TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 121
7.7
Geometric Design Parameters
The geometric parameters applicable to merges and diverges must be those in Tables 7.1 and 7.2
respectively. Figures 7.4 and 7.6 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 7.4 does
not require the use of larger radii. For diverges, the layout of the edge line should incorporate the radii
shown on Figure 7.6.
Table 7.1: Geometric Design Parameters for Merging Lanes
Road Class
Length of
entry taper
(m) (1)
Nose
Ratio
(see
Note
1) (2)
Nose
Length
(see
Note 1)
(3)
Minimum
auxiliary
lane
length (m)
(4)
Length
of
auxiliary
lane
taper (m)
(5)
Length
of
Ghost
Island
Tail (m)
(6)
Reduction Taper (See
Note 2) (7)
4m
lane
3.65m
lane
Preferred
Absolute
Min
Motorway
Rural Motorway
Mainline
220
205
1:40
115
230
75
180
n/a
Within
Interchange
150
130
1:25
75
160
55
150
n/a
Type 1 Dual Carriageway National roads
100 km/h
140
130
1:25
75
160
55
150
n/a
80 km/h
speed limit
80
75
1:12
40
100
40
n/a see
Note 3
1:40
1:30
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 7.4.1 to 7.4.5
Table 7.2:
Geometric Design Parameters for Diverging Lanes
Road Class
Length of exit taper (m)
Nose
Ratio (See
Note 1)
(3)
Nose
length (m)
(4)
Minimum
auxiliary
lane
length (m)
(5)
Length of
aux lane
taper (m)
(6)
4.00m
lane (1)
2x3.65m
lane (2)
2x3.00m
lane (2)
Motorways
Rural Motorway
Mainline
180
185
150
1:15
80
200
75
Within
Interchange
130
130
110
1:15
70
150
55
Type 1 Dual Carriageway National roads
100 km/h
130
130
110
1:15
70
150
55
80 km/h
speed limit
80
90
75
1:12
40
100
40
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 7.6.1 to 7.6.4
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 122
Where, in a merge on a rural motorway, it is anticipated that the connector road and mainline will
frequently be carrying traffic flows approaching their design capacities, it is desirable to extend the
minimum auxiliary lane length of 230 m (Table 7.1) to 370 m. As a guide, this should be considered
when connector road and mainline flows reach 85% of capacity, as defined in the Traffic Flows section
later in this chapter, for more than 1,000 hours per year. Figure 7.7 shows an example for the layout
of a ghost island merge with lane gain. Within larger interchanges, 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 considered as a safety hazard.
In order to allow merging drivers using an auxiliary lane to match their speed with those on the mainline
where there is an uphill section of road, the auxiliary lane must be extended beyond the crest
sufficiently to enable the end of the auxiliary lane to be clearly visible to drivers when:
a)
the uphill section of road would be sufficiently steep
to require a climbing lane; or
b)
the proportion of HGVs is greater than 10% and the uphill mainline gradient is in
excess of 2% and within 0.5 km of the crest.
For extended auxiliary lanes in merges, of length greater than that given in column (4) of Table 7.1, a
sign showing the number of lanes ahead, also that traffic in the slip road must merge into the mainline
nearside lane with a distance plate ‘200m’, should be placed 200m from the start of the taper. For very
long auxiliary lanes in merges consideration should be given to additional signs with the appropriate
distance plates, see Traffic Signs Manual.
Emergency telephones and other equipment that requires vehicles to stop for assistance or road
maintenance must be sited a minimum of 100m from the termination of merges where vehicles may
overrun onto the hard shoulder or hard strip.
Where a diverge connector road has a single lane, a single auxiliary lane is appropriate.
7.8
Successive Merges or Diverges Within Interchanges
Where there are closely spaced successive merges or diverges on mainlines and connector roads
within a junction or interchange (Figure 7.8), the minimum spacing between the tips of noses must be
3.75V m, where V is the design speed in km/h, 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).
At a fork within an interchange link, the taper must be developed as shown in Table 7.3 and Figure
7.9.