No 170: Urban cycle use, Safety in Numbers, and speed limits

Dr Adrian Davis

Top line: In the absence of cycle paths segregated from motorised traffic in urban areas the priority measure is to reduce the speed limit to 30km/h (20mph). In addition to built environment changes, including lower speed limits, Safety in Numbers also reduces injury risk as cycle use increases along a road link or at junctions.

In seeking to improve cycle safety, a question which arises is ‘which built environment factors improve cycle safety in urban areas?’

Research has found that both perceived and objectively assessed lower traffic speeds were associated with higher odds of cycling for transport.1, 2 In the absence of segregation of cycle users from motorised traffic (with a kerb or barrier between cycle users and motorised traffic) adjusting the speed limit of the traffic to 30km/h may ensure an increase in the street’s appeal for bicycle transport. In contrast, when a segregated cycle path is available, the authors found no additional effect on the street’s appeal of a 30km/h limit. The authors note that these findings should be communicated and included into policies at national and subnational level encouraging bicycle transport

In a UK study of cycling injury risk, Safety in Numbers (SiN)3 was identified as a factor in explaining lower cycle injuries.4 The paper provides new evidence for a SiN effect at a road link/junction level, where roads with more cyclists have lower injury risk. These results are consistent with, for instance, both a simple ‘physical’ explanation (more cyclists on a link means less exposure per cyclist) and a ‘behavioural’ explanation (drivers on routes with high cyclist volumes are more aware of cyclists and take more care). However, in this study, the researchers were able to report an effect of 20mph speed limits separate from SiN in protecting cyclists or reducing injury events.

The researchers found that road type affects injury risk. The key difference seems to be that residential roads are safer than other road types, controlling for other factors. Among non-residential roads, the differences are more minor and the highest risk was observed on secondary roads. There was a clear reduction in injury odds in 20mph compared to 30mph. In consequence, the data suggest that speed limits on 20mph help reduce cycling injury risk as does motor traffic reduction. The relationship between motor traffic volumes and cycling injury risk suggests that reducing motor traffic volumes by, for example, 5000 motor vehicles a day will have a much greater impact on relative injury odds on a road with 10,000 motor vehicles, than on a road with 30,000 motor vehicles. Further, building cycle routes that can generate new cycle trips will bring ‘safety in numbers’ benefits.

1 Mertens, L., et al. 2016. Which environmental factors most strongly influence a street’s appeal for bicycle transport among adults? A conjoint study using manipulated photographs, International Journal of Health Geographics, 15:31.
2 Mertens, L., et al, 2017 Built environmental correlates of cycling for transport across Europe, Health & Place, 44: 35-42.
3 See
4 Aldred, R., Goodman, A., Gulliver, J., Woodcock, J. 2018. Cycling injury risk in London: A case-control study exploring the impact of cycle volumes, motor vehicle volumes, and road characteristics including speed limits, Accident Analysis and Prevention, 117: pp.75-84.