TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 147
Merges with a flow imbalance, where the merging traffic flow is greater than the mainline traffic flow
can occur within an interchange. Priority should still be given to traffic on the mainline. If the merging
flow is over a lane capacity, there will need to be a lane gain. HGVs must be given an opportunity to
join the mainline safely. Operational problems have occurred where the left hand link has been on a
long downhill section and the right hand link uphill, with consequential disparity in vehicle speeds at
the merge, and this particular layout is not recommended.
Loops and certain links may require advisory speed limits (which should be discussed and agreed
with TII) to warn the driver of the safe negotiating speed for reasons of alignment and visibility. This
speed limit should be used in conjunction (where appropriate) with a bend warning sign and ‘chevron’
warning signs to reinforce the hazard warning. Only one level of speed limit should be used within an
interchange as steps down in speed limits may confuse the driver.
Single lane interchange links can have advantages in cost over 2 lane interchange links for
interchanges which contain structures of substantial length. However, where the predicted flows are
near the top of the range (Table 7.4) the uncertainty of the prediction should be recognised, as it may
be prohibitively expensive to convert later to a two-lane interchange link. A disadvantage is that single
lane interchange links may require closure during certain maintenance activities. Consequently, a
whole life cost assessment (including costs during maintenance) should be carried out to confirm the
cost effectiveness of proposed single lane links.
7.17
General
7.17.1
Maintenance
Designers should allow within their designs for facilities to maintain areas within interchanges which
are not readily accessible. Locations for access should be chosen having regard to visibility to and
from the proposed access location and the need to maintain traffic flow through the works. Any lay-by
should not be sited in an exposed position on the inside of connector roads on left hand curves with
radii below Desirable Minimum. They should be located on straights or right hand curve sections with
at least desirable minimum radius.
7.17.2
Connector Roads
Two way slip roads must be dual carriageway with opposing traffic separated by a physical central
reserve with vehicle restraint system. Two way single carriageway slip roads are not permitted. Two
way slip roads only occur at half-cloverleaf and trumpet junctions. Studies into the safety of tight loops
for 2 way slip roads, as compared to one way, indicated that a physical barrier will improve safety and
reduce cross-over collisions.
For motorway interchanges emergency telephones should not be sited in an
exposed position on the
inside of connector roads on left hand curves with radii below Desirable Minimum. They should be
located on straights or right hand curve sections with at least desirable minimum radius.
The collision risk for slip roads is similar whether the mainline is carried over or under. However, the
preferred treatment is to design diverge slip roads uphill and merge slip roads downhill, with the side
road over the mainline. This assists vehicles on the slip roads in matching their speeds to those of
mainline vehicles on merging and reducing their speeds at the approach to the side road junction on
diverging.
Private means of access and junctions on connector roads are not permitted.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 148
7.17.3
Merging and Diverging Lanes
Mainline lane drops within a junction on a 3-lane mainline (3 lanes prior to the diverge, 2 lanes between
diverge and merge and then back to 3 lanes) are not generally recommended on operational and
safety grounds. They severely impair future maintenance, especially at interchanges where no
reasonable diversion route is available. However, if such a layout becomes necessary the
requirements of Section 7.15 should be followed.
A lane drop at a junction diverge must be used when changing carriageway standards from 4 lanes
to 3 or 3 lanes to 2. Similarly, a lane gain at a junction merge must be used when changing carriageway
standards from 2 lanes to 3 or 3 lanes to 4. The layout of the diverge or merge should be selected
corresponding to the leaving or joining flow but under light flow conditions could be Figure 7.6.3
(Layout C) and Figure 7.4.2 (Layout B). Removal of a lane (excluding climbing lanes) must not take
place on the link between junctions.
7.17.4
On-line Service Areas
On-line Service Areas should be provided in accordance with the guidance in DN-GEO-03028, The
Location and Layout of Service Areas. Generally all vehicle types are permitted to enter an On-line
Service Area via a connector road directly from the mainline or as an integral part of a grade separated
junction. On-line Service Areas shall be designed in accordance with the merge and diverge layout
designs, including minimum length of diverge slips, and junction spacing parameters in this chapter.
Measures must be taken to reduce any ‘see
-
through’
effects when looking from a diverge slip to the
merge such as slip or internal road system of the On-line Service Area.
7.17.5
Other Service Areas
The merge and diverge layout design of all-purpose road service areas should be based on the
geometric parameters within this standard as set out in this Chapter or Chapter 5 of this Standard, as
appropriate for each site.
7.17.6
Emergency and Maintenance Accesses
Where an emergency or maintenance access is required, a suitable layout must be chosen from
Chapter 5 of this standard. The preferred layout is that shown as Direct Access Layout 1 but the
designer must check that this would be adequate for its likely use. The access must be gated and
locked to prevent unauthorised use. The entrance gate or gates must be set back to accommodate,
behind the hardstrip or hardshoulder, one vehicle of the largest type expected to use the access. For
a maintenance access, provision must be made for two vehicles of the largest type expected to use
the access to pass in opposite directions in the vicinity of the access. The design of the emergency or
maintenance access must comply with the requirements of Chapter 5 of this standard
with respect to
avoiding steep gradients on the access road in the immediate vicinity of its connection to the national
road.