction optimum design for comparison with other junction accordingly. Ease of use should be checked for night- types. Some major contributing issues are listed in time conditions. paragraph 5.4.

February 2006 2/1 Chapter 2 Volume 6 Section 2 Design Procedure Part 1 TD 22/06

Determine Strategic Network and Design Year. ➤

Decide whether urban or rural standards apply.

➤ Decide initial strategy for network and junction. ➤ ➤

Derive hourly traffic flows to be used for design, correcting for LGVs and gradients. ➤

Confirm whether All-Purpose or Motorway using network strategy and TA46 or TA79 initial standards. ➤ No Are lane requirements for mainline and connector roads achievable? ➤

➤ Yes No Are suitable merge/diverge and weaving layouts for the design flows achievable? ➤

Yes ➤

Is signing/motorway signalling possible? No Are lane drop/gains satisfactory? ➤ Is junction spacing satisfactory?

Yes ➤

Scheme preparation continues.

Figure 2/1 Flow Chart Showing the Junction/Interchange Design Process

2/2 February 2006 Volume 6 Section 2 Chapter 2 Part 1 TD 22/06 Design Procedure

Safety 2.12 The design of an at-grade junction within a grade separated junction is subject to the appropriate 2.10 The main objective of grade separated junction Standards and Advice Notes as follows: design is to provide a junction which is safe for the forecast traffic flows. Certain layouts are not TA 23 (DMRB 6.2) – (Advice Note – Junctions and recommended for safety reasons and should be avoided. Accesses: Determination of the Size of Roundabouts These are: and Major/Minor Junctions);

(i) grade separated junctions on single carriageways TD 16 (DMRB 6.2.3) – (Standard – Geometric Design (see TD 9 (DMRB 6.1.1) and TD 40 (DMRB of Roundabouts); 6.2.5)); TD 42 (DMRB 6.2.6) – (Standard – Geometric Design (ii) grade separation on dual carriageways within of Major/Minor Priority Junctions); 0.5 km of a changeover to single carriageway standard, measured from the end of the merge TD 50 (DMRB 6.2.3) – (Standard – Geometric Layout taper to the beginning of the right hand lane of Signal-Controlled Junctions and Signalised Roundabouts); hatching that removes the offside lane or lanes (see The Traffic Signs Manual, Chapter 4 and TD 51 (DMRB 6.3.5) – (Standard – Segregated Left TD 42 (DMRB 6.2.6)); Turn Lanes and Subsidiary Deflection Islands at (iii) offside merges and diverges; Roundabouts). A more detailed discussion of the layout options is (iv) major/minor junctions, particularly those with contained in Chapter 5. right turning movements, on an otherwise grade separated route. The Design Process Recommended Layouts 2.13 Following through the flow-chart, (Figure 2/1) 2.11 Recommended layouts for consideration in order the first stages would be to determine a network strategy, fix a design year, and decide whether urban or of increasing traffic flow level are: rural standards apply (see paragraphs 1.29, 1.34 and i) diamond or half clover-leaf – simple priority 1.35). The next stage would then be to decide on an junctions with the minor road; initial network and junction strategy, including the connections to be made, for example whether the ii) dumb-bell roundabout – junctions with the minor junction should be omni-directional. road are provided by two normal roundabouts which are connected by a central link road either 2.14 Having made those starting decisions, it is under or over the mainline; possible to derive hourly flows to be used in the design process following the guidance in Chapter 3. An iii) two roundabout – a single large examination of these flows, applied to the network roundabout with the circulatory carriageway strategy adopted, will enable a decision to be taken (or either under or over the mainline; confirmed) that the route should be Motorway or All- Purpose. Reference to TA 46 and TA 79 (DMRB 5.1.3), iv) 3 level roundabout – a junction usually between will give a starting point on the level of carriageway two roads of similar flow. The two mainlines are provision for the links on the network assumed. on the upper and lower levels of the junction with the roundabout on the central level; 2.15 The next stage, and the first step that could lead to iteration, is to assess the likely lane provision on the v) interchange – a junction between major roads mainline and the connector roads. If the basic scheme with all movements catered for by free flowing cannot be tailored to cope with demands, including connector roads. those likely to arise when maintenance work needs to With the exception of the Interchange these junctions be undertaken, then network and junction strategy will have merge and diverge slip roads which may be need to be reviewed and alternatives investigated; for signalised at their junction with the or example – reducing the number of junction accesses or roundabout. using link roads. Link roads reduce the frequency of

February 2006 2/3 Chapter 2 Volume 6 Section 2 Design Procedure Part 1 TD 22/06 direct access points along the mainline in order to at other decision type locations or in situations where eliminate sub-standard weaving lengths thus promoting the driver’s view may be obstructed by high traffic free flow to minimise the potential for accidents and to volumes or large proportions of LGVs. At these preserve the high capacity of the mainline. They can locations consideration should be given to the provision also be used where it is unsafe or not possible to make of gantries to mount the signs. Where these are direct connections. Link roads can be useful for proposed the design of the junction or interchange maintenance and diversions. should take the siting of the gantries into account, (see TD 18 (DMRB 9.1)). 2.16 The following stage may also lead to iteration. This is to determine the merge and diverge facilities and 2.22 It may also be that the predicted turning flows are to check that weaving sections at or above the desirable not realised in the proportions expected in the design minimum length can be provided. If these cannot be year and the consequences of being wrong should be achieved, then the junction strategy should be reviewed. examined. Sensitivity testing of differing flow proportions should be undertaken. 2.17 The next stage is to check that desirable geometric standards can be achieved with the junction 2.23 Correction factors to take account of gradients spacing, and any lane gains or drops proposed, and that and proportion of large goods vehicles, as detailed in an effective and economic signing system can be Tables 3/2 and 3/3, may need to be made to the flows to provided. Again the strategy may have to be adjusted. be entered in Tables 3/1a and 3/1b, and Figures 2/3, 2/5 Figure 2/2 is a flowchart showing the connector road and 4/14. design process. It refers to the particular paragraphs, figures and tables of this standard applicable to Merges – General Principles connector road design and to TD 27 (DMRB 6.1.2).

2.18 If the junction and interchange designs pass these 2.24 It is important on safety grounds and to limit stages, the scheme can then be taken to the next stage in interference to mainline traffic that joining traffic is its preparation which is likely to be a cost/benefit channelled into the merging area (i.e. from the tip of the assessment. Analysis may not be sufficiently fine to nose to the end of the taper(s)) and arrives there in an evaluate the performance of individual junction orderly fashion to perform a safe and comfortable elements. The best means of ensuring that a junction is merge with the mainline. effective is to carry out the operational check outlined 2.25 If joining flows are greater than one lane capacity above and in Figure 2/1. then an additional lane should normally be added to the mainline as a lane gain. The individual merging area for Junction and Interchange Design each joining lane within a merge should be separated from the previous one and there should be space General Principles between them for mainline traffic to adjust to the new traffic flow.

2.19 Where lane drops and lane gains occur, the lane 2.26 Where design flows are close to capacity on both configurations ahead should be made clear to drivers by the connector road and on the mainline it is important to the consistent use of signs and road markings as ensure that there is adequate provision for those outlined in TSRGD and TA 58 (DMRB 8.2.1). These merging. If the availability of merging opportunities is principles have been incorporated in the recommended estimated to be low for long periods of the day, layouts. improved merging opportunities could be provided by auxiliary lanes. 2.20 Where large traffic flows are joining the mainline in an interchange or junction, turbulence can occur, 2.27 There may be occasions when the merge flow is with short headways and sudden braking. A length of greater than the mainline flow. The junction should auxiliary lane may be necessary to provide increased nevertheless be set out so that mainline traffic has local capacity. This is covered more fully in paragraphs priority over traffic entering from the left, except at a 4.23 to 4.26. lane gain. 2.21 The signing of junctions and interchanges should give clear and timely information to drivers. This is particularly important at lane gains and lane drops and

2/4 February 2006 Volume 6 Section 2 Chapter 2 Part 1 TD 22/06 Design Procedure

Determine junction location ➤

Determine junction option ➤

Check proposed location for driver perception (Paragraph 2.4) ➤

Check protection of scheme elements and allow adequate space within scheme to preserve sightlines (Paragraph 2.5) ➤

Determine flows (Chapter 3) having used adjustment factors for LGVs and gradient. ➤

Determine cross sections for AP or MW (Tables 3/1a and 3/1b, TD 27 (DMRB 6.1.2)) and paragraph 3.5 ➤ ➤

Merge Diverge ➤ ➤

Enter merge flow in Figure 2/3 on the Enter diverge flow in Figure 2/5 on the vertical axis and the upstream mainline flow vertical axis and the downstream mainline on the horizontal axis and read off the flow on the horizontal axis and read off the appropriate layout at the intersection point appropriate layout at the intersection point (see Figure 2/4) (see Figure 2/6) ➤ ➤

Determine if there is a need for auxiliary lanes Determine if there is a need for auxiliary lanes ➤ ➤

Check widths of any ghost islands Check widths of any ghost islands (see Paragraph 2.32) (see Paragraph 2.53) ➤ ➤

Determine the lengths of slip roads Determine the lengths of slip roads (see Paragraph 2.34) (see Paragraph 2.46) ➤ ➤

Determine connector road design speed Determine connector road design speed ➤ ➤ Determine design parameters of the elements Determine design parameters of the elements of the design (see Table 4/3) of the design (see Tables 4/4, 4/5)

Figure 2/2 Flow Chart Showing the Connector Road Design Process

February 2006 2/5 Chapter 2 Volume 6 Section 2 Design Procedure Part 1 TD 22/06

2.28 There are many sites throughout the network 2.32 The minimum width of a ghost island is that have a two-lane taper merge layout; such 2.0m at its widest point and the minimum width of layouts are not now recommended. When junctions a chevron is 0.5m (TSRGD diagram 1042.1). If the that contain these features are to be improved, the ghost island marking is less than 1.2m wide it will layout must be altered to a standard layout as be too narrow to mark with chevrons. The length appropriate for the merge and mainline flow levels. of ghost island that is unmarked with a chevron When urban two lane taper merge layouts are to be could extend over a long distance. In order to improved, Figure 2/4.2, layout D must be used. prevent this problem, the minimum width of a Ghost island merge layouts must not be used on ghost island must be 1.2m at a distance of 50m urban roads. from the tip of the ghost island head or tail. It should be noted that ghost island layouts can Designing Merges require significant length to comply with the standard and this may be reflected in the land 2.29 Hourly flows, as determined from Chapter 3 requirement especially where the layout is being for the merge and the mainline upstream must be provided within an existing highway boundary. inserted in Figure 2/3 to select a merge layout as 2.33 The minimum length of a merge slip road shown in Figures 2/4.1 to 2/4.5. Where design will normally be dictated by the requirements flows lie close to, or on, a boundary between the given at paragraph 2.29 and the topographical flow regions, the probability of the particular flow layout of a junction. Where this is not the case, as actually occurring should be carefully reviewed. for instance at the merge slip road leading out of a The provision of a layout that differs from that service area, then the requirements set out in derived from the use of Figure 2/3 is a departure paragraph 2.34 must be followed. from standard, whether the proposed design is an under or over provision. 2.34 Gap finding is assisted when the merging traffic has the opportunity to match the speed of the mainline traffic. For all connector roads, a near 2.30 Where, for reasons of route continuity, the mainline capacity provided is in excess of the design straight at least equal in length to the nose length flows and a merging design flow of over one lane given in Table 4/3 column (3) for the appropriate capacity is expected, then layout C of Figure 2/4.2 may Road Class must be provided upstream of the back be substituted for layout F of Figure 2/4.4, but of the merge nose. This requirement will enable normally, with such a large flow expected to merge, a merging traffic to achieve a matching speed. lane would be added to the mainline. For layout C the meaning of ‘where design flows on the mainline are 2.35 Where the required length of near straight light’ (see Figure 2/4.2) is that there is sufficient cannot be achieved, it may be appropriate to capacity on the mainline to accept the flow from the provide an auxiliary lane instead or in slip road. Layout H (see Figure 2/4.5) may be combination. An application must be made for a considered as a departure from standards where it is not departure from standard. possible to use Layout F (see also paragraph 4.29). For Figure 2/4.4 layout F, Option 1 is the preferred option due to the likely usage of Lane 1 of the connector road 2.36 Platoons of traffic can enter a merge slip road if by the majority of large and/or slow vehicles and Lane junctions upstream are signal-controlled. This traffic 2 predominantly by light vehicles. Option 2 has been can have a significant effect on the mainline flow retained for use in circumstances where it is especially at peak times when available gaps in the appropriate. mainline traffic flow are few. Turbulence and congestion are the result. Care should be taken to 2.31 Ghost island road markings should be designed program such traffic signals with a view to reducing in accordance with TSRGD diagram 1042.1. their impact on the mainline flow.

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Notes: # Area of uncertainty – In this area the choice will depend on the downstream provision. If there is a lane gain then use Layout E or F. See paragraph 2.29 and the example above, for explanation of the usage of this diagram.

Figure 2/3 AP All-Purpose Road Merging Diagram

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Notes: * If Layout F Option 2 is used consider extended Auxiliary Lane (see paragraph 4.23). # Area of uncertainty – In this area the choice will depend on the downstream provision. If there is a lane gain then use Layout E or F. See paragraph 2.29 and example above, for explanation of the usage of this diagram. Figure 2/3 MW Motorway Merging Diagram

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N.B. Figures in brackets refer to columns in Table 4/3 Figure 2/4.1 Merge Lane Layouts for use with Figure 2/3

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N.B. Figures in brackets refer to columns in Table 4/3

Figure 2/4.2 Merge Lane Layouts for use with Figure 2/3

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N.B. Figures in brackets refer to columns in Table 4/3

Figure 2/4.3 Merge Lane Layouts for use with Figure 2/3

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N.B. Figures in brackets refer to columns in Table 4/3

Figure 2/4.4 Merge Lane Layouts for use with Figure 2/3

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N.B. Figures in brackets refer to columns in Table 4/3

Figure 2/4.5 Merge Lane Layouts for use with Figure 2/3

February 2006 2/13 Chapter 2 Volume 6 Section 2 Design Procedure Part 1 TD 22/06

Ramp Metering Designing Diverges

2.37 Ramp metering is a remedial measure to improve 2.43 Hourly flows, as determined from Chapter 3 the flow of traffic on a mainline by controlling traffic for the diverge and the mainline downstream must entering from the slip road. Currently it is only used on be inserted in Figure 2/5 to select a diverge layout motorways. Traffic signals installed on motorway entry as shown in Figures 2/6.1 to 2/6.4. Where design slip roads control vehicle flow on to the main flows lie close to, or on, a boundary between the carriageway at a ‘metered’ rate. The traffic signals are flow regions, the probability of the particular flow linked to sensors which measure speed, flow and lane actually occurring should be carefully reviewed. occupancy rates of traffic using the motorway and The provision of a layout that differs from that identify an appropriate metering rate for the traffic derived from the use of Figure 2/5 is a departure conditions. from standard, whether the proposed design is an 2.38 The use of ramp metering is still being tested at under or over provision. selected sites in and the initial signs are that it increases the flow of traffic on the motorway by 2.44 The provision of a Layout B parallel diverge reducing congestion that results from merging traffic. (Option 2) on a 4-lane mainline would create a 6-lane carriageway, contrary to the requirements of 2.39 Guidance on the use of ramp metering on TD 27 (DMRB 6.1.2). Any proposal for such a existing slip roads is available from the Overseeing layout must be referred to the Overseeing Organisation. Organisation.

Diverges – General Principles 2.45 The minimum length of a diverge slip road will normally be dictated by the requirements 2.40 Diverging traffic should be able to leave the given at paragraphs 2.41 and 2.43 and the mainline easily and without impeding the progress of topographical layout of a junction. Where this is through traffic. not the case, as for instance at the diverge slip road leading into a service area, then the requirements set out in paragraph 2.46 must be followed. 2.41 There is potential for accidents on diverge connector roads if the capacity of the connection to 2.46 For all connector roads, a near straight at the local road network is insufficient and causes least equal in length to the nose length given in queuing on the connector road. Drivers leaving the Table 4/4 column (4) for the appropriate Road mainline should have sufficient time to react and Class must be provided downstream of the back of brake safely before the end of any queue. The the diverge nose. This requirement will enable designer must therefore ensure that the drivers to comprehend the layout ahead and adjust downstream cross-section (designed in accordance their speed accordingly. with TD 27 (DMRB 6.1.2)) and junctions (see DMRB 6.2) do not cause queues that approach the 2.47 Where the required length of Near Straight back of the diverge nose. This will allow drivers to cannot be achieved, it may be appropriate to use the diverge area and length of nose to provide an auxiliary lane instead or in decelerate in reasonable comfort, as intended. combination. An application must be made for a departure from standard.

2.42 For existing junctions, if even after improvement 2.48 For diverges, the layout of the edge line to the downstream connection to the local road must incorporate the radii shown on Figure 2/6. network, there is a likelihood of queuing extending back onto the mainline carriageway, then auxiliary lanes should be considered as an exceptional case so that queues occur off the mainline. Motorway Incident Detection and Automatic Signalling (MIDAS) should also be considered in such circumstances. When auxiliary lanes are specified in this situation a departure from standards approval will be required.

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Ghost Island Diverges the relevant Overseeing Organisation. The layouts have also been developed for use at existing junctions and 2.49 Ghost Island diverge layouts are preferred to the there may be constraints at a particular site that prevent equivalent auxiliary lane layouts and should be selected the dimensions of the recommended layouts from being in preference to the auxiliary lane layouts except where achieved. Designers may need to consider amendments the Ghost Island layout may be unsuitable (see to the lengths and widths of the various elements of the paragraph 2.52). layouts. Where there are land constraints, for example, encroaching on the hardshoulder may be considered an 2.50 Ghost Island diverge layouts are for use when the acceptable means of achieving the additional capacity diverge flow is high and are designed to reduce the and safety offered by a ghost island diverge layout likelihood of queues of slow moving traffic in Lane 1 subject to obtaining agreement to a departure from together with ‘swooping’ movements (late manoeuvres standard. Figures 2/8(i) and 2/8(ii) are examples of the from Lane 2 or 3) to the slip road. By providing two conversion of existing layouts. Note that these layouts access points to a two-lane exit slip road, the capacity also require provision of a full sequence of gantry of the diverge is increased, congestion on the mainline direction signing plus two verge-mounted signs to the is reduced and swooping is discouraged. design illustrated in Figure 2/7.

2.51 A full sequence of gantry direction signing is essential for a Ghost Island diverge layout. The 2.53 The minimum width of a ghost island is Overseeing Organisation should be consulted for 2.0m at its widest point and the minimum width of guidance on the provision and location of sign and a chevron is 0.5m (TSRGD diagram 1042.1). If the signal gantries. In addition, it is essential that ghost island marking is less than 1.2m wide it will drivers are informed of the behaviour expected at a be too narrow to mark with chevrons. The length Ghost Island diverge. Two verge-mounted advance of ghost island that is unmarked with a chevron direction signs, to the design illustrated in Figure could extend over a long distance. In order to 2/7, must be provided. The first of these signs will prevent this problem, the width of a ghost island be between the 1 mile gantry and the ½ mile must be not less than 1.2m at a distance of 50m gantry; the second sign will be between the ½ mile from the tip of the ghost island head or tail. It gantry and the final gantry. The main objective of should be noted that ghost island layouts can these signs is to highlight to drivers the existence require significant length to comply with the of the second exit point and encourage its use. It standard and this may be reflected in the land has been found that the installation of these verge- requirement especially where the layout is being mounted signs improves the utilisation of the provided within an existing highway boundary. second exit with the effect of balancing the vehicle flows on the slip road lanes. Signs authorisation will be required for the non-standard signs designed for a particular site.

2.52 There may be occasions when the Ghost Island diverge layout is not suitable, for instance if signing is difficult to implement or if a high turning movement at the junction downstream of the diverge may lead to slip road queues in one or more lanes tailing back towards the mainline (see paragraphs 2.40, 2.41 and 2.42); in such cases the auxiliary lane layouts may be used instead. Note that for a Lane Drop at Parallel Diverge (Figure 2/6.3 Layout D Option 2), a full sequence of gantry direction signing should be provided in order to encourage utilisation of Lane 2 by diverging traffic. To date, ghost island diverges have been used only on rural motorways. For the application of ghost island diverge layouts to other roads, guidance should be sought from

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Example of use of Figure 2/5AP Given a downstream main line flow 4000vph and diverge flow 2000vph. 1 strike a perpendicular from 4000vph on the horizontal axis 2 strike a perpendicular from 2000vph on the vertical axis 3 the intersection point gives layout type D which also requires a lane drop (see Upstream Mainline axis above)

Notes:

See paragraph 2.43 and the example above, for explanation of the usage of this diagram.

Figure 2/5 AP All-Purpose Road Diverging Diagram

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Example of use of Figure 2/5MW Given a downstream main line flow 4000vph and diverge flow 2000vph. 1 strike a perpendicular from 4000vph on the horizontal axis 2 strike a perpendicular from 2000vph on the vertical axis 3 the intersection point gives layout type D which also requires a lane drop (see Upstream Mainline axis above)

Notes:

* If Layout D Option 2 is used consider extended Auxiliary Lane (see paragraph 4.24).

See paragraph 2.43 and the example above, for explanation of the usage of this diagram.

Figure 2/5 MW Motorway Diverging Diagram

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N.B. Figures in brackets refer to columns in Table 4/4

Figure 2/6.1 Diverge Lane Layouts for use with Figure 2/5

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N.B. Figures in brackets refer to columns in Table 4/4

Figure 2/6.2 Diverge Lane Layouts for use with Figure 2/5

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N.B. Figures in brackets refer to columns in Table 4/4

Figure 2/6.3 Diverge Lane Layouts for use with Figure 2/5

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N.B. Figures in brackets refer to columns in Table 4/4

Figure 2/6.4 Diverge Lane Layouts for use with Figure 2/5

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Figure 2/7 Typical Sign for Ghost Island Diverge Layout (“tiger-tail”)

Motorway Service Areas (MSAs)

2.54 The merge and diverge layout design and junction spacing parameters in this standard apply to MSAs.

2.55 Generally all vehicle types are permitted to enter an MSA via a connector road directly from the mainline or as an integral part of a grade separated junction.

2.56 Drivers wishing to make a stop at MSAs will have made a choice about their immediate destination and know that they will have to slow down. The provisions set out below should facilitate safe layouts for access to and egress from MSAs.

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Figure 2/8 Examples of Existing Parallel Diverges Converted to Ghost Island Diverges

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2.58 For online service areas, at the end of the MSA 2.57 The following requirements must apply to diverge slip road, it is recommended that a gateway be MSAs accessed directly from motorways: erected. The object of the gateway is to draw the attention of the driver to the change to the lower • The design speed of connector roads must standards of the MSA. It will also highlight the need for be the same as a slip road in Table 4/1. greater care and emphasise the probability of encountering slow vehicles and pedestrians using the • For diverge slip roads for MSAs, stopping internal roads of the MSA. The gateway must be sited sight distance and horizontal curvature may on the MSA side of the ‘End of Motorway’ signs. be reduced by one design speed step as a Relaxation (see paragraph 1.39 and TD 9 2.59 The gateway should include speed restriction (DMRB 6.1.1)). signs, which may be emphasised by the use of calming measures such as dragon’s teeth and coloured road • Near straights must be provided on the slip markings. Additional gateway features may also be used roads as described in paragraph 2.34 for provided that they do not create a road safety hazard. merges and paragraph 2.46 for diverges. 2.60 A similar or simpler gateway may be provided at • Street Lighting (see also paragraphs 5.33, the start of a merge slip road on leaving the MSA. The 5.34 and 5.35): gateway must be sited on the MSA side of the point where the motorway regulations start. – If the mainline is lit then the slip road must be lit Other Service Areas – If the mainline is not lit then: 2.61 The merge and diverge layout design of all- ~ for a merge, the lighting must purpose road service areas should be based on the be ended as soon as possible geometric parameters within this standard as set out in after the MSA boundary; paragraph 2.57 above or TD 42 (DMRB 6.2.6), as appropriate for each site. ~ for a diverge, the lighting must start before the point where Application to Maintenance Compounds MSA lighting occurs but such as not to cause light spillage onto the mainline. 2.62 Where maintenance compounds are • The layout must include a comprehensive accessed off the mainline, the standards set out in traffic sign and road marking scheme and paragraph 2.57 for MSAs must be used. consideration should be given to the inclusion of ‘chevron’ warning signs, reflective road studs, edge of carriageway Design for Maintenance markings, rumble strips and advisory speed limit signs. 2.63 Any area of pavement that can be driven over in • High containment kerbs must not be used on an emergency or during maintenance or other road slip roads as high speed impacts may lead to works should be designed to make it safe to do so. the overturning of vehicles. Although it is illegal to drive over noses and other chevron areas bounded by continuous edge lines, they • Measures must be taken to reduce any ‘see- may be trafficked during road works if drivers are through’ effects when looking from a directed to do so. diverge slip to the merge slip or internal road system of the MSA, e.g. suitable 2.64 Paragraph 4.32 gives advice relating to lane landscaping. drops and lane gains and the intervening length of carriageway through a junction.

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Emergency and Maintenance Accesses 2.69 In the case of ghost island merges and diverges, the examples in Figure 4/13 show the two points which must be used for the two 2.65 Where an emergency or maintenance access connector road lanes to provide the averaged is required, a suitable layout must be chosen from weaving lengths between junctions. Similar TD 41 (DMRB 6.2.7). The preferred layout is that techniques must be applied for diverges. shown as Layout 1 but the designer must check that this would be adequate for its likely use. The access must be gated and locked to prevent 2.70 In the case of wide (5 lane or more) unauthorised use. The entrance gate or gates must carriageways, there should be no reduction below the be set back to accommodate, behind the hardstrip desirable minimum weaving length. A vehicle on a 5- or hardshoulder, one vehicle of the largest type lane carriageway requires at least 1km to cross between expected to use the access. For a maintenance Lanes 5 and 1 in safety to leave at a diverge and the access, provision must be made for two vehicles of driver will need advance warning. The formula in the largest type expected to use the access to pass paragraph 2.71 should still be used, but non-weaving in opposite directions in the vicinity of the access. traffic may be excluded from the calculation if it travels The design of the emergency or maintenance in a reserved lane. access must comply with the requirements of TD 41 (DMRB 6.2.7) with respect to avoiding steep gradients on the access road in the immediate 2.71 For weaving sections on motorways and vicinity of its connection to the trunk road. dual carriageway roads, design flows must be calculated as in Chapter 3. In measuring Lact, it will be necessary to consider whether distance is Designing Weaving Sections available to adequately sign the second junction and allow adequate visibility to the sign from all lanes. To calculate the number of traffic lanes 2.66 The principle of calculating weaving sections is required for weaving the following equation must that the length is fixed using paragraphs 4.34 to 4.37 and the width is calculated from the formula in be used (and see Figure 2/9): paragraph 2.71. This determines the number of lanes N = 1 (Q +Q +Q (2 L /L + 1)) and can indicate the addition of one or two auxiliary nw w1 w2 min act lanes. The formula shows that the minor weaving flow D has an impact on the traffic demand of up to 3 times its Where N = Number of traffic lanes numerical value.

Qnw = Total non-weaving flow in vph 2.67 An actual weaving length less than the Q = Major weaving flow in vph desirable minimum must not be entered into the w1 formula. Qw2 = Minor weaving flow in vph 2.68 Weaving lengths for taper layouts must be D = Maximum mainline flow from measured between the end of the merge and start paragraph 3.3 in vph per lane of the diverge tapers, see Figure 4/9A. For auxiliary lane layouts, the auxiliary lane is ignored Lmin = Desirable Minimum weaving and the length between the end of the notional length for the road class as in merge and and the start of the notional diverge paragraphs 4.34 to 4.37 must be measured as illustrated in Figure 4/9B. In the case of lane gains and lane drops, the methods Lact = Actual weaving length set out in Figures 4/10, 4/11 and 4/12 must be available used.

(Lact must always be greater than or

equal to Lmin)

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2.72 In calculating the number of traffic lanes i the number of lanes required for merging required (paragraph 2.71) a fractional part will and diverging (paragraphs 2.29 and 2.43); inevitably require a decision to round up or down. If it is possible to vary the position of the junctions ii when the fractional part is about 0.5 the and thus increase or decrease the weaving length, uncertainty of the design flows (Chapter 3) the fractional part will converge approximately to a suggests always rounding up from 2 to 3 lanes; whole number of lanes and the decision is simplified. However, if this is not possible the iii on recreational routes there can be a high decision becomes more difficult. Where the proportion of drivers who are not local and fractional part is small and is combined with a low therefore behave less efficiently than weaving flow rounding down is suggested, commuters would at the same flow levels; whereas a high fractional part with a high weaving volume suggests rounding up. For example the iv the consequences of under provision should addition of a fourth lane would have operational be borne in mind, as the acquisition of land advantages in releasing the two middle lanes for at a later date could be costly or impossible; weaving traffic. Other factors which may influence the decision are: v. relevant environmental factors should be taken into account.

Qnw (non-weaving flow) = Flow 1 + Flow 4 Qw1 (major weaving flow) = greater of Flow 2 or Flow 3 Qw2 (minor weaving flow) = lesser of Flow 2 or Flow 3

Figure 2/9 Terms used in Weaving

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3. TRAFFIC FLOWS

Introduction

3.1 At the time of publishing this standard, the procedure for determining traffic flows for use in design is undergoing change and development.

3.2 Until such time as guidance has been published, designers must contact the Overseeing Organisation for instructions on how to proceed for individual schemes.

3.3 The Highway Code advises that a minimum two-second headway should be maintained between vehicles on roads carrying fast traffic. For the purpose of designing grade separated junctions and interchanges, the maximum flow per lane for motorways must be taken as 1,800 vehicles per hour (vph) and for all-purpose roads as 1,600 vph. These flows do not represent the maximum hourly throughputs but flows greater than these will usually be associated with decreasing levels of service and safety. These values have been used in Figures 2/3 and 2/5 in this standard.

Design Flow Ranges and Connector Road Cross Sections

3.4 Connector road cross sections are set out in TD 27 (DMRB 6.1.2) and the corresponding design traffic flow ranges are given in Tables 3/1a and 3/1b.

3.5 Designers should consider the possible benefits of providing greater widths for connector roads than those derived solely from Tables 3/1a and 3/1b and the standard cross-sections in TD 27 (DMRB 6.1.2). This would be, for example, to provide for future maintenance activities.

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Connector Merge Merge Diverge Diverge Interchange Interchange Road (Rural) (Urban) (Rural) (Urban) Link/Loop Link/Loop Flow+ (Rural) (Urban)

0-800 MG1C MG1D DG1C DG1D IL1C IL1D Single lane Single lane Single lane Single lane Single lane* Single lane* with with with with with with hardshoulder hardshoulder hardshoulder hardshoulder hardshoulder hardshoulder

801-1200 MG1C MG1D DG2E DG2F IL1C IL1D Single lane Single lane Two lanes Two lanes Single lane* Single lane* with with with with with with hardshoulder hardshoulder hardstrip hardstrip hardshoulder hardshoulder

1201-2400 MG2E MG2F DG2E DG2F IL2C IL2D Two lanes Two lanes Two lanes Two lanes Two lanes Two lanes with with with with with with hardstrip hardstrip hardstrip hardstrip hardstrip hardstrip

2401-3200 MG2E MG2F DG2E DG2F IL2C IL2D Two lanes Two lanes Two lanes Two lanes Two lanes Two lanes with with with with with with hardstrip hardstrip hardstrip hardstrip hardstrip hardstrip

Table 3/1a Cross-Sections for Connector Roads To/From Mainline All-Purpose Roads

Connector Merge Merge Diverge Diverge Interchange Interchange Road (Rural) (Urban) (Rural) (Urban) Link/Loop Link/Loop Flow+ (Rural) (Urban)

0-900 MG1A MG1B DG1A DG1B IL1A IL1B Single lane Single lane Single lane Single lane Single lane* Single lane* with with with with with with hardshoulder hardshoulder hardshoulder hardshoulder hardshoulder hardshoulder

901-1350 MG1A MG1B DG2A DG2B IL1A IL1B Single lane Single lane Two lanes Two lanes Single lane* Single lane* with with with with with with hardshoulder hardshoulder hardstrip hardstrip hardshoulder hardshoulder

1351-2700 MG2C MG2D DG2A DG2B IL2A IL2B Two lanes Two lanes Two lanes Two lanes Two lanes Two lanes with with urban with with with with urban hardshoulder hardshoulder hardstrip hardstrip hardshoulder hardshoulder

2701-3600 MG2C MG2D DG2C DG2D IL2A IL2B Two lanes Two lanes Two lanes Two lanes Two lanes Two lanes with with urban with with urban with with urban hardshoulder hardshoulder hardshoulder hardshoulder hardshoulder hardshoulder

Table 3/1b Cross-Sections for Connector Roads To/From Mainline Motorways

Notes For Tables 3/1a and 3/1b * See paragraph 4.3 for restrictions on use of single lane interchange links + Design flow (vehicles per hour) adjusted for gradients and LGVs Refer to TD 27 (DMRB 6.1.2) for actual dimensions of cross-section components These tables can indicate, for low connector road flows, that a single lane connector road should be provided for a Layout C diverge, which has two connector road lanes. In such cases, two lanes should be provided.

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Flow Adjustments for Uphill Gradients and for % LGVs Merge Connector Gradient LGVs on Merge Connector 3.6 Before using Figures 2/3 and 2/5 for the <2% 2% – 4% >4% selection of merge and diverge layouts respectively, the design flows must be adjusted for 5 – 1.15 1.30 uphill gradients and the presence of LGVs by 10 – 1.20 1.35 using Tables 3/2 and 3/3. Note that adjustments are made to flows on the mainline and on merge 15 1.05 1.25 1.40 connector roads but not to flows on diverge connector roads. 20 1.10 1.30 1.45

3.7 To establish the mainline gradient at merges Table 3/3 Adjustment Factors for Uphill or diverges, a 1 kilometre section must be used, Gradients and for the presence of Large 0.5 km either side of the merge or diverge nose tip, Goods Vehicles and the average gradient determined. The merge Merge Connector connector road gradient must be based on the average of the 0.5 km section before the nose tip.

3.8 Before using Figure 4/14 and the weaving formula in paragraph 2.71, the design flows must be adjusted for uphill gradients and the presence of LGVs by using Table 3/2.

3.9 To establish the mainline gradient at a

weaving section, the weaving length, Lact, must be determined and the average gradient calculated over that length.

%LGVs Mainline Gradient on mainline <2% ≥2%

5 – 1.10

10 – 1.15

15 – 1.20

20 1.05 1.25

Table 3/2 Adjustment Factors for Uphill Gradients and for the presence of Large Goods Vehicles Mainline

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4. GEOMETRIC STANDARDS

Cross Sections two lane slip roads. One solution could be the reduction from two lanes to one lane near the end of the 4.1 For the purpose of designing junctions and interchange link in accordance with TA 58 (DMRB interchanges, cross sections for the mainline and all 8.2.1). This may not be possible on loops where the connector roads are given in TD 27 (DMRB 6.1.2). The accident risk of a lane reduction on a tight bend should design flow ranges corresponding to these cross be avoided, normally by providing a length of near sections are shown in Table 3/1a and 3/1b. straight at the end of the loop. Alternatively, on loops it may be preferable to adopt a one-lane interchange link throughout (subject to the requirements of paragraph Maximum Lengths of Slip Roads and Interchange 4.3) or remove one lane prior to the loop. Links Design Speed 4.2 A Slip Road longer than 0.75 km must be designed as an Interchange Link. 4.5 Design speeds for the mainline must be determined from TD 9 (DMRB 6.1.1). The design 4.3 Single Lane Interchange Links must only be speeds of connector roads must be as given in provided: Table 4/1. The design speed for link roads should normally be one design speed step below that of • when their length does not exceed 1 km and the mainline, as shown in Table 4/1 and this they are on an average uphill grade of up to reduced design speed will need to be made clear to 3%, are level or on a downhill grade; or the vehicle driver. To help achieve this, link roads should be subject to an appropriate speed limit, • where their length does not exceed 0.5 km either mandatory or advisory. Where the proposed and they are on an average uphill grade of link road design speed is one design speed step 3% or steeper. below that of the mainline and this cannot be made obvious to the driver, the higher design speed 4.4 Where two lane interchange links are proposed, should be used. Where the link road is a care will be needed to ensure that any subsequent merge connection to a motorway, motorway merge can be designed in accordance with this standard. parameters apply, regardless of the design speed. Layout A and Layout B merges are not permitted for

Mainline Urban Urban Rural Rural Design Speed 100 kph 85 kph 120 kph 100A kph

Interchange 70 70 85 85 Link Connector Road Slip Road 60 60 70 70 Design Speed Link Road 100 or 85 85 or 70 120 or 100A 100A or 85 (kph) Dumb-bell 70 70 70 70 Link Road

Table 4/1 Connector Road Design Speed

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4.10 Research into loops carried out from 1985 to 4.6 Any transition curves at locations where the 1994 did not reveal any systemic safety problems. design speed changes must be designed to the Accident levels at sites surveyed were generally low higher design speed value. with approximately a third of the sites having no personal injury accidents over the study period. If the general decrease in accident rates over time is Horizontal and Vertical Alignment considered, then the overall accident rates in the study are consistent with those found in the earlier study that 4.7 The geometric standards for horizontal and underpinned the design advice in the now superseded TD 22/92 and TA 48/92. vertical alignment and stopping sight distance for the mainline through a grade separated junction 4.11 The research looked at whether non-compliance and for the connector roads must be provided in with existing standards gave rise to safety problems and accordance with TD 9 (DMRB 6.1.1). TD 9 a variety of non-complying loops were examined. The specifies an absolute maximum gradient for study examined the following loops which are shown in motorways of 4%. For motorway connector roads, Figure 4/1: this may be increased to 6%. Basic merge A loop that passes through 4.8 Low radius connector roads must be approximately 270o where traffic widened on curves in accordance with merges with the mainline flow. This TD 9 (DMRB 6.1.1) and TD 42 (DMRB 6.2.6). Basic Merge, when combined with the Hook Diverge, forms the layout Loops in Figure 4/2b. Basic diverge A loop that passes through 4.9 In the case of the horizontal curvature and approximately 270o where traffic superelevation for loops (as defined in paragraph diverges from the mainline flow. 1.21), there is evidence to suggest that the radii of This Basic Diverge, when combined loops (Figure 4/1) can safely be much less than for with the Hook Merge, forms the curves turning through lesser angles, provided that layout in Figure 4/2a. adequate warning is given to drivers and clear sight lines are maintained. For loops the minimum D merge The loop commences at a T-junction radii may therefore be those given in Table 4/2. or roundabout and merges with the Within the loop, successive radii of the same hand mainline flow. The angle turned is o must not reduce in radius. The standards for typically approximately 180 . superelevation for loops are set out in TD 9 D diverge The loop commences at a diverge (DMRB 6.1.1). Superelevation greater than 7% from the mainline flow and ends at a and up to 10% may be provided as shown in TD 9 T-junction or roundabout. The angle (DMRB 6.1.1) but superelevation greater than 7% turned is typically approximately should be used with caution where there is a risk of 180o. prolonged icy conditions. Where loops leave or join the mainline, crossfall alongside the nose must Hook merge This layout is classified as a loop be the minimum required for drainage design as and the notional total angle is laid down in TD 9 (DMRB 6.1.1). Widening on between 180o and 270o. This Hook loops must be as set out in TD 42 (DMRB 6.2.6). merge, when combined with the Basic diverge, forms the layout in Motorway All-Purpose Figure 4/2a.

On/Off Mainline On to Mainline Off Mainline Hook diverge This layout is classified as a loop and the notional total angle is o o 75 30 50 between 180 and 270 . This Hook diverge, when combined with the Basic merge, forms the layout in Table 4/2 Minimum Loop Radii – (m) Figure 4/2b.

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Figure 4/1 Types of Loop

Note: A near straight is required beyond the back of each nose (see paragraphs 2.34 and 2.46)

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4.12 Motorway to motorway one-way loops and Sight Distances motorway to all-purpose road two-way loops were identified as the loop types with the highest average accident rate. Rates on loops on all-purpose routes and 4.16 Desirable Minimum Stopping Sight between all-purpose routes and motorways tended to be Distances must be provided on all connector roads lower. in accordance with the design speed selected and TD 9 (DMRB 6.1.1). 4.13 The findings give support to the argument that average speed of approach to a loop may have an impact on its safety record. It is possible that the higher speeds on motorways on the approach to loops may be a contributory factor to accidents, particularly on diverge loops. Measures to maintain safety are necessary, and measures to consider include: i. provision and maintenance of clear visibility over the whole of the loop on the approaches, especially beyond an underbridge (see paragraph 4.19); ii. advisory speed limits and/or bend signs and “chevron” warning signs; iii. widening of lanes on the loops as appropriate for lower radii in accordance with TD 42 (DMRB 6.2.6); iv. the provision of vehicle restraint systems on the outside of the curve; v. physical separation of opposing traffic streams (see paragraph 5.27 for mandatory requirements); vi. lighting; vii. high skid resistant surfacing.

4.14 The provisions for loops in this document must apply only to the layouts shown in Figure 4/1, which may be used in combination as shown in Figure 4/2.

4.15 The provisions of paragraphs 2.33 and 2.34 for merges must be applied and will assist drivers to adjust their speed and join the mainline traffic. Similarly, the provisions of paragraphs 2.45 and 2.46 for diverges must be applied and will assist drivers to adjust their speed and to comprehend the layout of the loop in front of them.

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a) Basic diverge plus hook merge

b) Basic merge plus hook diverge

Figure 4/2 Examples of Combinations of Different Types of Loop

Note: A near straight is required beyond the back of each nose (see paragraphs 2.34 and 2.46)

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4.17 For merges, the Stopping Sight Distance on 4.19 For loops, in addition to the general the connector road must be that related to the stopping sight distance requirements, there must design speed selected for that road (see paragraph also be no obstruction to sightlines across the full 4.5 and Table 4/1). This will apply along the length extent of loops of low radius. This includes where of the connector road until the driver’s eye is the loops connect to the main carriageway as square with the back of the merge nose. From that shown in Figure 4/2. This is to ensure that drivers point forward, the Stopping Sight Distance must be are able to perceive the whole of the loop layout on that for the mainline design speed. There must be their approach from upstream and adjust their no obstruction to sight lines between the connector speed and conduct accordingly. The only available road and the mainline and vice versa for the length relaxation to these requirements is when the of the merge nose. There is a minimum approach necessary vehicle restraint systems obstruct the angle at which drivers can merge on direct sight, view to the 0.26m object height, in which case a otherwise blind spots to the rear of the vehicle will clear sightline must be available above the vehicle be troublesome. Below this minimum approach restraint system to the 1.05m object height. angle drivers will be moving nearly parallel to the mainline carriageway and will have to merge using mirrors. It follows that there is a minimum width 4.20 For the connections to the local road system, of merge nose and this can be derived from guidance on sight distance standards at major/minor geometric parameters (paragraph 4.22). The width junctions is given in TD 42 (DMRB 6.2.6) and for of the back of the nose must be sufficient to roundabouts in TD 16 (DMRB 6.2.3). Advice on accommodate the mainline hardshoulder/hardstrip signal-controlled junctions is contained in TD 50 (DMRB 6.2.3). and the connector road off side hardstrip.

4.18 For diverges, the Stopping Sight Distance Hardstrip and Hardshoulder related to the mainline design speed must be maintained into the diverge until the drivers eye is square with the back of the diverge nose. The 4.21 Where the hardshoulder has to taper into a Stopping Sight Distance can then be progressively slip road or interchange link hardstrip or vice reduced to that for the design speed selected for versa, this must be done in accordance with TD 27 the connector road in the manner illustrated in (DMRB 6.1.2). The slip road or interchange link Figure 4/3A i.e. an object at a distance from the hardstrip must terminate prior to an at-grade back of the nose equal to mainline SSD must junction in accordance with the requirements of remain visible as the vehicle moves forward along TD 16 (DMRB 6.2.3) for terminating edge strips the connector road. If the length of the connector on the approach to a roundabout. road between the back of the nose and the give way line of the at-grade junction at the end of the connector road is less than the mainline Stopping Sight Distance, then a 0.26m object at the give way line must be visible from a distance equal to the mainline Stopping Sight Distance. See Figure 4/3B. Similarly, for a diverge leading into an MSA, a 0.26m object at the downstream end of the slip road, the minimum length of which has been determined from paragraphs 2.46 and 2.57, must be visible from a distance equal to the mainline Stopping Sight Distance. Beyond that point drivers will expect a reduction of standards to that of the MSA.

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Figure 4/3A Illustration of Stopping Sight Distance on Slip Road

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Figure 4/3B Illustration of Stopping Sight Distance on Slip Road

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Merges and Diverges

4.22 The geometric parameters applicable to merges and diverges must be those in Tables 4/3 and 4/4 respectively. Figures 2/4 and 2/6 illustrate their use in typical layouts. Lengths are measured along the left edge of the carriageway. For merges, the layout of the edge line shown on Figure 2/4 does not require the use of larger radii. For diverges, the layout of the edge line should incorporate the radii shown on Figure 2/6. Ghost island merges are not permitted for urban roads (see Table 4/3).

Road Class Length Nose Ratio Nose Minimum Length of Length of Reduction of (See Note 1) Length Auxiliary Auxiliary Ghost Taper Entry Lane Lane Island Length Taper Length Taper Tail m mmmmm

(1) (2) (3) (4) (5) (6) (7)

Rural Motorway Mainline 205 1:40 115 230 75 180 n/a Within Interchange 130 1:25 75 160 55 150 n/a

Rural All-Purpose Design Speed

120kph 150 1:30 85 190 55 150 n/a

100A kph or less 130 1:25 75 160 55 150 n/a

Urban Road Speed Limit

60 mph 95 1:15 50 125 40 n/a see Note 2 see Note 2

50 mph or less 75 1:12 40 100 40 n/a see Note 2 see Note 2

Note 1 Nose Ratio is the ratio of nose back width to nose length for minimum angle at nose. The maximum angle will be limited by the ability of vehicles to negotiate the change in direction.

Note 2 Ghost islands for merges on urban roads are not permitted (see paragraph 4.22) and the layout in Figure 2/4.2D should be used for all new or improvement work. For slip road reduction taper, (7) on Figure 2/4.2D, tapers are as given in Table 10-3 of Traffic Signs Manual Chapter 5. When the angle is less or the ratio is greater than the preferred minimum taper in Table 10-3, it is a departure from standards.

Table 4/3 Geometric Design Parameters for Merging Lanes (See also Figure 2/4)

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Road Class Length of Nose Ratio Nose Minimum Length of Length of Exit Taper (See Note 1) Length Auxiliary Auxiliary Ghost m Lane Lane Island Length Taper Head

1 lane 2 lane m m m m (1) (2) (3) (4) (5) (6) (7)

Rural Motorway Mainline 170 185 1:15 80 200 75 180 Within Interchange 130 130 1:15 70 150 55 n/a

Rural All-Purpose Design Speed

120kph 150 150 1:15 70 170 55 160

100A kph or less 130 130 1:15 70 150 55 140

Urban Road Speed Limit

60 mph 95 110 1:15 50 125 40 100

50 mph or less 75 90 1:12 40 100 40 80

Note 1 Nose Ratio is the ratio of nose back width to nose length for minimum angle at nose. The maximum angle will be limited by the ability of vehicles to negotiate the change in direction.

Table 4/4 Geometric Design Parameters for Diverging Lanes (See also Figure 2/6)

4.23 Where, in a merge on a rural motorway, it is Option 1, 2/6.3D Option 1, 2/8(i) and 2/8(ii) cannot be anticipated that the connector road and mainline will fitted. For layout 2/6.1B Option 2 it is desirable to frequently be carrying traffic flows approaching their project a single auxiliary lane upstream for 400m prior design capacities, it is desirable to extend the minimum to the diverge (an example is shown in Figure 4/4), auxiliary lane length of 230 m (Table 4/3) to 370 m. As connected by a taper of length as shown in Table 4/4 a guide, this should be considered when connector road column 6 to the two lane section as shown in layout and mainline flows reach 85% of capacity, as defined in 2/6.1B Option 2. The single auxiliary lane should also paragraph 3.3, for more than 1,000 hours per year. commence with such a taper. The same guidance as in Figure 4/4 shows an example for the layout of a ghost paragraph 4.23 may be taken to indicate when an island merge with lane gain. Within larger interchanges, extended auxiliary lane should be considered. the length may be increased to 500 m. The auxiliary lane should be extended to the next diverge if this is close and the termination of the auxiliary lane is 4.25 In order to allow merging drivers using an considered as a safety hazard. auxiliary lane to match their speed with those on the mainline where there is an uphill section of 4.24 Where, in a diverge on a rural motorway it is road, the auxiliary lane must be extended beyond anticipated that the connector road and the mainline the crest sufficiently to enable the end of the will frequently be carrying traffic flows approaching auxiliary lane to be clearly visible to drivers when: their design capacities, layouts which encourage orderly use of the diverge by the use of ghost islands should be • the uphill section of road would be used in preference to layouts which do not have this sufficiently steep to require a climbing lane; feature. Use of layouts 2/6.1B Option 2 and 2/6.3D or Option 2 is restricted to locations where layout 2/6.1B

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Figure 4/4 Extended Auxiliary Lanes for Rural Motorways

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At the time of publishing this standard, the standards • the proportion of LGVs is greater than 10% for safety barriers are under review and designers and the uphill mainline gradient is in excess should consult the most recent publication for the of 2% and within 0.5 km of the crest. recommended dimensions of clear zones, seeking advice from the Overseeing Organisation in the For advice on the signing of auxiliary lanes, see meantime. paragraph 4.26.

4.29 Parallel merges and diverges (Figure 2/4.1 4.26 For extended auxiliary lanes in merges, of length (Layout B) and Figure 2/6.1 (Layout B Option 2)) greater than that given in column (4) of Table 4/3, a must be used in preference to taper merges and sign to TSRGD diagram 872.1 (reversed in a mirror diverges (Figure 2/4.1 (Layout A) and Figure 2/6.1 image) with a diagram 876 distance plate ‘200yds’, (Layout A)) if one or more of the following apply: should be placed 200yds from the start of the taper. For very long auxiliary lanes in merges consideration i) the mainline has a horizontal radius less than should be given to additional diagram 872.1 signs with the Desirable Minimum (Table 3 of TD 9 the appropriate distance plates. (DMRB 6.1.1)) for merges in the left hand curve direction and for diverges in the right hand curve direction; 4.27 Emergency telephones and other equipment that requires vehicles to stop for assistance or ii) the mainline is on an upgrade of 3% or highway maintenance must be sited a minimum of steeper for longer than 1.5 km prior to the 100m from the termination of merges where start of the taper; vehicles may overrun onto the hard shoulder or hard strip. iii) the mainline is on a downgrade of 3% or steeper for longer than 1.5 km prior to the start of the taper; 4.28 The consequences of an incident can be severe if hazards are present within the verge area immediately iv) the connector road entering a merge is on an downstream of the diverge nose. It is therefore desirable upgrade of 3% or steeper for longer than to provide a clear zone at the back of diverge noses 500 m before the merge. such that the physical nose is free from all hazards, including safety barriers, to minimise the risk to errant Where a diverge connector road has a single lane, vehicles. Creating a clear zone will normally require the a single auxiliary lane is appropriate. vertical alignment for the connector road to follow that of the main carriageway for a short distance to allow the cross-section to be reasonably level, although it is normal practice to demarcate the paved area from the verge using kerbs. If creation of a clear zone is not achievable due to site constraints, the risk needs to be reduced to as low as is reasonably practical, for example:

- by the use of passively safe sign posts and lighting columns, although following the advice in TA 89 (DMRB 8.2.2) this would require approval from the Overseeing Organisation;

- if a safety barrier is required to protect errant vehicles from any hazards, including height differences between adjacent carriageways, the use of P4 terminals or crash cushions is recommended as end treatments to barriers. Sufficient space should be allowed for any safety barriers and end treatments.

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Successive Merges or Diverges Within Interchanges

4.30 Where there are closely spaced successive merges or diverges on mainlines and connector roads within a junction or interchange (Figure 4/5), the minimum spacing between the tips of noses must be 3.75V m, where V is the design speed in kph, subject to the minimum requirements for effective signing and motorway signalling. If the merges or diverges are on a connector road, the design speed must be that for the connector road. This paragraph applies to successive merges (merge followed by a merge) or successive diverges (diverge followed by a diverge). It also applies to a diverge followed by a merge but not to a merge followed by a diverge (the latter is a weaving section).

4.31 At a fork within an interchange link, the taper must be developed as shown in Table 4/5 and Figure 4/6. Problems may be encountered if a broken down vehicle were to be situated alongside the nose where a single lane fork passes to the right. The only additional width of carriageway available is the hardstrip. Under such circumstances the offside verge should be hardened (and marked using road markings to TSRGD diagram 1040.3) opposite the nose and for a length before and after, in order that vehicles can make their way past the disabled vehicle (see Figure 4/7). The maximum width of the hardened verge should be that of the appropriate hardshoulder. Any vehicle restraint system will require setting back behind such a length of hardened verge.

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Figure 4/5 Example of Successive Merges/Diverges

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Figure 4/6 Development of Taper at Fork

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Figure 4/7 Hardened Verge at Single Lane Fork

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Interchange Link Length of Taper L Nose Ratio Nose Length Design Speed (metres) (metres)

1 lane 2 lane

70/85kph 75 90 1:12 40

Note Nose Ratio is the ratio of nose back width to nose length for minimum angle at nose. The maximum angle will be limited by the ability of vehicles to negotiate the change in direction.

Table 4/5 Geometric Design Parameters for a Fork Within an Interchange Link

Lane Drop/Lane Gain and Through Carriageway

4.32 Where a 3-lane carriageway is reduced to 2 lanes by means of a lane drop at a junction as shown in Figure 4/8, provision must be made on the link between the lane drop and subsequent lane gain for maintenance activities, incident management and temporary traffic management systems. Therefore the pavement must be constructed to a width of 3 lanes (plus hardshoulder if a motorway) and the pavement adjacent to the 2 running lanes must be hatched out to leave a normal width of hardstrip (or hardshoulder if a motorway) adjacent to the running lane as shown in Figure 4/8. The diverge and merge areas must be designed to provide sufficient pavement to allow conversion of the junction from a lane drop/lane gain to a 3-lane link with taper diverge and merge.

4.33 Advice on the signing of lane gains and lane drops is given in TA 58 (DMRB 8.2.1).

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Figure 4/8 Lane Drop to Two Lanes and Subsequent Lane Gain Showing Hatched Pavement for Maintenance and Traffic Management

- See Paragraph 4.32

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Weaving Lengths around 2 kilometres and this should be taken as the maximum weaving length. The weaving formula is 4.34 Weaving lengths must be measured as not to be used for weaving lengths above 2km. shown in Figures 4/9 – 4/13. 4.37 For Urban Roads as defined in Chapter 1, 4.35 For Rural Motorways, the desirable the design flows must be inserted in Figure 4/14 to minimum weaving length must be 2 kilometres. obtain a minimum weaving length (L ). This must Above about 3 kilometres apart, merges and min then be compared to the Design Speed related diverges tend not to interact and can be considered Absolute Minimum weaving length in Figure 4/14 as separate entities, since weaving ceases to occur. and the greater of the two lengths taken as the The maximum possible weaving length can thus be minimum length of weaving section, provided that taken as 3 kilometres. This would appear to be the signing requirements can be met. case up to and including weaving sections 5 lanes wide. The weaving formula is not to be used for 4.38 For All-Purpose Roads, the minimum weaving lengths above 3 kilometres. The length between a grade separated junction requirements for weaving for MSAs on rural designed to this standard and an at-grade junction motorways are as for rural motorway junctions. (including roundabouts), service area, lay-by or direct access must be the desirable minimum 4.36 For Rural All-Purpose Roads the desirable weaving length as defined in paragraph 4.36 for minimum weaving length must be 1 kilometre. On rural roads or the minimum length of weaving carriageways up to 3 lanes wide, the maximum section as derived from paragraph 4.37 for urban distance over which successive merges and roads. diverges are likely to interact and cause weaving is

A - Merge, Weaving Length and Diverge

B - Parallel Merge/Diverge as for Taper Merge/Diverge by Notional Layout

N.B. See Figure 4/13 for measurement of weaving length for ghost island layouts.

Figure 4/9 Definition of Terms used in Weaving and Measurement of Weaving Length for Taper and Auxiliary Lane Layouts

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N.B. See Figure 4/13 for measurement of weaving length for ghost island layouts.

Figure 4/10 – 4/12 Definition of Terms used in Weaving and Measurement of Weaving Length for Lane Gain and Lane Drop Layouts

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1. * Total Weaving Length Lact is the distance to point 2 plus half the distance between 1 and 2. 2. Figures in brackets refer to Table 4/3. 3. For diverges, the mirror image shall apply.

Figure 4/13 Measurement of Weaving Length for Ghost Island Layouts

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Figure 4/14 Weaving Length Diagram for Urban Roads

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5. LAYOUT OPTIONS

INTRODUCTION • land take;

5.1 There are two forms of grade separation, namely, • capital cost; grade separated junctions and interchanges. • economic assessment; 5.2 The most efficient form of grade separation is that which presents the driver with the minimum • provision for non-motorised users (this should be assessed using TA 91 (DMRB 5.2.4) and number of clear unambiguous decision points as they HD 42 (DMRB 5.2.5)). drive through the junction and in merging and diverging. Additionally, on a motorway or an all- 5.5 The provision of vehicle restraint systems within purpose road that is generally grade separated, a junction should be in accordance with the current consistency of design for successive junctions is an Requirements for Road Restraint Systems. important consideration involving the adoption of the same Design Speed. This need for consistency also applies to the signing and road markings to be adopted GRADE SEPARATED JUNCTIONS particularly at the boundary of responsibility between different highway authorities. 5.6 A grade separated junction involves the use of an at-grade junction at the commencement or termination 5.3 The siting of a grade separated junction on a hill of slip roads. The at-grade junction element, whether a top should be avoided if possible as approach gradients major/minor junction or roundabout and slip roads, can can cause operational problems in the diverge area, produce 3 main types of grade separated junction: even when the percentage of LGVs is small. Hill top Diamond, Half-Cloverleaf and Roundabout; these are locations could be environmentally damaging to the discussed below. skyline and might present difficulty to drivers in comprehending road signs which are silhouetted against the sky. There is also the risk of drivers being blinded when the sun is low in the sky.

5.4 Among the aspects of design which should be taken into account and included in a decision framework are:

• efficiency;

• safety;

• consistency;

• location;

• maintenance;

• environmental effects;

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Figure 5/1.1 Typical Layouts of Grade Separated Junctions

Note: See also Figure 5/2

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Figure 5/1.2 Typical Layouts of Grade Separated Junctions

Note: See also Figure 5/2

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Figure 5/1.3 Typical Layouts of Grade Separated Junctions

Note: See also Figure 5/2

Diamond movements which for difficult sites, especially in urban areas, may create severe environmental problems. 5.7 A diamond is the simplest form of grade separated junction and the normal layout will provide 5.8 A junction such as the half diamond (Figure turning movements to and from the slip roads by two 5/1.1b), can be designed for restricted traffic staggered junctions (see Figure 5/1.1a). The use of movements. However, if there is a possibility that future non-signalised crossroads is not recommended – see conversion to provide all movements will be required, TD 42 (DMRB 6.2.6). The diamond has the advantage then the original design should be capable of that land take is minimised and slip road design is conversion without alteration to the built layout. simple. Costs are minimised as only one bridge is required, but consideration should be given to inclusion Half-Cloverleaf of a ghost island on the road which crosses the bridge, either at the outset or in the future, as bridge widening 5.9 A half-cloverleaf is used at similar flow levels to at a later stage will be expensive. The disadvantage is a diamond, particularly where site conditions are that all four quadrants are used to provide turning difficult and the use of all four quadrants is not possible

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(see Figures 5/1.2c, 5/1.2d). The at-grade junction element normally utilises two ghost island junctions. Whereas the diamond utilises all four quadrants, which can be a problem in urban or environmentally difficult situations, the half-cloverleaf overcomes this problem by requiring the use of only 2 quadrants, which if possible should be chosen so as to minimise the right turn movements. This layout has similar advantages to the diamond but similarly consideration of future improvement should be given.

Roundabout

5.10 The two most common forms of grade separated junction are the two bridge roundabout and dumb-bell roundabout types – see Figure 5/2 and TD 16 (DMRB 6.2.3). The dumb-bell roundabout which is an intermediate layout between the diamond and the two bridge roundabout has the advantages of reduced cost (only one bridge) and less land take than the two bridge roundabout. It can be adapted to fit either a diamond or half cloverleaf. It also has increased junction capacity and reduced land take compared with the diamond. In urban locations where large flows have to be accommodated, signalised gyratories can be considered.

5.11 For the dumb-bell layout, it is possible that the distance between the two roundabouts may be less than the desirable minimum SSD for the design speed of the connecting link road. In that case, a low (0.26m) object at the give way line of the next roundabout must be visible from a vehicle as it leaves the circulatory carriageway of the previous roundabout. Attention must be given to the needs of future maintenance of the connecting link road to avoid the need for closure of the road. One lane dual carriageways should, therefore, be avoided and single carriageways would often be preferable.

5.12 The most common type of grade separation is the two bridge roundabout. Observation has shown that if they are constructed too large, high circulating speeds on the roundabout can be induced leading to difficulties for joining traffic. Therefore every effort should be made to achieve a design with a small footprint (TD 16 (DMRB 6.2.3)).

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Figure 5/2 Typical layouts of Grade Separated Junctions

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5.13 Where two major roads cross, a 3 level moved into the path of oncoming traffic. This effect has arrangement with a roundabout sandwiched between been most noticeable at junctions where drivers have the two major flows, should be considered as an left long lengths of fully grade separated road. In such alternative to an interchange, see Figure 5/1.3e. Its situations a dumb-bell roundabout is recommended. advantages are that both the overall land take and the Alternatively, a ghost island major/minor junction carriageway area are greatly reduced. The layout can be considered in accordance with TD 42 disadvantages are that structure costs are high and if the (DMRB 6.2.6). If neither option is achievable, the turning movements become greater than predicted, priority junction should be made square to the side road operational problems such as queuing on the as shown in Figure 5/3b, with no merging lanes or roundabout entries can result. If queuing does become a splitter islands and with corner radii in accordance with problem, segregated left turn lanes and restricted TD 42 (DMRB 6.2.6) to emphasise to the driver the circulatory carriageway width should be considered impression of a two way single carriageway, rather than before traffic signals are installed. The inclusion of a a dual carriageway. This needs to be reinforced by clear specific link, as a remedial measure to remove a heavy signing. right turn movement, is rarely a practical solution on either cost or environmental grounds. INTERCHANGES

Variants 5.17 An interchange does not involve the use of an at- grade junction and so provides uninterrupted 5.14 Variants on the three basic types of grade movements for vehicles moving from one mainline to separated junctions (diamond, half cloverleaf and another, by the use of connector roads with a succession roundabout) can be provided if: of diverging and merging manoeuvres. Good design minimises conflict points and ensures that the path a. the junction is 3 way i.e. a T junction; between them is easily understood by drivers, by effective signing and road marking. This design b. not all movements need catering for e.g. a half objective should be assessed within the overall diamond; framework of the points in paragraph 5.4. c. traffic signals, either full-time or part time, are 5.18 Figure 5/4 shows three different 4 way included to remove congestion on an existing interchanges. grade separated junction. It is recommended that they should only normally be considered as an • The 4 level interchange layout has the advantages alternative to a priority junction; of reduced land take, absence of loops and low structural content, but is visually highly intrusive, d. large flows are to be handled and a signalised has the greater number of conflict points and has gyratory is used. therefore been used infrequently. See Figure 5/4.1a. Compact Grade Separated Junctions • The 3 level interchange introduces two loops and 5.15 An alternative for low flow situations in rural and reduces conflict points but increases both environmentally sensitive areas, is a Compact Grade structural content and cost, whilst still being Separated Junction (TD 40 (DMRB 6.2.5). This visually intrusive. A disadvantage is that it provides a junction to a standard intended to enforce requires separate diverge points for left and right low traffic speeds and minimise land take. movements from one of the mainlines, which can be difficult to sign. See Figure 5/4.1b.

At Grade Junction Design A variant of Figure 5/4.1b is shown at Figure 5/5 and is an example of how environmental impact 5.16 Poor design of priority junctions at the end of and structural content can be substantially uphill diverge slip roads can create safety problems. An reduced without a great increase in land take, by example is shown in Figure 5/3a where drivers, taking advantage of the skew of the intersecting approaching the left turn splitter island with a merging mainlines. taper, have misperceived the facing vehicle restraint systems (required by the height of the embankment) as being on a dual carriageway central reserve and have

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See Paragraph 5.16

Figure 5/3 Example of Poor Design Reducing Safety at Diamond Junction

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• A 2 level or ‘cyclic’ interchange is shown in Figure 5/4.2c. This utilises reverse curves and a low number of conflict points, the land take is extensive and there is a high structural content. However, since this form of interchange fits easily into the topography it is a suitable solution for schemes where land is not at a premium. A disadvantage is that it requires separate diverge points for left and right movements from both mainlines, which can be difficult to sign.

Figure 5/4.2c shows two successive diverges off and one merge on to the mainline. A variant of this uses one diverge and two merges but the distance between the merges should be as great as possible to avoid potential conflicts. One principal connection on the mainline for the diverge, and one for the merge, is actually to be preferred with the final route selection occurring on the slip road. This reduces turbulence on the mainline. It would need a suitable multiple lane layout for the actual connection. Site constraints sometimes make it impossible to have the one connection.

5.19 The three way ‘trumpet’ interchange (Figure 5/4.2d) should be designed to enable future conversion to a four way without alteration if this is considered a possibility. It has a 2 way slip road which requires careful design for safety. Figure 5/4.2e shows a three way interchange with restricted movement. This enables high vehicle speeds to be maintained with low land take, but it requires a costly skew structure and prohibits any future conversion.

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(See Paragraph 5.18)

Figure 5/4.1 Typical Layouts of Interchanges

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(See Paragraph 5.19)

Figure 5/4.2 Typical Layouts of Interchanges

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(See Paragraph 5.18)

Figure 5/5 Variant of Figure 5/4.1b Restricted in Height to Reduce Environmental Impact

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5.20 Merges with a flow imbalance, where the Maintenance merging traffic flow is greater than the mainline traffic flow can occur within an interchange. Priority should 5.24 Designers should allow within their designs for still be given to traffic on the mainline. If the merging facilities to maintain areas within interchanges which flow is over a lane capacity, there will need to be a lane are not readily accessible. Locations for access should gain. LGVs must be given an opportunity to join the be chosen having regard to visibility to and from the mainline safely. Operational problems have occurred proposed access location and the need to maintain where the left hand link has been on a long downhill traffic flow through the works. Any lay-by should not section and the right hand link uphill, with be sited in an exposed position on the inside of consequential disparity in vehicle speeds at the merge, connector roads on left hand curves with radii below and this particular layout is not recommended. Desirable Minimum, as vehicles have been observed to move into the hardshoulder on such sections. They 5.21 Loops and certain links may require advisory should be located on straights or right hand curve speed limits (which should be discussed and agreed sections with at least desirable minimum radius. with the Overseeing Organisation) to warn the driver of the safe negotiating speed for reasons of alignment and 5.25 It may be necessary to provide access to isolated visibility. This speed limit should be used in land by means of an underpass linked to adjacent land. conjunction (where appropriate) with a bend warning This is an expensive solution to this problem and the sign and ‘chevron’ warning signs to reinforce the ownership of isolated land should be acquired to hazard warning. Only one level of speed limit should be prevent the need for regular access by an owner other used within an interchange as steps down in speed than the highway authority. limits may confuse the driver. 5.26 Any lay-bys for maintenance vehicles should be 5.22 Single lane interchange links can have provided clear of the hardshoulder or hardstrip. The advantages in cost over 2 lane interchange links for lay-by should be adequate for the maximum number of interchanges which contain structures of substantial vehicles expected to use it at one time. The surfacing of length. However, where the predicted flows are near the the lay-by should not be attractive to other road users top of the range (Tables 3/1a and 3/1b) the uncertainty and should be signed for its purpose. Its surfacing need of the prediction should be recognised, as it may be only be adequate for its expected use. prohibitively expensive to convert later to a two-lane interchange link. A disadvantage is that single lane Connector Roads interchange links may require closure during certain maintenance activities. Consequently, a whole life cost assessment (including costs during maintenance) should 5.27 Two way slip roads must be dual be carried out to confirm the cost effectiveness of carriageway with opposing traffic separated by a proposed single lane links. physical central reserve with vehicle restraint system. Two way single carriageway slip roads are GENERAL not permitted. Two way slip roads only occur at half-cloverleaf and trumpet junctions. Studies into the safety of tight loops for 2 way slip roads, as Link Roads compared to one way, indicated that a physical barrier will improve safety and reduce cross-over 5.23 When two grade separated junctions with high accidents. flows are closely spaced, potential weaving problems caused by the short length of carriageway available can be removed by the inclusion of link roads. No link 5.28 For motorway interchanges emergency should be provided between carriageways other than at telephones should not be sited in an exposed position the start and finish of the segregated lengths of on the inside of connector roads on left hand curves carriageway. An example of such a junction is shown at with radii below Desirable Minimum, as vehicles have Figure 5/6 where weaving is separated from the been observed to move into the hardshoulder on such mainline flow. sections. They should be located on straights or right hand curve sections with at least desirable minimum radius. Advice on the provision of emergency telephones on motorways is given in TA 73 (DMRB 9.4.2). Note that there are separate Annexes for England, Scotland, Wales and Northern Ireland.

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Figure 5/6 Example of Link Road Interchange

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5.29 The accident risk for slip roads is similar whether 5.34 The lighting of the main carriageway will depend the mainline is carried over or under. However, the on an appraisal carried out in accordance with TA 49 preferred treatment is to design diverge slip roads uphill (DMRB 8.3). The design of the lighting will then be and merge slip roads downhill, with the side road over carried out in accordance with TD 34 (DMRB 8.3). the mainline. This assists vehicles on the slip roads in matching their speeds to those of mainline vehicles on merging and reducing their speeds at the approach to 5.35 It is normal practice to light grade separated the side on diverging. junctions (i.e. the roundabout, the T-junction etc). The lighting of the grade separated junction would normally extend 60m along each entry or exit slip 5.30 Private means of access and junctions on road without lighting the mainline carriageway. connector roads are not permitted. However, sometimes a decision may be taken to extend the slip road lighting to include the full length of the slip roads. When the full length of the Merging and Diverging Lanes slip road is lit, the mainline carriageway must be lit all through the junction. Drivers approaching on 5.31 Mainline lane drops within a junction on a 3-lane the mainline carriageway may otherwise think that mainline (3 lanes prior to the diverge, 2 lanes between they are coming to a lit area and drive up the slip diverge and merge and then back to 3 lanes) are not road thinking that it is the mainline carriageway. generally recommended on operational and safety grounds. They severely impair future maintenance, 5.36 The provision and layout of traffic signs and especially at interchanges where no reasonable road markings is an integral part of the junction diversion route is available. However, if such a layout design process and must be considered at an early becomes necessary the requirements of paragraph 4.32 stage. Advance direction and warning signs must should be followed. be provided. Positioning of signs within the junction must be carefully considered so that they do not interfere with drivers’ visibility. It is 5.32 A lane drop at a junction diverge must be essential that there is no over-provision of signing. used when changing carriageway standards from 4 lanes to 3 or 3 lanes to 2. Similarly, a lane gain at a 5.37 For grade separated junctions two or three junction merge must be used when changing advance direction signs must be provided. These carriageway standards from 2 lanes to 3 or 3 lanes are to be located at the start of the diverging lane, to 4. The layout of the diverge or merge should be 1 ½ mile ( 3 mile in difficult circumstances) from selected corresponding to the leaving or joining the junction and additionally for motorways and flow but under light flow conditions could be some all-purpose roads 1 mile ( 2 mile in difficult Figure 2/6.2C and Figure 2/4.3E. Removal of a 3 circumstances) from the junction. On motorways lane (excluding climbing lanes) must not take either a confirmatory gantry sign or a route number place on the link between junctions. confirmatory sign (TSRGD diagram 2910), located at the back of the nose, must be provided.

Signing and Lighting 5.38 Countdown markers (TSRGD diagram 823, 824 and 825) must be provided on the approaches 5.33 Signing and lighting should be considered at the to all diverges. They must not be provided for lane earliest stage of design to ensure the satisfactory drops. operation of a grade separated junction for all users, including cyclists and pedestrians and to ensure that allowances are included for signing and lighting 5.39 Further requirements and advice for signing are equipment such as columns, feeder pillars, buried given in the TSRGD, The Traffic Signs Manual, DMRB cables, cable ducts and draw pits. Volumes 8 and 9 and Local Transport Note 1/94.

February 2006 5/15 Volume 6 Section 2 Chapter 6 Part 1 TD 22/06 Facilities for Non-Motorised Users

6. FACILITIES FOR NON-MOTORISED USERS

Introduction 6.6 Where slip road traffic joins or leaves the local network in free flow conditions, uncontrolled 6.1 This chapter gives guidance on the provision for non-motorised users (NMUs) crossing grade separated NMU crossings must be avoided. NMUs must be junctions. It considers the needs of pedestrians provided for separately and it may be necessary to (including the disabled), cyclists and equestrians. provide grade separation for these users.

6.2 NMUs have a legal right to use the public highway, unless specifically prohibited, as in the case of 6.7 For all-purpose trunk roads, where grade Special Roads (including Motorways). All-purpose separated junctions are provided, the provision for trunk roads typically carry high flows of fast moving cyclists will depend on whether cyclists are using a traffic, are designed primarily for such use and are broadly parallel off carriageway route (OCR) or are generally unattractive to NMUs. A better standard of travelling on the trunk road carriageway. provision for NMUs may encourage modal shift from motorised vehicles and may play a part in creating a 6.8 Where an OCR is provided, it should allow more integrated and sustainable transport system along cyclists to cross only at the downstream end of diverge trunk road routes that often provide the most direct slip roads or at the upstream end of merge slip roads. route between key destinations. Scheme designs should These will be at similar locations as the crossing points take account of opportunities to provide safe and for other NMUs and will avoid proliferation of crossing attractive provision. points. At the crossing locations there should be adequate visibility for both drivers and NMUs. Advice 6.3 The need for facilities for NMUs, will be on the design of such crossings is given in TA 90 identified by the NMU Audits see HD 42 (DMRB (DMRB 6.3.5). 5.2.5). 6.9 At grade separated junctions, cyclists using the 6.4 The design of facilities for NMUs is addressed in main line and crossing the mouths of slip roads are at the relevant sections of the DMRB and particularly in risk of coming into conflict with merging or diverging the advice notes: traffic. Traffic Advisory Leaflet TAL 1/88 Provision for Cyclists at Grade Separated Junctions contains advice • The Geometric Design of Pedestrian, Cyclist and on how to take a cycle route through these junctions but Equestrian Routes – TA 90 (DMRB 6.3.5); and cyclists should also be offered the alternative of leaving the main line and being diverted around the junction. In • Provision for Non-Motorised Users – TA 91 this case, a cycle by-pass route running broadly parallel (DMRB 5.2.4). to both slip roads should be linked by the local road network or some other dedicated provision. A cycle Provision for Cyclists route running alongside an on-slip road but segregated from it, should not rejoin the carriageway until it reaches a point beyond the end of the merge taper 6.5 Grade separation at junctions is provided to where it is safe to do so. allow vehicles to join or leave the main line with minimum disruption to through traffic. The speed Other Advice on Facilities for NMUs of diverging and merging traffic is similar to that of the mainline flow and at on-slips in particular, 6.10 Advice on the provision of facilities for NMUs drivers are concentrating on the merge. It is can be found in the following documents: essential therefore that NMU crossings of slip roads are only located where traffic is moving – Local Transport Note 1/86 Cyclists at Road relatively slowly, i.e. away from the main line. Crossings and Junctions.

– Department of Transport Traffic Advisory Unit Leaflet 1/88 Provision for Cyclists at Grade Separated Junctions.

February 2006 6/1 Volume 6 Section 2 Chapter 7 Part 1 TD 22/06 References

7. REFERENCES

HD 42 DMRB 5.2.5 Non-Motorised Users Audit

TA 23 DMRB 6.2 Junctions and Accesses: Determination of Size of Roundabouts and Major/Minor Junctions

TA 30 DMRB 5.1.4 Choice Between Options for Trunk Road Schemes

TA 46 DMRB 5.1.3 Traffic Flow Ranges for use in the Assessment of New Rural Roads

TA 48/92 DMRB 6.2.2 Layout of Grade Separated Junctions (SUPERSEDED)

TA 49 DMRB 8.3 Appraisal of New and Replacement Lighting on Trunk Roads and Trunk Road Motorways

TA 58 DMRB 8.2.1 Traffic Signs and Road Markings for Lane Gains and Lane Drops on All-Purpose Dual Carriageways and Motorway Trunk Roads

TA 73 DMRB 9.4.2 Motorway Emergency Telephones

TA 79 DMRB 5.1.3 Traffic Capacity of Urban Roads

TA 89 DMRB 8.2.2 Use of Passively Safe Signposts, Lighting Columns and Traffic Signal Posts to BS EN 12767

TA 90 DMRB 6.3.5 Geometric Design of Pedestrian, Cyclist and Equestrian Routes

TA 91 DMRB 5.2.4 Provision for Non-Motorised Users

TD 9 DMRB 6.1.1 Highway Link Design

TD 16 DMRB 6.2.3 Geometric Design of Roundabouts

TD 18 DMRB 9.1 Criteria for the Use of Gantries for Traffic Signs and Matrix Traffic Signals on Trunk Roads and Trunk Road Motorways

TD 22/92 DMRB 6.2.1 Layout of Grade Separated Junctions (SUPERSEDED)

TD 22/05 DMRB 6.2.1 Layout of Grade Separated Junctions (Interim Revision February 2005) (SUPERSEDED)

TD 27 DMRB 6.1.2 Cross-Sections and Headrooms

TD 34 DMRB 8.3 Design of Road Lighting for the Strategic Road Network (IN PREPARATION)

TD 39 DMRB 6.2.4 The Design of Major Interchanges

TD 40 DMRB 6.2.5 The Layout of Compact Grade Separated Junctions

TD 41 DMRB 6.2.7 Vehicular Access to All-Purpose Trunk Roads

TD 42 DMRB 6.2.6 Geometric Design of Major/Minor Priority Junctions

TD 50 DMRB 6.2.3 Geometric Layout of Signal Controlled Junctions and Signalised Roundabouts

February 2006 7/1 Chapter 7 Volume 6 Section 2 References Part 1 TD 22/06

TD 51 DMRB 6.3.5 Segregated Left Turn Lanes and Subsidiary Deflection Islands at Roundabouts

Traffic Signs Manual; Chapters 1 to 8

The Traffic Signs Regulations and General Directions (TSRGD)

Local Transport Note 1/94: The Design and Use of Directional Informatory Signs

Local Transport Note 1/86: Cyclists at Road Crossings and Junctions

DfT Traffic Advisory Unit Leaflet: 1/88 Provision for Cyclists at Grade Separated Junctions

BS 6100 Subsection 2.4.1: 1992 Glossary of building and civil engineering terms. Civil engineering. Highway, railway and airport engineering. Highway engineering

The Highway Code

7/2 February 2006 Volume 6 Section 2 Chapter 8 Part 1 TD 22/06 Enquiries

8. ENQUIRIES

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

Chief Highway Engineer The Highways Agency 123 Buckingham Palace Road London G CLARKE SW1W 9HA Chief Highway Engineer

Chief Road Engineer Scottish Executive Victoria Quay Edinburgh J HOWISON EH6 6QQ Chief Road Engineer

Chief Highway Engineer Transport Wales Welsh Assembly Government Cathays Parks M J A PARKER Cardiff Chief Highway Engineer CF10 3NQ Transport Wales

Director of Engineering The Department for Regional Development Roads Service Clarence Court 10-18 Adelaide Street G W ALLISTER BT2 8GB Director of Engineering

February 2006 8/1

Ionad Ghnó Gheata na Páirce, www.tii.ie +353 (01) 646 3600 Stráid Gheata na Páirce, Baile Átha Cliath 8, Éire

Parkgate Business Centre, Parkgate Street, [email protected] +353 (01) 646 3601 Dublin 8, Ireland