TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 142
Figure 7.19C: Illustration of Stopping Sight Distance on Slip Road
Notes:
This figure shows the situation when the distance from the back of the nose to any stop or yield line is less than the Desirable Minimum
Stopping Sight Distance for the mainline (SSD1).
This figure does not apply to loop roads.
For measurement of Stopping
Sight Distance to the yield line of
a roundabout, as described in
this standard
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 143
7.15
Lane Drop/Lane Gain and Through Carriageway
Where a 3-lane carriageway is reduced to 2 lanes by means of a lane drop at a junction as shown in
Figure 7.20, provision must be made on the link between the lane drop and a 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 nearest paved edge 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 7.20. 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.
Advice on the signing of lane gains and lane drops is given in the Traffic Signs Manual.
Figure 7.20: Lane Drop to Two Lanes and Subsequent Lane Gain Showing Hatched Pavement for
Maintenance and Traffic Management
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 144
7.16
Interchanges
An interchange does not involve the use of an at- grade junction and so provides uninterrupted
movements for vehicles moving from one mainline to another, by the use of connector roads with a
succession of diverging and merging manoeuvres. Good design minimises conflict points and ensures
that the path between them is easily understood by drivers, by effective signing and road marking.
This design objective should be assessed within the overall framework of the points below:
a)
efficiency;
b)
safety;
c)
consistency;
d)
location;
e)
maintenance;
f)
environmental effects;
g)
land take;
h)
capital cost;
i)
economic assessment;
j)
provision for non-motorised users (crossing the junction)
Figure 7.21 shows three different 4 way interchanges.
a)
The 4 level interchange layout has the advantages of reduced land take, absence
of loops
and low structural content, but is visually highly intrusive, has the greater
number of conflict points and has therefore been used infrequently. See Figure
7.21.1a.
b)
The 3 level interchange introduces two loops and reduces conflict points but
increases both structural content and cost, whilst still being visually intrusive. A
disadvantage is that it requires separate diverge points for left and right
movements from one of the mainlines, which can be difficult to sign. See Figure
7.21.1b.
A variant of Figure 7.21.1b is shown at Figure 7.22 and is an example of how environmental impact
and structural content can be substantially reduced without a great increase in land take, by taking
advantage of the skew of the intersecting mainlines.
The three way ‘trumpet’ interchange
(Figure 7.21.2c) should be designed to enable future conversion
to a four way. It has a 2 way slip road which requires careful design for safety. Figure 7.21.2d shows
a three way interchange with restricted movement. This enables high vehicle speeds to be maintained
with
low land take, but it requires a skew structure and prohibits any future conversion.