TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 137
The average speed of approach to a loop may have an impact on its safety record. It is possible that
the higher speeds on motorways on the approach to loops may be a contributory factor to collisions,
particularly on diverge loops. Measures to maintain safety are necessary, and measures to consider
include:
a)
provision and maintenance of clear visibility over the whole
of the loop on the
approaches, especially beyond an underbridge (see Paragraph 7.13.3);
b)
advisory speed limits and/or bend signs and “chevron” warning signs;
c)
widening of lanes on the loops as appropriate for lower radii in accordance with
Chapter 5 of this standard;
d)
the provision of vehicle restraint systems on the outside
of the curve;
e)
physical separation of opposing traffic streams (see Section 7.17 Connector Roads
for mandatory requirements);
f)
lighting;
g)
high skid resistant surfacing.
The provisions for loops in this document must apply only to the layouts shown in Figure 7.17, which
may be used in combination as shown in Figure 7.18.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 139
7.13
Sight Distances
7.13.1
Merges
The Stopping Sight Distance on the connector road must be that related to the design speed selected
for that road (see Table 7.6). This will apply until the driver reaches the Stopping Sight Distance from
the back of the merge nose. From that point forward, the Stopping Sight Distance must be that for the
mainline design speed as illustrated in Figure 7.19A. There must be no obstruction to sight lines
between the connector road and the mainline and vice versa for the length of the merge nose. There
is a minimum approach angle at which drivers can merge on direct sight. Below this minimum
approach angle drivers will be moving nearly parallel to the mainline carriageway and will have to
merge using mirrors. It follows that there is a minimum width of merge nose and this can be derived
from geometric parameters (Section 7.7). The width of the back of the nose must be sufficient to
accommodate the mainline hardshoulder/hardstrip and the connector road off side hardstrip.
7.13.2
Diverges
The Desirable Minimum Stopping Sight Distance for the mainline design speed shall be maintained
until the driver reaches the tip of the diverge nose. The stopping sight distance can then be reduced
to the Desirable Minimum for one design speed step below the mainline design speed (but not below
70km/h). When the driver reaches the back of the diverge nose, the stopping sight distance can then
be reduced to the Desirable Minimum for the design speed of the connector road as illustrated in
Figure 7.19B. On lengths where two
sight distances overlap, the requirements of the longer Stopping
Sight Distance shall be met. If the length of the connector road between the back of the nose and the
stop or yield line of the at-grade junction at the end of the connector road is less than the mainline
Stopping Sight Distance, then a 0.26m object at the stop or yield line must be visible from a distance
equal to the mainline Stopping Sight Distance from either the connector road or the mainline
carriageway. See Figure 7.19C.
7.13.3
Loops
In addition to the general stopping sight distance requirements, there must also be no obstruction to
sightlines across the full extent of loops of low radius. This includes where the loops connect to the
main carriageway as shown in Figure 7.18. This is to ensure that drivers are able to perceive the
whole of the loop layout on their approach from upstream and adjust their speed and conduct
accordingly. The only available relaxation to these requirements is when the necessary vehicle
restraint systems obstruct the view to the 0.26m object height, in which case a clear sightline must be
available above the vehicle restraint system to the 1.05m object height.
At connections to the local road system and where at-grade junctions form part of the grade separated
junction, the sight distance requirements for various junction types as outlined in this standard shall
be adopted for roundabouts, traffic signals and priority junctions respectively. Approaching drivers
shall have unobstructed visibility of the at-grade junction from a distance corresponding to the relevant
Stopping Sight Distance on the approach road.
7.14
Hardstrip and Hardshoulder
Where the hard shoulder has to taper into a connector road hard strip or vice versa, this shall be done
over the length of the diverge or merge taper.