Mini-roundabout sign

Mini-roundabouts - Getting them Right!
Main issues

I was always taught not to separate issues pertaining to safety and capacity; yet I hear that
"there has to be a trade-off between capacity and safety"
again and again!

Crucial safety aspects

Safety is a constant theme throughout the book and on the website. It is important to distinguish between real safety issues and perceived safety. The TRL study Accidents at Urban Mini-roundabouts Report 281 tracks down the many factors that lead to accidents at mini-roundabouts and I have reviewed some of it.

The key findings at all mini-roundabouts are that:

Pedestrians are relatively safe, and
Two-wheeled vehicles are relatively vulnerable.

The reasons are open to interpretation, but a common theme concerning accidents seems to be a failure by drivers to appreciate the presence of the mini-roundabout in good time, more than likely caused by layouts that fail to grasp their attention, particularly on the approaches. Too much emphasis is also placed on signing and insufficient attention to road layout and correct deflection.

It is the vehicles which cross one another's paths which get involved in almost all vehicle/vehicle accidents at mini-roundabouts. In effect these are the right turners (UK); their speeds must be controlled by the imposition of 60m radius or less on vehicle paths across the junction. This applies to the the right (UK) turn/ahead (crossing) movements, but as an alternative, vertical deflections may be used.

For the merging conflicts left turns run at slower speed anyway but the movement along a straight kerbline can run quickly. Bulging out the kerbline is rarely an answer as drivers can still pass at speed, only this time closer to the mini-roundabout island. There is mounting evidence that kerbline bulges can create accidents.

when (mini-) roundabouts would work better!!
Many Highway Authorities waste huge sums by installing, operating and maintaining traffic signals at sites where (mini-) roundabouts would be much cheaper to install, run and maintain, have greater capacity, reduce delays to pedestrians, and be safer all round including for pedestrians. At a time when financial constraint is severe, local highway authorities should be looking more carefully at this issue distinguishing between perceived safety of signals and real safety of roundabouts.

Pedestrian Safety

It is a myth that signal junctions guarantee safety for pedestrians. Recent research shows that the provision of red/green man pedestrian crossing stages does not enhance safety; in many cases such facilities actually make matters worse.

Design aspects of mini-roundabouts
(for more details click on the links)

  • Clear simple layout of junction - avoid clutter
  • Conspicuous central island
  • Approaches must NOT resemble any other form of junction
  • Place Mini-roundabout give-way (yield) signs (dia 611.1 UK) where best seen
  • Never sink island in a hollow - drain outwards
  • Give-way/Yield lines follow outer swept paths NOT normally ICC (inscribed circle circumference)
  • Central island position based on equally deflected inner swept paths
  • Speed of crossing streams must be closely controlled
  • Speed of merging streams need not be so controlled
  • Avoid kerbline bulges, they rarely slow the traffic down but just get hit
  • Ensure all visual continuity across junction is broken e.g. centre lines shouldn't line up
  • Light the junction clearly
Lop-sided small roundabout at Tunbridge Wells
Most of these rules apply to normal roundabouts too.  I am finding quite a number of sites in the UK where the designers have avoided draining down to the central island of quite large roundabouts and it is noticeable how circulation speeds are reduced. The central island is relatively conspicuous and there are many advantages to this form of construction. Such adverse camber is not thought to be related to the problems of HGVs or trucks overturning on roundabouts.  Excessive speed, rapid steering switching combined with sudden changes in the crowns, often on non-circular roundabouts, all seem to add up to this risk.

Far too many UK small roundabouts look like this, just because the designer has to install inward drainage and it falls to a gully; avoid this.

Some excuses for not installing a (mini-) roundabout

"Flows have to be in balance" The first public road mini-roundabout had right turning flows in the ratio 1:5:11 and worked extremely well.
"Queues will merely transfer to another arm" This will depend entirely upon the relative turning movements and demand flows but the junction operation is so different that this is rare.
"Dominant streams will develop" This can happen in the virtual absence of a turning movement, but the junction operation should still be improved. Check each arm's operation.
"Tidal flows won't work" Very tidal flows can be disruptive - examine flows and apply any formulas sensibly to see the likely effects; quite a severe degree of tidality may still be acceptable.
"The passage of an HGV compromises the integrity of the junction" At the site concerned a long vehicle turning would overrun the whole junction and both sides of an approach road. An HGV will usually occupy the whole junction anyway; the mini-roundabout will be likely to have reduced queues and therefore allow the HGV a better chance to complete its manoeuvre.
"Signals enable [the Traffic Engineer] to maintain control" Control of what?  Yes, it may be possible to cause longer queues on one arm than another during peak periods by using signals.  But off-peak this is virtually impossible and it would be wrong to deliberately cause a long queue on one arm when other arms are significantly free of traffic as some form of deterrent.
"What about pedestrians; surely they are better off with signals?" Just because pedestrians do not get priority this does not mean that they are worse off. Pedestrian accidents are much lower at roundabouts. Delays to pedestrians using a signal system at a busy junction are much higher than at a busy roundabout where splitter islands allow the road to be crossed in two stages. Better designs which keep speeds down will further enhance the safety of pedestrians and cyclists.
"What about cyclists?" While cyclists are vulnerable at roundabouts, this is true of most forms of non signal junction control.  The use of outward crossfalls on circulating carriageways and relatively narrow lanes on the approaches to mini-roundabouts all help cyclists to be part of the traffic rather than obstructions that "get in the way". For mini-roundabouts in particular, the repeated failure at many sites of drivers to appreciate the presence of the roundabout in the first place put circulating cyclists in great danger - ensure your designs get ALL drivers out of "auto-pilot".

Mini-roundabouts and Traffic Calming

There has been an upsurge in the use of mini-roundabouts in the UK for traffic calming. While it is true that mini-roundabouts are effective at traffic calming this has been because they have generally been justified in their own right at the site concerned. 

It is not satisfactory to install a mini-roundabout at any junction just to reduce speeds.  

If the side-road at a site has very low flows then the mini-roundabout will not perform well and drivers will start to treat it merely as an obstruction to be negotiated.  At one site I came across recently a series of mini-roundabouts had been installed along a road that needed traffic calming but there was very little side-road flow.  One of the minis was a two-arm mini-roundabout!!  Avoid this.
See mini-roundabouts and traffic calming.

The long term aims and hopes

More schemes will come on line.
TRL studies similar to existing studies on links and other junctions will clarify design points.
We will gradually learn not to worry about adverse crossfalls on larger roundabouts.
Network management using calming devices with roundabouts at all key junctions will become the norm in large areas especially residential and small towns; these areas have the poorest UK child safety records.
Bus-friendly vertical deflections with mini-/small roundabouts will bring effective traffic calming to main road and village environments; especially abroad...
An international design standard will emerge.
Huge global casualty reductions possible if roundabouts adopted world-wide.
(Annual savings of about 8000 deaths and 100,000 casualties in USA alone.)

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Penntraff - August 2018
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