TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 149
7.18
Safety
The consequences of an incident can be severe if hazards are present within the verge area
immediately downstream of the diverge nose at a junction. It is therefore desirable to provide a clear
zone at the back of diverge noses such that the physical nose is free from all hazards, including safety
barriers, to minimise the risk to errant vehicles. Creating a clear zone will normally require the vertical
alignment for the connector road to follow that of the main carriageway for a short distance to allow
the cross-section to be reasonably level. If creation of a clear zone is not achievable due to site
constraints, the risk needs to be reduced to as low as is reasonably practical.
For example, if a safety barrier is required to protect errant vehicles from any hazards, including height
differences between adjacent carriageways, the use of full height terminals or crash cushions with
appropriate performance levels is recommended as end treatments to barriers. Sufficient space
should be allowed for any safety barriers and end treatments.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 150
8.
Layout of Compact Grade Separated Junctions
8.1
Design Procedure
8.1.1
General Principles
The introduction of design standards for compact grade separation is primarily aimed at improving
safety for all road users. At all stages in the design and construction of the junction it is of paramount
importance that safety aspects are fully investigated and considered.
Compact grade separation can be used on single carriageways and Type 2 and 3 Dual Carriageways
as defined in Table 6.1 of DN-GEO-03031.
This Chapter sets out the level of provision to be considered for low traffic flow situations on the minor
road and effectively extends downwards the range of flows and conditions over which grade
separation could be justified economically to around 12,500 AADT on the major road.
On existing dual carriageways the provision of compact grade separation may be applied to a route
which consists of a number of grade separated junctions and roundabouts interspersed with priority
at grade priority junctions. Introduction of compact grade separation on a route such as this is intended
to remove all of the right turn manoeuvres associated with the mainline resulting in a fully grade
separated route. Some left-in/left-out junctions may be retained or introduced.
Where a compact grade separated junction or junctions are being considered the following shall be
taken into account:-
a)
the closure of certain minor road junctions which have
very low flows and for which
there are alternative routes. The additional journey length, delay and
inconvenience resulting from the closure of a route must be considered in the
context of the improved safety which will be achieved by the removal of all the right
turn manoeuvres;
b)
limiting remaining at grade junctions to left in left out only;
c)
where there is no alternative route it may be necessary to reconsider the location
and
number of junctions;
d)
improvements to the network to assist the closure of certain junctions;
e)
the provision of accommodation roads, and NMU facilities;
f)
the removal of agricultural accesses from the mainline;
g)
collecting a number of minor roads into a single compact grade separated junction;
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 151
8.1.2
Flow Standards
On Type 3 Dual Carriageways with relatively high traffic flows, it may be appropriate to provide
compact grade separated junctions. On a length of Type 3 Dual Carriageway with compact grade
separated junctions there shall be no breaks in the central reserve. All major junctions shall be
compact grade separated, or roundabout, while minor ones shall be left-in/left-out only. U-turns will
only be possible at the compact grade separated junctions, or at roundabouts. Accesses should be
severely limited or avoided altogether.
Where practicable, layouts should be designed so that merging occurs on two-lane sections, thereby
avoiding the problems of merging into a single lane. This can be achieved if the junction is at a non-
critical changeover. Roundabouts may be provided at the most significant junctions on a length with
compact grade separated junctions.
Analysis indicates that for the improvement of an existing priority junction to compact grade separation
standards an increase in capacity of 70% for some movements can be achieved at the mainline
junctions.
The layout of the compact grade separation should be chosen to suit the traffic movements. In certain
conditions the redistribution of the turning traffic can result in significant turning flows at the minor road
junction entry and exits. It is important therefore to assess the capacity of these junctions when
considering the layout to be adopted.
8.1.3
Provision for Non-Motorised Users
Provision must be made for the specific requirements of non-motorised users in the design of compact
grade separated junctions.
To assist cyclists, gradients would ideally be limited to 5%, however the objectives of compact grade
separations will in many instances prevent this being achieved. Gradients should only be increased
above this figure with careful consideration of all factors in accordance with Section 8.2.6.
At junctions, where the NMU facility along the mainline crosses a connector road, the geometric
requirements as prescribed for major priority junctions should be provided for NMU’s, including the
provision of a 3.5 metre central island if traffic flows warrant it.
8.2
Geometric Standards
8.2.1
General
This standard provides a geometric standard for compact grade separation incorporating established
design standards for priority junctions.
The geometric standards for the mainline and for those minor roads which pass directly through a
compact grade separation shall be provided in accordance with DN-GEO-03031.
The standards for compact grade separation in this standards have been set in order to minimise the
variation in designs to prevent confusion for the road user. Those items which do permit a degree of
variation at the discretion of the designer are:
a)
Junction configurations;
b)
Horizontal radii on compact connector road (desirable minimum radius with a
relaxation of one design speed step);