20mph speed limits and their effectiveness

There is a growing use of 20mph (30km/h) speed limits across London, which have been implemented as part of the drive to improve road safety, particularly for pedestrians and cyclists. However, there is a lack of a coherent evidence base which could be used to inform future 20mph policy in London.

It is clear that—all other things being equal—slower vehicles speeds result in fewer and less severe collisions. This is because lower speeds give more time to avoid a collision, whilst impact speeds are lower for those collisions that still occur, meaning that any injuries are likely to be less severe. However, implementing a 20mph speed limit does not automatically reduce actual vehicle speeds.

Steer Davies Gleave was therefore commissioned in 2014 by the London Borough of Merton on behalf of the London Environment Directors’ Network (LEDNet) to conduct research into the impacts of 20mph speed limits. Our work involved a desktop review of the available evidence, from both the UK and abroad.

Impacts of 20mph speed limits

One important distinction that applies in the UK is between the two different ways in which 20mph speed limits can be implemented:

• 20mph zones require physical traffic calming features (such as speed humps) every 100m.
• 20mph limits can be implemented using only signage, and are therefore much cheaper than 20mph zones.

Our study found that 20mph zones are more effective at reducing vehicle speeds and collisions than signed-only 20mph limits. This is largely due to the inclusion of physical traffic calming measures as part of 20mph zones. Nevertheless, 20mph limits (as opposed to 20mph zones) are now the more common approach, due to their lower implementation cost and because they do not require physical traffic calming measures which can be controversial.

Achieving compliance with 20mph limits is therefore an ongoing challenge. Some targeted physical measures such as speed humps can be implemented, along with enforcement using police resources. However, these methods can be resource intensive and are only part of the answer. One of our key findings was that achieving cultural change, so that driving at 20mph in urban areas feels ‘normal’, is the key to sustained vehicle speed reductions.

Current context in London

To understand the context of 20mph speed limits in London, we sent out a survey to all local authorities. Based on the responses, we found that that the City of London, Camden and Islington have implemented 20mph speed limits across all of their borough roads. In addition, a number of inner London boroughs also have a policy of adopting blanket 20mph speed limits, whilst outer London boroughs still tend to implement 20mph speed limits on a case-by-case basis.

Conclusion

In conclusion, it is clear that reducing vehicle speeds results in fewer and less severe collisions, particularly for vulnerable road users. However, whilst 20mph zones have been successful at reducing speeds by using physical traffic calming measures, limited resources and relaxed regulations mean that signed-only 20mph limits are now preferred. These tend to achieve smaller decreases in vehicle speeds and therefore smaller improvements in road safety.

The challenge is therefore to find ways to achieve larger reductions in speeds in signed-only 20mph limits, so that road safety benefits are maximised. Enforcement is required, but this is only a partial solution. The key to achieving sustained and meaningful speed reductions is to change drivers’ attitudes to urban driving speeds. This suggests that supporting measures that foster cultural change need to be an integral part of all 20mph schemes.

Whilst our study focused on London, by bringing together the available evidence it should help authorities around the UK and across the globe in their decision making regarding 20mph speed limits. Our study is also informing the wider debate taking place across the country regarding the effectiveness of 20mph speed limits, for example as recently reported in The Guardian.

The study report can be downloaded. In addition, the Department for Transport has commissioned a separate and larger three-year study on the effectiveness of 20mph limits, which will bring together data from different regions as well as new research carried out on the roads. Results are due in 2017, and this will provide further evidence on the impacts of 20mph limits.