TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 17
Figure 2.10: Compact Grade Separated Junction with roundabouts
Figure 2.11: Compact Grade Separated Junction with Priority Junctions
Compact Grade Separated Junction types are discussed in further detail in chapter 8.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 19
3.
Road Safety
3.1
General
TII is responsible for securing a safe and efficient national road network. The performance and safety
of roads are strongly influenced by the layout and frequency of junctions and accesses.
A junction layout should give road users (including NMUs) a clear understanding of what is required
of them. Poor layouts lead to road user confusion, indecisiveness and rash decisions that could
contribute to collisions.
The design should provide:
a)
advance notification of the layout on the approach to a junction;
b)
conspicuous junction locations
and layouts;
c)
an understanding of permitted changes to the direction of travel;
d)
an understanding of other traffic movements;
e)
avoidance of potential hazards.
Thus, in considering the design components, designers should ensure that as road users approach a
junction they are able to easily understand the junction form and layout so that they can select their
path through the junction accordingly. Ease of use should be checked in night-time conditions.
Road safety should be considered at the earliest stage of design to ensure the satisfactory operation
of a junction for all users, including cyclists and pedestrians, and to ensure that allowances are
included for physical elements, such as signing, lighting equipment, columns, feeder pillars, buried
cables, cable ducts, draw pits, etc.
3.2
Road User’s Limitations
An appreciation of road users’ performance is essential to proper
road design and operation. The
suitability of a design rests as much on how safely and efficiently road users are able
to use the road
as on any other criterion.
Motorist’s perception and reaction time set the standards for sight distance and length of transitions.
The driver’s ability to understand and interpret the movements and crossing times of the other
vehicles,
drivers and NMU’s using the junction and their associated reactions is equally important
when making decisions. The designer needs to keep in mind the user’s limitations and therefore
design junctions so that they meet user expectation.
Sequences of junctions should not involve many different layout types. Safe road schemes are usually
straightforward, containing no surprises for the road users. A length of route or bypass containing
roundabouts, ghost islands, simple priority junctions, grade separation and different provision of NMU
facilities would inevitably create confusion and uncertainty for drivers and may result in collisions.
The sun can detrimentally affect a road user’s vision. Designers should attempt to avoid the need for
drivers to approach a manoeuvre or a decision point looking into the rising or setting sun. The designer
should consider the potential for dazzle and silhouetting of signs when the sun is low in the sky.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 20
3.3
Visibility
For safety, road users shall be able to see any potential hazard in time to slow down or stop
comfortably before reaching it. It is necessary therefore to consider the driver's line of vision in both
the vertical and horizontal planes and the stopping sight distance for the vehicle or NMU at the relevant
design speed. Visibility requirements for the different junction types are included in the relevant
chapters of this standard.
3.4
Road Marking and Signing
Signs and road markings are provided at junctions to warn, regulate and guide traffic. The provision
and layout of traffic signs and road markings is an integral part of the junction design process and
must be considered at an early stage and it may affect the junction geometric design. Traffic signs
and road markings can significantly affect the safety and the capacity of a junction. All road markings
and signage for national roads shall be designed in accordance with the requirements of the Traffic
Signs Manual (TSM) issued by the Department of Transport, which includes guidance on the
appropriate use and requirements of road markings at various types of junctions.
Positioning of signs within the junction and on the mainline approach to junctions must be carefully
considered so that they do not interfere with road user’s visibility. It is essential that there is no
over-
provision of signing
leading to ‘sign clutter’
.
Road markings are
used to channelise traffic and, where required, to indicate a dedicated lane. Lane
direction signs complementing the advance direction signs at entries can be beneficial where heavy
flows occur in a particular direction.
Where cycle facilities are provided road markings and signs must be adequately provided at the entry
and at suitable distances along the cycle route. Yield signs and road markings shall be provided to
indicate vehicle priority at junctions.
3.5
Lighting
Road lighting at junctions shall be provided in accordance with DN-LHT-03038 and IS EN 13201.
It is a requirement on all rural motorways and dual carriageways to light the conflict points at grade
separated junctions i.e. the roundabouts at the end of the slip roads. The lighting shall extend 60m
from the roundabout along each entry or exit slip road without lighting the mainline carriageway.
In general, the lighting layout should provide the highest levels of illumination at traffic conflict areas
and NMU crossing areas including the immediate traffic approach where illegal movements are most
likely.
3.6
Landscaping
The design of landscaping and street furniture within the road boundaries shall be carried out in
consultation with appropriate specialists. The Designer shall consider the maintenance implications
and where the responsibility for maintenance is passed
to a third party, maintenance standards must
be agreed. If third parties wish to enhance the standard of planting or landscaping at a junction, this
shall be with the agreement of TII, and shall not compromise visibility sightlines or safety.
Apart from the amenity benefits, the landscape treatment of junctions can have practical advantages
from a traffic engineering point of view. These can be: