Research into the impacts of 20mph speed limits and zones

This report says that while there is clear evidence that reducing vehicle speeds results in fewer and less severe collisions, getting drivers to comply with signed only 20mph limits is challenging.

The desktop research, carried out by the consultants Steer Davies Gleave for the London Borough of Merton and the London Environment Directors’ Network (LEDNet), was commissioned to “inform future 20mph policy in London”.

The report says that historically 20mph zones have been successful at reducing speeds by using physical traffic calming measures. However, at present limited resources and relaxed regulations mean that signed-only 20mph limits are now preferred, but these tend to achieve smaller decreases in vehicle speeds.

The challenge, it says, is to find ways to achieve reductions in vehicle speeds in signed-only 20mph limits, so that safety benefits are still achieved.

The report says enforcement is a partial solution, but describes changed driver attitudes so that 20mph is seen as the appropriate speed in urban areas as the key to achieving sustained reductions in vehicle speeds.

While acknowledging this will take time, is says there are precedents such as attitudes towards drink driving. It also highlights a potential role for new technologies such as Intelligent Speed Adaptation.

The report says at present publicity and marketing are seen as ancillary measures rather than a core part of 20mph schemes. Going forward, it says supporting measures that foster cultural change need to be an integral part of all 20mph schemes.

On vehicle speeds and collisions, it says that these are generally monitored over a relatively short period, but a longer period would provide more robust information.

On enforcement, it says that while police are becoming more willing to enforce 20mph, their position remains that there should be no expectation for additional police resources.

It adds that the evidence on vehicle emissions is mixed, with the effect dependent on fuel type and driving styles, and any impact on traffic noise is likely to be negligible.

The report also says there is some evidence that 20mph zones can reduce traffic volumes and increase the use of sustainable modes, such as walking and cycling, especially where 20mph is implemented as part of a wider package of measures.