Mini-roundabouts - getting them right!
Your problem sites

These sites are taken from some of  those presented at my seminars.  They either consist of existing (mini-) roundabouts that are not performing as well as they should, or are prospective roundabouts about which there have been concerns.

Problem sites at seminars

I look at the problem sites referred to me before the seminars if we have sufficient time to get there. We record stills and video of what is going on and these are played back as part of the afternoon session. The Engineer whose site we are inspecting will be expected to present the site at the seminar. He/she should provide overheads of the site and any other information to be able to illustrate the difficulties which the site presents.


Oxfordshire

Mini-roundabout at Bicester
Bicester

One of my most interesting sites was at the A421/A4095 a converted T-junction in Bicester where the accident record had increased considerably following resurfacing and changes to the layout.  It became apparent that there had originally been two approach lanes on the critical approach from the south (arms at south, west and north) but that with re-surfacing this had been dropped in favour of a single lane approach.

This photo shows the scars of the original approach layout beyond the limit of the resurfacing (just behind the red car).

Problems:

  • Drivers "running" the junction from the south,
  • Sharp increase in accidents.

The skid marks indicate clearly the failure of drivers to assess the junction in sufficient time.

This site has proved what I strongly recommend that the approach layout has to give drivers sufficient "information" about the imminent approach onto the mini-roundabout and two narrow lanes do that better than one wide one.

Recommended action:
  • Re-instate the two lane approach as before re-surfacing
  • Ensure central island is sufficiently raised to deter over-running

    Lessons to be leant:

    • Take very seriously what your approach layout is saying to drivers - ALL drivers!
      It is understood that the two lane system has been re-instated, but the central island remains flat.
Bicester mini-rbt

Wiltshire Here we examined a number of schemes funded by developers.  At the sites in Warminster and Chippenham the mini-roundabouts were probably justified, but at Wootten Bassett there must be considerable doubt.
Wootton Bassett mini-rbt
Wootton Bassett

The development served about 12 properties.  The flow in and out of this development would hardly be sufficient to justify the scheme and it was apparent that "through" drivers were learning to ignore the roundabout and some were even observed driving to the wrong side of the islands altogether and it was this problem that brought the site into the seminar.

The original scheme was welcomed initially as the "major" road was straight and there had been some speeding problems; also the developer was unable to provide sufficient sight-line for a priority junction so the offer of a mini-roundabout seemed a good idea at the time particularly as it was thought that it would deal with the speeding problem.

 

Problems:

  • Wilful disregard by some drivers for the mini-roundabout - passing right of the islands as looking in the direction of this image.

  • Speeding

Recommended action:
  • Re-locate approach island to "capture" approaching vehicles better.
  • Re-locate mini-roundabout sign
  • Consider additional measures to traffic calm the road along its length if possible.

Lessons to be learnt:

  • Avoid using isolated mini-roundabouts where not justified by side-road flow,
  • If you must, consider using additional speed controlling features.
Wootton Bassett mini-rbt
Chippenham mini-rbt
Chippenham

The site comprises a mini-roundabout to serve a development but originally built as a small roundabout.  Problems:

  • The design might be much more suitable for a larger junction but this is only 20m ICD,
  • The central island has been demolished by the HGVs over-running it.
  • The kerbs on the overrun area are beginning to break out.
  • The drainage towards the central island means that it is not very conspicuous.
  • The exit direction signs (still to be erected on the splitter islands) will be poorly sited and only visible by a driver already circulating which is too late.
  • A kerbline build-out by the red car is regularly hit.
Recommended action:
  • Make central island traversable anyway - if using kerbs be sure that they are designed for the purpose - most are not;
  • Re-profile surface to eliminate crowns and hollow thus raising the central island;
  • Remove or reprofile kerbline build-out;
  • Re-design direction signing, left and right flags opposite entries.

Lessons to be learnt:

  • Don't try to design a mini-roundabout like a reduced version of a larger roundabout. They don't work that way.

Submit your problem site:

Here is a great opportunity to optimise your site and help others to learn at the same time. There is so much to getting the details right which may cause confusion so contact me if you would like to consider examining a site and placing it here. I have suggested a review of some of the sites which were used in the extensive TRL study. If you are the "owner" of a problem site which featured there, I would like to hear from you.

Contact me:- penntraff@aol.com

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Penntraff - Dec 2014
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