Transport for London (TfL) has revealed that the introduction of 20 mph speed limits on borough roads between 1989 and 2013 was followed by a 34% reduction in the number of people killed or seriously injured (KSI) in collisions, compared with a baseline trend across all London borough roads Transport for London.

This landmark analysis of more than 150 individual 20 mph schemes is the first study of its kind to assess long‑term impacts across the capital’s sprawling road network.

Digging deeper into the data, TfL’s report shows a 40% reduction in fatalities on streets where 20 mph limits were introduced, set against a background decline of just 7 % in deaths on other borough roads over the same period.

Collisions fell by 35% and overall casualties by 36%, compared with background trends of 12% reductions.

Notably, motorcycle fatalities remained unchanged, while all other user groups—drivers, pedestrians and cyclists—benefited from fewer severe outcomes.

 

 

Moving ahead

Today, over half of London’s streets carry a 20 mph limit.

Since 2018 alone, TfL has rolled out more than 164 miles (264 km) of 20 mph road markings, bringing the total to more than 50% of all London carriageways.

Twenty‑one of the 33 London boroughs now enforce a default 20 mph speed limit, and TfL expects that figure to rise in the coming years in light of these safety gains.

The capital’s progress sits against the broader UK picture.

National safety

In 2023, official Department for Transport (DfT) figures estimate 1,624 people were killed on Great Britain’s roads—a 5% drop compared to 2022—while 29,711 were recorded as killed or seriously injured, little changed year‑on‑year.

Overall, there were 132,977 casualties of all severities, down 2% on 2022.

At five  road deaths per billion vehicle miles travelled, the fatality rate also fell by 7%, reflecting improved vehicle safety and enforcement measures.

Despite these successes, road safety has plateaued in recent years.

Since 2014, annual KSI figures in Great Britain have hovered around 29,000, and slight injuries remain stubbornly high at over 100,000 annually.

The resilience of these numbers underlines the urgency for new interventions to break through the stagnation.

Many road safety professionals regard the previous Conservative Government’s abandonment of road safety targets as having a seriously negative effect on progress.

Austerity and severe cuts to local authorities, road traffic police numbers, and road safety organisation funding have all stalled the momentum of road safety gains achieved over previous decades.

Beyond borders

Beyond London, Wales provides a compelling case study in default 20 mph speed limits.

Since the Welsh Government reduced the urban default from 30 mph to 20 mph on September 17, 2023, the first-year evaluation shows around 100 fewer people killed or seriously injured.

This represents a 28% reduction compared with the same period in 2022.

Most residential roads (about one‑third of the network) now carry the new limit.

In the first quarter of 2024, casualties on Welsh roads plunged 26% year‑on‑year, with KSI figures down 23% and slight injuries by 27%, though enforcement activity surged in the initial months.

However, the public backlash prompted a review of the policy, particularly in the initial period after their introduction, and councils are currently considering whether more roads can be switched back to 30mph.

Results over a more extended period will provide a better gauge of success.

In Scotland, a spike in fatalities—up 26% in the year to early 2024—has prompted consultations on rolling out urban 20 mph limits by the end of 2025.

Transport Secretary Fiona Hyslop has earmarked £36 million for road safety improvements, including lower speed limits, while stakeholders debate targeted versus blanket approaches.

 

 

Research, statistics and results

The effectiveness of 20 mph schemes is backed by decades of academic research.

A landmark time‑series analysis of nearly 400 London zones found a 42% reduction in all casualties within 20 mph areas, adjusting for annual background declines.

Government studies have also shown that 20 mph zones cut fatal and serious child injuries by over 50 % and overall collisions by 60%, with no displacement of crashes onto nearby streets.

Lilli Matson, TfL’s Chief Safety, Health and Environment Officer, hailed these latest findings: “Lower speed limits save lives,” she said, noting the analysis “shows the difference that 20 mph speed limits have made to road safety in London”.

Matson 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 period. Lower speed limits save lives.”

Walking and Cycling Commissioner Will Norman added: “Slower speeds not only protect the most vulnerable; they help create safer, more welcoming streets”.

TfL has committed to extend its 20 mph programme in 2026 and will publish additional research on emerging schemes.

At the national level, the Government’s tougher March 2025 guidance on 20 mph zones and low‑traffic neighbourhoods aims to empower local authorities to adopt these measures, reinforcing the growing belief that lower speeds are central to Vision Zero objectives across the UK.