Transport for London have released new data that quantify the safety benefits of the 20mph speed limits in force across the capital between 1989 and 2013.*
The analysis uses collision data from three years before and after each scheme’s implementation to assess changes in road safety.
A total of 157 schemes were reviewed, using data from London’s Traffic Accident Diary System (TADS), and compared against overall trends on borough roads to control for wider safety improvements during the same period.
The results show that 20mph measures led to reductions in collisions across all injury categories and travel modes, with the exception of motorcycle fatalities, which showed no change.
The main findings were:
- A 40% reduction in the number of people killed (from 15 to 9), compared against the background trend of 7% fewer fatalities across borough roads
- A 34% reduction in the number of people killed or seriously injured (from 395 to 260), compared against the background trend of a 15% fall in people killed or seriously injured across borough roads
- A 75% reduction in the number of children killed (from 4 to 1), compared with no change across the control group.
- A 50% reduction in children’s casualties (from 517 to 280), against the background trend within the control group of 20%.
- A 35% reduction in collisions and 36% reduction in casualties on borough roads, against a background trend of 12% fewer collisions and casualties across all borough roads
21 out of 33 London boroughs now have a default 20mph speed limit and TfL have provided £87.6m in Local Implementation Planning (LIP) funding that was confirmed for 2025-26, for the implementation of more 20mph zones and other road safety measures.
Lilli Matson, TfL’s Chief Safety Health and Environment Officer said: ‘It is brilliant to see the long-term analysis in this report. It contains hugely important findings which show the difference that 20mph speed limits have made to road safety in London, and we are able to fully understand the impact of road safety measures across the city over a longer time span. Lower speed limits save lives.
‘We’re committed to eliminating unsafe speeds across London, and the analysis in this report reinforces our approach. No death or serious injury on London’s streets is acceptable or inevitable, and we are determined that London continues to be at the forefront of making real and lasting change.’
Emissions
The emission impact of a 20mph speed limit was investigated by The Transport and Environmental Analysis Group at Imperial College London in 2013. They found that NOx emissions from diesels would fall while those from petrol vehicles would increase.
For both petrol and diesel cars with an engine size of 2.0 litres or less, PM emissions would be reduced by around 8-10%
Non-exhaust emissions from brake and tyre wear were also predicted to be reduced because of the lower demand for power at reduced speed.
Jeremy Leach from 20’s Plenty for Us said: ‘Over the past 30 years London’s boroughs have pioneered the widespread use of 20mph speed limits making streets feel safer and be safer for residents and visitors alike. This latest research from TfL backs up other studies that show streets are safer with 20mph speed limits and more people able to walk, wheel and cycle and to be active.
‘We hope that the small number of London’s boroughs that have yet to make widespread use of lower speed limits will not miss this chance to bring in 20mph limits on many of their streets and roads.’
The full report can be read here.
* ‘The report only considers schemes delivered by 2013 because in 2016, the Department for Transport changed the way the severity of injuries was recorded. In order to provide a rigorous analytical comparison using the industry standard of three years’ collision data before and after schemes were implemented, this report did not look at any schemes introduced after 2013. Our dataset runs until 2013 and not 2016 because we required a full 3 years of monitoring data after implementation of the last 2013 scheme. Ending the analysis in 2013 guarantees consistency and a fair comparison.’
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