FOI request detail

20mph limits for london roads

Request ID: FOI-1361-1819
Date published: 24 August 2018

You asked

Have you modelled the impact of the 20mph plan on restricted speed of traffic flows and the impact on gridlock and increased air pollution? If the answer is no, then why not? if you have can you provide me with the detailed modelling data what would be the set up costs and annual costs of enforcing the 20mph limit have the Met police committed to enforcing it?

We answered

TfL Ref: 1361-1819

Thank you for your request received by Transport for London (TfL) on 13 August 2018 asking for information about the 20 mph plan for roads in London.

Your request has been considered in accordance with the requirements of the Freedom of Information Act and our information access policy. I can confirm that we hold the information you require.

Designing for lower speeds in London is vital for achieving our Vision Zero ambition to eliminate deaths and serious injuries from our transport network by 2041. Speed is the most important determinant in both the likelihood of a collision occurring and the severity of the outcome. For each 1mph reduction in speed there is an associated 5 per cent reduction in collisions[1]. A study of 20 mph zones in London found they resulted in a 42 per cent reduction in all casualties and a 53 per cent reduction in killed or seriously injured (KSI) casualties, adjusting for background trends.

London has already taken action to reduce speeds, and around a third of streets in London now have a 20mph speed limit. Transport for London (TfL) and the boroughs are working to make streets in London more welcoming and safe for people travelling by bike, on foot and by public transport, so speed limits and corresponding speeds need to adjust.

TfL is proposing to consult on a programme to progressively address road risk through lowering speed limits on parts of the TLRN.  Under these proposals, and in line with its broader transformation, the TLRN in central London would become a low-speed environment suitable for walking, cycling and public transport use.  Speed limits would also be lowered in town centres and other high-risk locations on the TLRN to reduce road danger for all road users. 

TfL carries out modelling where changes to junctions are proposed, in order to determine the impact on traffic flow and other potential impacts. Many of the new lower speed limits proposed as part of our programme can be introduced using simple changes to signs and line markings. However, where an engineering intervention is required to introduce a lower speed limit and changes to a junction are required, traffic modelling will be carried out to ensure there are no adverse impacts.

Lowering traffic speeds reduces the dominance of motor vehicles and making our streets more inviting, less polluted, and more attractive for walking, cycling and public transport trips.

An evaluation of 20mph zones in London, carried out by Imperial University, suggested slowing traffic had no net negative impact on exhaust emissions. In 20mph zones vehicles moved more smoothly, with fewer accelerations and decelerations, than in 30mph zones. This smoother driving style reduces particulate emissions from tyre- and brake-wear.

Further evidence of the impact of vehicle speed on emissions and health is set our in Transport for London’s (2018) Speed Emissions & Health evidence summary available at: http://content.tfl.gov.uk/speed-emissions-and-health.pdf.

The MPS Roads and Transport Policing Command (RTPC) enforce 20mph zones where there is evidence and/or intelligence that shows that speeding occurs.

Successful 20 mph speed limits are generally self-enforcing. Designs will ensure that the 20 mph speed limits are self-enforcing, i.e. the existing conditions of the road together with measures such as traffic calming or signing, publicity and information as part of the scheme, lead to a mean traffic speed compliant with the speed limit. To achieve compliance there should be no expectation on the police to provide additional enforcement beyond enforcement activity they would provide in other speed limits.

If this is not the information you are looking for, or if you are unable to access it for any reason, please do not hesitate to contact me.

Please see the attached information sheet for details of your right to appeal as well as information on copyright and what to do if you would like to re-use any of the information we have disclosed.

Yours sincerely

Sara Thomas

FOI Case Management Team

General Counsel

Transport for London



[1] Taylor, M. C. Lynam, D. C. and Baruya, A. (2000) The effects of drivers’ speed on the frequency of road accidents. TRL Report 421. Transport Research Laboratory: Crowthorne

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