Traffic Signs Manual/Chapter 5/2009/10

10GRADE SEPARATED JUNCTIONS

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DESIGN

10.1 Grade separated junctions may involve merging and diverging lanes, or the gain or loss of lanes, or a combination of these. As junctions become more complex, so road marking layouts become more complicated. It is not practicable to detail all possibilities, but the standard principles should be followed. Reference should be made to TD 22 / 92 in Volume 6 of the Design Manual for Roads and Bridges (see para 1.4) for further guidance. When designing a complex layout, it should be borne in mind that it must be capable of being signed and marked in a way that drivers can readily understand.

MERGING AND DIVERGING SLIP ROADS

10.2 The layout of markings and road studs at the simplest grade separated junction is shown in figure 10-1. The principal dimensions are detailed in tables 10-1 and 10-2. This arrangement is suitable forone or two-lane exit and one-lane entry slip roads.

10.3 The nosing of the slip road is marked using diagram 1042 (varied where appropriate to reverse the direction of the chevrons, see para 4.62). Red studs are provided at 3 m centres along the outside of both edges.

Table 10-1 Merge and lane-gain markings

Road type	Speed limit (mph)	Length of entry taper (m)	Taper for minimumangle at nose	Nose length (m)	Length of ghost island tail (m)	Width of diag 1010 marking (mm)
		(1)	(2)	(3)	(4)
Rural motorway	70	205	1 in 40	115	180	200
Rural dual carriageway	70	150	1 in 30	85	150	200
	60 or less	130	1 in 25	75	150	150
Urban road	60	95	1 in 15	50	n/a	150
	50 or less	75	1 in 12	40	n/a	150

NOTE: Numbers (1) to (4) at the head of the above columns relate to features shown on figures 10-1 to 10-6.

Table 10-2 Diverge and lane-drop markings

Road type	Speed Limit (mph)	Length of exit taper(m)		Taper for minimum angle at nose	Nose length(m)	Width of diag 1010 marking (mm)
		1 lane	2 lanes
		(5)1	(6)1,2	(7)2	(8)2
Rural motorway	70	170	185 (150)	1 in 15	80	200
Rural dual carriageway	70	150	150 (120)	1 in 15	70	200
	60	130	130 (110)	1 in 15	70	150
Urban road	60	95	110 (90)	1 in 15	50	150
	50 or less	75	90 (75)	1 in 12	40	150

NOTES

1. Taper lengths refer to 2×3.65 m lanes, or, in brackets, 2×3.00m lanes.
2. Numbers (5) to (8) at the head of the above columns relate to features shown on figures 10-1 and 10-7.

10.4 The edge line should be continued along the slip road. Where raised rib markings are used, the rib spacing should be reduced to 250 mm on motorway slip roads. Red reflecting road studs should be used on the near side and amber on the off side, both at 18 m centres except as detailed in para 10.5.

10.5 The main carriageway edge line should change to diagram 1010, with green reflecting road studs at 8 m centres, across the slip road over which traffic joins or leaves. Along the same length, the spacing of the red studs associated with the edge line to diagram 1012.1, 1012.2 or 1012.3 at the near side of the slip road is reduced from 18 m to 9 m. The bifurcation arrow to diagram 1039 should be used as shown in figure 13-8 at the lengths specified in para 13.12.

10.6 Conventional lane lines to diagram 1005 or 1005.1 (see table 10-4) on the main carriageway should be continued through the junction.

10.7 An entry layout for two-lane slip roads is shown in figure 10-2, allowing each lane of the slip road to join separately. The ghost island between the joining lanes should be to diagram 1042, bordered by red studs. This layout is suitable where the main line flow is light, the main carriageway is three or more lanes wide and the merging flow exceeds the capacity of a single lane.

LANE GAINS

10.8 The principal dimensions for lane-gain layouts are given in table 10-1. There are two basic situations; the number of lanes gained will either be the same as, or less than, the number of lanes on the entry slip road.

10.9 Where the number of lanes gained equals the number of lanes on the entry slip road, the markings in figure 10-3 should be used. All lane markings to diagram 1005.1 on both the main carriageway and the slip road should change to 1004.1 as indicated (or from 1005 to 1004 on roads where the speed limit is not more than 40 mph; see table 10-4). If only one lane is gained, the ghost island is not used.

10.10 Where the number of lanes gained is less than the number of lanes on the entry slip road, there are three methods of carrying out the merge:

(i) the two-lane slip road is reduced to one lane in advance of the back of the nose, so that the number of lanes on the entry slip road equals the number of lanes gained. The lane reduction should be effected by the use of hatched markings to diagram 1040.3 (see para 4.53 and figure 10-4). The taper length should be determined from table 10-3 and the full reduction in width achieved 50 m in advance of the back of the nose. The hatching should then be extended to the end of the slip road to form part of the nose. Additionally, at least two deflection arrows to diagram 1014 should be placed in the lane which is to be discontinued. The tip of the final arrow should be 30 m before the start of the taper and preceding arrows at intervals of 30 m in advance of that. The markings should be supplemented by signs to diagram 872.1,

(ii) the right hand slip lane merges with the through carriageway before the left hand slip lane is added (figure 10-5). All lane markings to diagrams 1005 or 1005.1 should change to 1004 or 1004.1 respectively (see table 10-4), commencing at the first sign to diagram 874 on the slip road and at least 200 m prior to the merge nose tip on the main carriageway. These markings should be continued for at least 50 m beyond the termination of the ghost island. Road markings to diagram 1010 should extend from the tip of the merge nose to the point where it meets the ghost island. Ahead arrows to diagram 1038 should be placed on the joining (additional) lane and on the original left lane of the main carriageway at the point they come together, to discourage premature lane changing,

(iii) the left hand slip lane merges with the additional main carriageway lane after the right hand slip lane has been added to the through carriageway (figure 10-6). In this option it is not easy for slower slip road traffic to merge into the faster lanes; it is also difficult to sign. Reference should be made to the appropriate overseeing authority before it is used.

Table 10-3 Taper for slip road lane reduction

85 percentile speed (mph)	Preferred minimum taper	Absolute minimum taper
30	1 in 40	1 in 20
40	1 in 40	1 in 30
50	1 in 45	1 in 40
60	1 in 50	1 in 50
70	1 in 55	1 in 55

NOTE: The preferred minimum taper should be used wherever practicable; the absolute minimum should be used only where unavoidable.

Table 10-4 Size of markings in figures 10-1 to 10-7

Diagram number		Size of marking (Length x Gap x Width)
Speed limit (mph)		Speed limit (mph)
40 or less	50 to 70	40 or less	50	60	70
1004	1004.1	4 m x 2 m x 100 mm	6 m x 3 m x 150 mm	6 m x 3 m x 150 mm	6 m x 3 m x 150 mm
1005	1005.1	1 m x 5 m x 100 mm	2 m x 7 m x 150 mm	2 m x 7 m x 150 mm	2 m x 7 m x 150 mm
1010	1010	1 m x 1 m x 100 mm	1 m x 1 m x 150 mm	1 m x 1 m x 150 mm	1 m x 1 m x 200 mm

LANE DROPS

10.11 A standard lane-drop layout for high speed roads is shown in figure 10-7. The lane markings dividing the through lanes from the lane or lanes to be dropped should change to diagram 1004.1 (see table 10-4 for width) at the one mile advance direction sign. From the half-mile advance direction sign to the diverge nose tip, the marking changes again to diagram 1010. Road studs used in conjunction with the diagram 1010 marking must be green. The studs will be spaced 18 metres apart from the half-mile sign to the final advance direction sign, then closed up to 8 metre centres to the tip of the diverge nose. On lower-speed roads, or where the advance direction signs are sited at two thirds and one third of a mile from the junction, the lane-drop markings will commence later and the stud spacings adjusted to suit. The appropriate widths and modules for these markings are detailed in table 10-4.

The principal dimensions for lane-drop layouts are given in table 10-2. The table indicates the length of the exit taper depending upon the number of extra lanes provided on the slip road. In the case of the lane drop shown in figure 10-7, two lanes on the slip road represents an increase of one over the approach lane and therefore the taper length shown in the table should be as indicated under the heading (5).

AUXILIARY LANES

10.13 Auxiliary lanes are additional lanes at the side of the main carriageway between the nose and the entry or exit taper at a junction. They are used at merge and diverge junctions and also at lane gain and lane drop junctions, and increase capacity by providing increased merge or diverge opportunity and additional space for weaving. They are separated from the main carriageway by the marking to diagram 1010. Further details may be found in TD 22 / 92 in Volume 6 of the Design Manual for Roads and Bridges (see para 1.4).

This work has been mirrored or sourced from material provided on the UK Government website at https://www.gov.uk/government/publications/traffic-signs-manual, which states material is available under the terms of the Open Government License unless indicated otherwise. (Crown copyright acknowledged.)

This is NOT an official transcription, and should NOT be considered in any way a controlled copy of the work in question.

This document no longer necessarily represents current practice, as it relates to an earlier version of the regulations (TSGRD 2002 as amended at publication date) rather than the current TSRGD 2016 regulations. It should not be used for actual design or operational use without consulting a more recent edition.

This version was transcribed in May 2015, from a 2009 impression.