Bicycle Network: Good Design Guides

3 June 2009. In the past traffic engineers regarded roundabouts as a magic bullet; cheaper than signals with better motor vehicle flow and reduced motor vehicle collisions.

Bike riders hate them with good reason. They know a roundabout intersection will be the place where there will be close shaves and collisions.

Today bikes are common and in most places the numbers are growing at a startling rate. Riders are normal, everyday people, riding to school, riding to work, and saving the Treasury millions in health and transport infrastructure costs.
 
And they are asking: "Why are we still building all these roundabouts when the studies show that current designs are hostile to bike riders?"

 

One lane roundabouts on local bike routes can be improved significantly by installing speed humps and zebra crossings. These pictures both from Cecil Street South Melbourne.

Risk

Studies around the world have shown that roundabouts increase risk for cyclists while decreasing them for drivers.
 
A detailed investigation of the New Zealand road crash database shows 26 per cent of cyclist injuries occur at roundabouts compared with 6 per cent at signalised intersections. In a multi-lane roundabout your chances of being involved in an accident goes up two to three times than at a traffic signal.
 
The research shows that more than 50 per cent of bike accidents in roundabouts occur when a vehicle hits a bike already in the roundabout.
 
What is it about roundabouts that make them such a poor choice for bike riders?
 
Roundabouts require road users to exercise judgement while in motion whereas signals and stop signs require the driver to stop and then weigh up the situation.
 
The assessment of a driver while in motion may be cursory and their expectation of seeing a bike rider may be low: they may only be looking out for cars; they may underestimate the approach speed of the rider; they are likely to arrive at the roundabout with a preference for passing through it without stopping.
 
Most Australian roundabouts exacerbate these negative characteristics because they have been designed in a way that encourages drivers to speed up rather than be more careful.

Deflection

The critical factor is the deflection angle -- it's something we have been using less of when we should have been using more.
 
Reducing the deflection required of the driver allows higher speeds through the intersection. In general a lower deflection angle means more collisions with bike riders. Specifically, the roundabouts that have the most casualty accidents with riders in Victoria have low or no deflection, and consequently, high speeds.
 
It is not appropriate to install roundabouts along or across a bike route, they will suppress usage and increase bicycle motor vehicle collisions.
 
If a roundabout has to be installed, the design should include sufficient deflection to significantly reduce speed through the intersection.
 
This is broadly the approach taken in Europe. 

Retrofitting a roundabout on a bike route

The most common challenge for those installing or upgrading on-road bicycle routes is not whether to install a roundabout but how to deal with an existing one that is causing the problems outlined above. Roundabouts are expensive to remove or upgrade to signalisation.
 
There are a number of low cost initiatives that can be taken that will significantly improve the comfort and reduce the risk of riders using a route that goes through a roundabout. The aim of all these techniques is to reduce the entry speed of motor vehicles and increase the chance of a sound assessment being made by the driver.
 
For single lane roundabouts the following treatments are recommended:  Zebra crossings on the approach legs; raised platforms across the approach leg; combining the two features above; narrowing and increasing the deflection of the approach leg by kerbing and/or painted chevrons.

There is a growing practice of painting circumferential lanes with line marking and or green paint around roundabouts. Research shows this is not effective, and in some cases has increased risk.

Multi lane roundabouts

Fatalities and serious casualty accidents are likely to occur at busy multi lane roundabouts on bike routes. For these locations signalisation is likely to be the solution.

See the page on multilane roundabouts