What is the stopping distance at 20mph vs 30mph?

At 20mph, the total stopping distance is 12 metres (6m thinking distance + 6m braking distance). At 30mph, it's 23 metres (9m thinking + 14m braking) - nearly double the distance.

What is the pedestrian survival rate at different speeds?

At 20mph, pedestrians have a 97% chance of survival if hit. At 30mph, this drops to 80%. At 40mph, it falls to just 10%. The difference in speed dramatically affects survival rates.

Has 20mph been proven to reduce casualties?

Yes. Edinburgh saw a 38% reduction in casualties after implementing 20mph limits. Brighton and Hove recorded a 42% reduction. Multiple UK cities have demonstrated significant safety improvements with 20mph zones.

Education

Why 20mph?

The science behind safer streets

The difference between 20mph and 30mph isn't just 10mph – it's the difference between life and death.

Understanding Stopping Distance

When you're driving, your total stopping distance consists of two parts: the distance you travel whilst reacting (thinking distance), and the distance needed to actually stop once you've hit the brakes (braking distance).

At 20mph, your thinking distance is 6 metres and braking distance is 6 metres – a total of 12 metres.

At 30mph, these increase to 9 metres and 14 metres respectively, giving a total of 23 metres.

This means you need almost twice the distance to stop, and in a residential street, that could be the difference between stopping short of a child or hitting them.

The physics is unforgiving: kinetic energy increases with the square of velocity, meaning a vehicle at 30mph carries 2.25 times more energy than at 20mph. This exponential increase in force translates directly into injury severity.

Evidence from UK Cities

Edinburgh implemented a city-wide 20mph limit in 2018 and saw a 30% reduction in casualties in the first two years. Similarly, Brighton and Hove reported a 25% reduction in child pedestrian casualties following their 20mph rollout.

These aren't anomalies – they're consistent with international evidence showing 20mph zones deliver real safety benefits.

Debunking the Myths

"It doesn't make a difference"

The evidence overwhelmingly contradicts this. Lower speeds mean drivers have more time to spot hazards, pedestrians have more time to react, and impacts – when they do occur – are significantly less severe.

"It increases emissions"

Studies by Transport for London found no significant increase in emissions from 20mph zones, and some showed slight decreases due to smoother traffic flow with less harsh braking and acceleration. Modern engines are efficient across a range of speeds.

"Journey times increase massively"

On a typical 2-mile urban journey with traffic lights, junctions, and other vehicles, the difference between averaging 20mph and 30mph is typically 1-3 minutes. The time cost is minimal; the safety benefit is substantial.

20mph: The Human Speed

Residential streets aren't motorways. They're where children play, where elderly residents cross, where someone might reverse out of a driveway unexpectedly. Twenty miles per hour gives drivers the time and space to respond to the unpredictable nature of urban environments.

It's not about making driving difficult – it's about making streets safer for everyone who uses them.

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Last updated: January 30, 2026