TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 55
The direct taper length is the length over which the width of a right turning lane is developed. For ghost
island junctions, right turning lanes shall be introduced by means of a direct taper whose length is part
of the deceleration length, and whole length depends on the design speed. This taper length is given
in Table 5.10. In difficult circumstances, the Designer may relax the taper value by one
design speed
step as a Relaxation.
Table 5.9: Tapers for Ghost Islands
Design Speed (km/h)
Taper for Ghost Island
50
1:20
60
1:20
70
1:20
85
1:25
100
1:30
Note: In difficult circumstances, the Designer may relax the taper value by one design speed step as a Relaxation.
Table 5.10: Direct Taper Length
Design Speed (km/h)
Direct Taper Length (m)
50
5
60
5
70
15
85
15
100
25
120
30
Note: In difficult circumstances, the Designer may relax the taper value by one design speed step as a Relaxation.
5.6.11.3
Turning lane width
For new junctions, the width of a ghost island turning lane shall be 3.5m, but a Relaxation to 3.0m is
permissible. On rural roads, with design speeds above 85km/h or where hard strips are present, widths
greater than 3.65m are not permitted.
5.6.11.4
Turning Lanes
The overall length of a turning lane provided at ghost island junctions depends on the major road
design speed and the gradient. It consists of the turning length and the deceleration length, and shall
be provided in accordance with Table 5.11. The gradient is the average for the 500m length before
the minor road. In difficult circumstances, the Designer may relax the deceleration length by one
design speed step as a relaxation.
Table 5.11: Deceleration Length (m) for Ghost Island
Design Speed
(km/h)
Up Gradient
Down Gradient
0-4%
Above 4%
0-4%
Above 4%
50
25
25
25
25
60
25
25
25
25
70
40
25
40
40
85
55
40
55
55
100
80
55
80
80
Note:
In difficult circumstances, the Designer may relax the deceleration length by one design speed step as a relaxation.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 56
5.6.11.5
Ghost Island Junction located on a curve
Where junctions are located on curves, ghost islands should be introduced asymmetrically on the
outside of the curve to suit the circumstances (as indicated in Figure 5.25). Where a Departure from
Standard has been granted for a junction on a climbing lane, the same principles would apply. It is
also appropriate to introduce islands asymmetrically in other circumstances. This can have the benefit
of avoiding expense (for example Statutory Undertakers' works). If the widening is developed to the
minor road side, through traffic will be deflected where crossing movements at the minor road take
place, which may be a benefit.
It should be noted that with asymmetrical ghost islands that the taper ratio will be the same as for a
symmetrical island but the taper length will be considerably greater. Designers shall ensure that with
any design whether, with symmetrical or asymmetrical ghost islands; a smooth flowing alignment is
achieved.
Figure 5.25: Ghost Island Junction with Curve on major road
5.6.12
Channelising islands and Refuges
Merging and diverging movements can usefully be separated by channelising islands so that the
number of traffic conflicts at any point is reduced (as indicated in Figure 5.20, 5.21 and 5.22).
Channelising islands shall be provided in the mouth of the minor road at ghost island junctions, and
may be similarly provided
at simple junctions, to:-
a)
Give guidance to long vehicles carrying out turning movements.
b)
Channelise intersecting or merging traffic streams.
c)
Warn drivers on the minor road that a junction is ahead.
d)
Provide shelter for vehicles / cyclists waiting to carry out manoeuvres such as
waiting to turn right.
At rural ghost island junctions channelising islands shall be used in the mouth of the minor road. The
recommended layout of channelising islands is shown in Appendix B. This layout shall be designed in
accordance with the step by step approach also provided within Appendix B.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 57
Rural channelising islands shall be raised and kerbed and constructed in accordance with Standard
Construction Detail CC-SCD-01110.
Channelising islands shall have a width of at least 2.5 metres, and shall be conspicuous in poor lighting
conditions. Smaller islands may be defined by road markings. The risk of overriding the islands can
be reduced by offsetting the approach nose from the edge of the vehicle paths.
Rural crossroads are regarded as a Departure from Standards, however details are also included in
Appendix B for use if considering upgrading existing rural cross-roads.
5.7
Diverging/Merging Tapers and Lanes at Junctions
Merge and diverge auxiliary lanes and tapers shall not be provided on new single carriageway roads.
5.8
Priority Junctions on climbing lanes
The provision of a priority junction on a climbing lane is a Departure from Standards.
5.9
Drainage and Crossfall
From considerations of surface water drainage and road user comfort, the road camber on the major
road shall be retained through the junction and the minor road graded into the channel line of the
major road. Checks shall be made for flat areas at all changes of gradient, superelevation or crossfall
in accordance with DN-GEO-03031.
5.10
Roads with Hard Shoulders
Where hard shoulders are provided on roads with priority junctions, particular care should be taken to
ensure safe designs for the junctions. The layouts should be in accordance with the geometric
requirements described in the preceding paragraphs of this Chapter, together with the following
additional requirements.
5.10.1
Major road requirements opposite a junction
On the side of the major road opposite a simple T- junction, the hard shoulder shall be maintained
through the junction.
On the side of the major road opposite a ghost island junction the hard shoulder shall be tapered to
form a 1.5m hard strip as illustrated in Figure 5.26. On the approach side the taper shall be at an angle
of 1:30 relative to the adjacent lane. This taper shall terminate at the start of the deceleration length.
On the departure side, the hard shoulder may be reintroduced over the length of the ghost island
taper. The running lane should be a constant width through the junction. Where the required paved
width is less than the full paved width away from the junction, the full paved width should be continued
through the junction, with any excess area hatched.
When considering upgrading an existing simple priority junction to provide a ghost island right turn
lane, the hard shoulder may be reduced to 0.5m if the existing pavement is to be used to facilitate
widening through the junction or there are issues with the required visibility splays from the minor
road. If a 1.5m hard strip can be achieved however, this shall be provided.