Mini-roundabout sign

Mini-roundabouts - Getting them Right!
Networks & Ring Junctions

Many small towns have several mini- or small roundabouts and over quite a period of time, I introduced several in Wokingham, Berkshire (UK) and I am aware of many other towns that have networks of roundabouts.

But first let's look at mini-roundabouts that are quite close together:

The double mini-roundabout

These are 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 operating
successfully in Londonderry. Both axes
line up and care was needed to ensure adequate deflection was provided.

The Ring Junction

First 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 internally.

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 roundabout database
These open in a new window...

Swindon - the Original Magic Roundabout

Swindon - the Second Magic Roundabout

The Plough - Hemel Hempstead

M40/A40/A412/A4020 Denham - 4 roundabouts and 4 sets of traffic signals

A13/A130 Sadlers 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.

Hatton Cross Roundabout, near Heathrow airport, London

A133 Colchester

A40/A404 High Wycombe - 5-arm former single roundabout - added Dec 2011

A51 Tamworth "The Egg"

Linear networks

I am very excited by the prospect of getting more schemes under way rather like Shenley Road, Borehamwood. This shows enormous potential yet to be realised. Once installed ongoing costs relate to maintenance of the road structure and surfacing - very little electrical work is needed when compared with traffic signals. Borehamwood shows the way forward for many towns and villages in the Uk and abroad.

Town networks

Near me in Devon the development of the town of Newton Abbot was planned as a roundabout town but has recently been spoiled by the addition of a second set of traffic signals at a main junction. It seems that Devon CC may have decided that pedestrian facilities are best provided by signals. I shall be watching developments and noting pedestrian delays at the new/revised installations. It is already very obvious that pedestrians will not wait at the signals when displaying the red man.

Penntraff - August 2018
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