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T.P.D.M.V.2.4
4.3.2 Types of Priority Junction
4.3.2,1 There are three types of priority junction appropriate to single
carriageways viz simple, shadow island and physical island and
they can be applied to three junction configurations viz
crossroads, T-junctions and staggered junctions*
4.3*2.2 Simple junctions are those without any shadow or physical
islands on the major road and without channelising islands in
the minor road approach* They are appropriate for most accesses
and minor junctions on single carriageways but are unsuitable
for junctions with substantial minor road flows (see Diagram Mo.
4.2.3.1). Right turning traffic from the major road can be
particularly problematic as there is no right turn diverging
lane to prevent queues from delaying major road through traffic.
4.3.2.3 Shadow island junctions use a painted hatched island in the
middle of single carriageway roads to provide a diverging lane
and waiting space for vehicles turning right from the major road
and thus overcome the problems mentioned in the previous
paragraph. The same island, if wide enough, also offers
protection to the right turn from the minor road thus allowing
this traffic to complete its manoeuvre in two stages. It can be
seen from Diagram No. 4.2.3.1, that even with relatively light
major road flows the simple junction is only viable up to a
minor road flow of about 500 vehicles 2 way MDT, and above this
level a shadow island layout should be considered. Shadow
islands are effective in improving safety, relatively cheap and
should be considered for busy accesses and junctions on single
carriageway roads.
4.3.2.4 Physical island layouts simply replace the painted central
island described in the previous paragraph with a kerbed island,
offering the same facility to the right turning traffic streams.
Physical island layouts are more appropriate on higher speed
roads, as the through lanes are physically restricted to cater
for single file traffic thus discouraging overtaking through the
junction. Physical islands may give the appearance of dual
carriageway sections and appropriate signing should be erected
to dispel this notion. In particular dual carriageway signs
must not be erected at physical island layouts.
4.3.2.5 In addition to the three layouts described for single
carriageways, priority junctions may also be appropriate for
dual carriageway roads. Local widening of the central reserve
provides sufficient width to harbour right turning traffic.
Priority junctions on dual two lane roads are appropriate for
minor road flows up to approximately 3000 vehicles AADT 2 way
(N.B. much less than physical island layouts on single
carriageway roads). On dual three lane roads priority control
should never be used.