TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 88
Figure 6.23: Visibility to Right along Circulatory Carriageway in Advance of Yield Line
6.7.5
Circulatory Visibility
Drivers on the circulatory carriageway shall be able to see the full width of the circulatory carriageway
ahead of them for the Visibility Distance given in Table 6.1. This visibility must be provided at a
distance of 2m in from the central island, as shown in Figure 6.24. The envelope of visibility must be
obtainable from
a driver’s eye height of between 1.05m and 2m to an object height of between 1.05m
and 2m.
The circulatory visibility needs to be checked to ensure it is not obstructed by landscaping or other
features. Normally, at least the outer 2m of the central island should be hard
standing or planted with
grass or similar low level vegetation.
Figure 6.24: Circulatory Visibility Required
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 89
6.7.6
Pedestrian/Cyclist Crossing Visibility
Drivers approaching a roundabout with a pedestrian/cyclist crossing across the entry shall be able to
see the full width of the crossing so that they can see whether there are pedestrians/cyclists wishing
to cross. The visibility required is the Desirable Minimum Stopping Sight Distance for the design speed
of the link included in DN-GEO-03031.
Pedestrians must be able to see and be seen by the approaching traffic. The visibility should not be
obscured or restricted by parked vehicles, trees or street furniture etc. This is particularly relevant at
roundabouts with two lane entries, where there are stationary vehicles in lane 1; these can obstruct
the visibility of pedestrians and cyclists.
At the yield line, drivers must be able to see the full width of a pedestrian/cyclist crossing across the
next exit if it is within 15m of the yield line on that arm.
6.7.7
Exit Visibility
On the circulatory carriageway, the exit visibility shall conform to Table 6.1. Once a vehicle has
crossed the inscribed circle at the exit from the roundabout, the Stopping Sight Distance shall conform
to DN-GEO-03031.
The provision of new priority junctions or direct accesses shall not be permitted within 90m of the exit
from a roundabout.
6.7.8
Visual Intrusion
Signs and street furniture should be located and designed so as not to obstruct road users’ visibility.
However, isolated objects less than 550mm wide such as lamp columns, sign supports or bridge
columns within the visibility envelopes are acceptable.
6.7.9
Visibility at Grade Separated Junctions
At roundabouts on grade separated junctions in particular, care is needed to ensure that the yield line
is clearly visible to approaching drivers. This shall be achieved by the provision of a section of level
approach road, with a maximum longitudinal gradient of 2%, from 25m prior to the yield line (subject
to the requirements for minimum crossfall and longitudinal gradient for drainage set out in DN-GEO-
03031 and as outlined in the following paragraphs). Visibility can also be impacted by the position of
safety barriers and parapets. Designers shall ensure that the design of safety barriers and parapets
does not reduce the visibility requirements outlined in this Chapter.
6.8
Crossfall
6.8.1
Crossfall and Longitudinal Gradient
Steep gradients should be avoided at roundabout approaches or flattened to a maximum of 2% for a
minimum distance of 25m before entry. Crossfall and longitudinal gradient combine to provide the
necessary slope to drain surface water from the carriageway. The minimum resultant gradient of any
part of the road surface within 50m of the roundabout shall be 1% in accordance with DN-GEO-03031.
This may be reduced locally to 0.5% at the direct interface between the approaching road and the
circulatory carriageway in difficult circumstances.
On
the approaches and exits, superelevation can assist drivers in negotiating the associated curves.
It’s value, when used, should be appropriate to the speed of vehicles, and equal to or greater than
those necessary for surface drainage, but should not exceed 5% (1 in 20). Superelevation should be
reduced to 2% at 20m from the yield line, since with adequate advance signing and entry deflection,
speeds on approaches should be reducing. The minimum longitudinal gradient of the circulatory