TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 21
a)
To make the layout of a junction more obvious to approaching traffic.
b)
To provide reference points or features for road users waiting to enter the junction,
aiding them in judging the speed of drivers approaching or in the junction. This can
be useful where a junction is located in an open landscape, where there is a lack
of natural reference points.
c)
To provide a positive background to the road signs around the junction.
It is essential that visibility within the required envelopes remains unobstructed by vegetation and
street furniture. The areas required for visibility envelopes should be either hard surface or planted
with grass or species having a low mature height and low maintenance characteristics.
Too much visibility can be as problematic as too little and this can also be addressed by careful
landscape treatment.
3.7
Enhancing road layouts at existing priority junctions to improve
safety
Various methods which have been shown to enhance safety at junctions include:-
a)
Where there is a history of collisions involving right turning vehicles, the installation
of a ghost island on single carriageway roads to shelter right turning
traffic and
discourage overtaking.
b)
The replacement of a rural crossroads by a staggered junction. This has been
shown to reduce collisions by 60%.
c)
The installation of channelising islands on the minor road approaches at existing
rural crossroads. This has been shown to reduce collisions (mainly minor road
overrun) by about 50%.
3.8
Safety Barriers
Safety Barriers shall be provided at junctions in accordance with DN-REQ-03034 where required.
Designers must consider the position of such barriers and their potential for obstructing visibility when
assessing the visibility requirements at a junction as outlined in this standard.
3.9
Detailing of Severed Roads
Where an existing road is severed by a new road scheme, resulting in the creation of a cul-de-sac,
the approach to the cul-de-sac should be detailed so as to direct through traffic on to the through road.
The access to the cul-de-sac therefore, shall be detailed so as to require a deliberate turning
manoeuvre to prevent drivers assuming the previous road layout still applies.
A typical well designed cut-off should include the following:
a)
The character of the road on the approach to the cut-off point shall be changed by
narrowing to between 3 and 4m for the last 100m (depending on the volume of
traffic on the cut-off road).
b)
All lining shall be removed within 100m of the cut-off point.
c)
All old signage relevant to the existing road shall be removed.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 22
d)
New signage shall be put in place to inform a driver that the road they are entering
is now a cul-de-sac. The signage should be placed such that it is conspicuous to a
driver approaching the junction that leads to the cul-de-sac.
e)
The possibility of see-through between old road and new road must be assessed
both by day and by night. If a see-through issue exists then
a screening fence of
suitable height needs to be erected, possibly supplemented by planting
of mature
scrubs, to remove the problem.
f)
At some locations creation of a turning circle at the end of the road may be the
best treatment; in other cases just narrowing the road, removing the lining and
treating the area in front of the cut-off point may suffice.
Figure 3.1 illustrates both good and bad practice examples for the detailing of severed roads.
Figure 3.1: Detailing of severed road arrangements
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 23
4.
Selection of Junction Type
4.1
General
As an overarching principle TII supports a junction strategy which seeks to prevent a proliferation of
side road junctions along national roads with speed limits of greater than 60 km/h. The application of
this strategy will maintain
the capacity, efficiency and safety of the national road network.
Recommendations are given in this Standard on the geometric design of the important elements of
any junction or access onto the national road network, and how the individual elements can be brought
together to produce an overall scheme.
Among the aspects of design critical to junction selection which should be taken into consideration
and included in the decision framework are:
a)
traffic flows (operational efficiency);
b)
safety;
c)
collision history;
d)
sight distances;
e)
consistency;
f)
location;
g)
maintenance;
h)
environmental effects;
i)
land take;
j)
capital cost;
k)
economic assessment;
l)
provision for NMUs.
The operation of junctions on the national road network must be readily understood by all road users
and therefore sequences of junctions should ensure a consistency of junction type application and not
involve many different layout types. It is therefore essential that designers prepare a junction strategy
when introducing or modifying a junction(s) on a road scheme and evaluate their effect upon the safety
and operational performance of the network as a whole.
The siting of a junction will require careful consideration of the local demand taking into account the
existing road network so as to ensure an adequate degree of access is provided to the national road.
4.2
Selection of Junction Type
The flow chart shown in Figure 4.1 illustrates the typical stages involved in the selection of a suitable
junction on a national roads scheme. As these stages are being completed the Designer shall
complete the junction analysis procedure form included in Figure 4.3.
If, at any point in the design procedure, the junction design is unsatisfactory, then the designer should
return to the previous stage in the procedure to refine the design. In certain cases, this process could
result in a change in junction type or form.