now very familiar and they have been in use since
the first one was installed at Upton Cross,
Dorset in 1970. Where the junction is a
crossroads with straight axes on both arms the
layout will rely on the central dividing island
and the position of the two (raised)
mini-roundabout centres to cause sufficient
deflection for "ahead" traffic. Care
must be taken, whatever the configuration, that
all crossing streams are subject to 60m radius
maximum unless vertical deflections provide
adequate speed control.
A double mini-roundabout now
successfully in Londonderry. Both axes
line up and care was needed to ensure adequate
deflection was provided.
installed at a four-arm roundabout crossroads at
Colchester in 1972, the ring junction has seen
limited service; but the principles that it
espoused have been taken much further in larger
networks using roundabouts of any size.
A possible problem with Ring
junctions lies in their potential for locking.
The one at Swindon used to lock up in the early
days because one arm of the five-way junction
carried relatively little traffic. The flow that
circulated around the outside network was often
less than that which tried to turn right and
internal locking occurred; once some drivers
learnt to take the longer clockwise route to
their exit the system locked less.
At Swindon, one of the arms
carried relatively little flow.
Here it is arm 1; as a result flows across arms 2
& 5 are low.
Traffic from these arms tends therefore to enter
the system too
freely and this can, and did, cause lock-up
Don't be put off ring junctions
though - this was a minor issue that needed
watching. The scheme at Hemel Hempstead -
originally 6-way was very successful.
Links to some satellite
views of Ring junctions from the PixPlot
These open in a new window...
Swindon - the
Original Magic Roundabout
Swindon - the
Second Magic Roundabout
The Plough - Hemel
Denham - 4 roundabouts and 4 sets of traffic signals
farm Roundabout (this has now been bypassed and has reverted
to a single roundabout. The ring junction was apparently
capable of handling approx 8,000 vehicle per hour.
to see the present layout.
Roundabout, near Heathrow airport, London
Wycombe - 5-arm former single roundabout - added