Roundabout Crash Prediction Models - NZTA RR386

Roundabouts are a popular choice for intersection control around New Zealand, particularly to replace priority controlled intersections where traffic volumes are high and safety has deteriorated. However, safety problems can occur at poorly designed roundabouts, particularly where speed is not managed well and where cycle volumes are high.

Safety deficiencies in existing and proposed roundabouts have received considerable attention from safety auditors over the last 10 years or more. The reoccurrence of common deficiencies in the design of new roundabouts in New Zealand culminated in the publication of the guide The ins and outs of roundabouts, which was published by Transfund New Zealand (2000). This guide provides a list of problems that have been encountered in 50 safety audit reports. Visibility and geometric design features, particularly inadequate deflection and marking, feature as problems in many of the safety audit reports. While not specifically mentioned in this report, approach and negotiating speed have the potential to exacerbate any geometric and other deficiencies present at a roundabout.

Roundabouts, particularly large and two-lane roundabouts, have a poor safety record with respect to cyclists. This is illustrated in the proportion of injury crashes involving cyclists at roundabouts (25%), compared with signalised crossroads (8%) and priority crossroads (11%). Many cycle advocates have strong opinions on this matter and strongly oppose the use of roundabouts, particularly larger roundabouts, on cycle routes. Two main reasons are given for this increased crash risk to cyclists:

• As roundabouts become larger, with more lanes and often higher speeds, they become more complex to negotiate by motor vehicle drivers, and motorists are less likely to see cyclists because of the relatively small size of cyclists.  

• As motor vehicle speeds increase, the relative speed between cyclists and motor vehicles increases and drivers are more likely to overtake cyclists in an unsafe manner, while cyclists are more likely to misjudge the gap/space required for various manoeuvres.   

It is expected that reduced vehicle speeds and complexity (single-lane circulating) should improve safety for cyclists.

The research presented in this report, which was carried out in 2006, focuses on the relationship between crashes, speed, traffic volume and sight distance for various approach and circulating movements at roundabouts. The flow-only models developed by Turner (2000) are extended in this study to include observed speed, sight distance and intersection layout variables in various forms. Given the impact vehicle speed is expected to have on ‘active’ mode (walking and cycling) crashes, compared with motor vehicle only crashes, separate models have been developed for the major crash type for each mode.

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