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T.RD.M. V.2.6
(Al/2002)
63.6 Junctions
6.3.6.1 Junctions on Expressways will always be grade separated. Detailed requirements for the
layout of grade separated junctions are given in Section 4.6 of Chapter 4 of this Volume.
6.3.6.2 Junctions on Expressways should preferably be spaced at about 5km intervals but where
circumstances require a closer spacing, this should not be reduced to below 2km.
6.3.6.3 New junctions giving access onto an existing Expressway should only be permitted to allow
an extension of the Expressway system or to link a primary distributor road to an
Expressway. Junctions or accesses, even when provided with full acceleration and
deceleration lanes, just to serve adjacent developments, however large, should not be
permitted. Such developments should be connected via the minor road network to the
Expressway by means of an existing junction.
6.3.6.4 Where it is essential to adopt a close spacing for junctions, particular attention must be paid
to the weaving length between the end of the merge taper and the start of the diverge taper,
as detailed in Section 4.6.10 of Chapter 4 this Volume.
6.3.6.5 Junctions should be designed with the minor road crossing over the major road so that entry
slip road gradients are down towards the major road of the Expressway and exit slip roads
are up away from the major road of the Expressway. This aids the acceleration of vehicles
entering the Expressway and likewise assists their deceleration as they leave.
6.3.6.6 Where large traffic flows with nearly full capacity are joining the mainline in an interchange
or junction, turbulence can occur with short headways and sudden braking. A parallel
merge lane should be provided to increase local capacity.
6.3.6.7 If the joining flows are greater than one lane capacity then an additional lane should
normally be added to the mainline as a lane gain.
6.3.6.8 Any queueing back to rejoin the local network to impede the mainline traffic should be
prevented by providing an auxiliary lane.
6.3.6.9 Whilst it may be possible to justify lane drops within grade separated junctions on capacity
grounds, the number of lanes through the junction should not be reduced to less than three
and lane drops should not differ by more than one. Furthermore, the Expressway
Legislation requires drivers 'to keep to the left unless overtaking' and thus lane drops will
necessitate additional signing and marking. Advice on this is included in Chapters 2 and 5
respectively of Volume 3.