TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 90
carriageway shall be 0.5%. The maximum longitudinal gradient shall be 2.5%. Gradients outside these
limits will require a Departure from Standard.
6.8.2
Crossfall on the Circulatory Carriageway
Crossfall is required to drain surface water on circulatory carriageways. The normal value is 2% (1 in
50). It should not exceed 2.5% (1 in 40). To avoid ponding, longitudinal edge profiles should be graded
at not less than 0.67% (1 in 150), with 0.5% (1 in 200) considered the minimum. The design gradients
do not in themselves ensure satisfactory drainage, and, therefore, the correct siting and spacing of
gullies is critical. Gullies shall not be located at any pedestrian crossing location.
At roundabouts on high speed roads, shall be arranged for crossfall to assist vehicles. To do this, a
crown line is formed. This line can either join the ends of the channelising islands as shown in Figure
6.25, or divide the circulatory carriageway in the proportion 2:1 internal to external (Figure 6.26). In
some cases a subsidiary crown line may assist in achieving appropriate values of crossfall without
giving excessive changes at the main crown line (Figure 6.27).
Figure 6.25: Using One Crown Line to Join Channelising Islands
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 93
6.10
Kerbing and Verge Width
Roundabout entries and exits should be kerbed, and hard strips or hard shoulders on each approach
should terminate where entry widening begins. Where kerbs are not present on approach links, the
kerbs shall start on the approach at the back of the hardstrip and then terminate the hard strip edge
line in a short smooth curve or taper (see Figures 6.28 and 6.29). On the exit, the kerbing should
terminate where the hard strip starts.
The verge width should be at least 2.5m and should generally be consistent around the roundabout.
Further advice is given in DN-GEO-03036. Factors that should be taken into account in determining
verge width include:
a)
visibility requirements;
b)
space required to
accommodate buried services, road signs and other street
furniture;
c)
maintenance access;
d)
any likely future traffic increases that could require an increase in carriageway
width.