20mph zones and speed limits

Introduction of 20mph zones and speed limits

Slough Borough Council believes that reducing the speed of traffic in residential areas will reduce the frequency and severity of collisions, reduce congestion, discourage rat-running and make Slough’s communities safer for residents.

20mph zones reduce accidents by an average of 60% (TRL report 215, 1996). In London a review of 78 sites found that they reduced injury accidents by about 42% and fatal or serious accidents by 53%.

On average, for every 1mph speed reduction, there is a 6.2% accident reduction.

A pedestrian hit by a car, travelling at a lower speed is much more likely to survive.

If a child runs out three car lengths ahead, when you are travelling at 20mph, you should just be able to stop in time. But at 30mph, you will hit the child, with a significant chance of injuring or killing them.

30mph 20mph
30mph 20mph
45% of pedestrians are killed 5% of pedestrians are killed

Zones and Speed Limits – what’s the difference?

There is a significant difference between the characteristics of a 20mph speed limit and a 20mph zone.

20mph limits

  • Signing alone is used, these are more appropriate for road where the average speed is already low.

20 mph zones

  • Use traffic calming measures to reduce speeds.

There are four main techniques to traffic calming programmes:

  • vertical deflections (speed cushions, speed tables)
  • horizontal deflections (chicanes)
  • road narrowing
  • central islands.

Will 20mph speed limits be enforced?

Thames Valley Police have a commitment to enforcing the law and therefore, will be enforcing all speed limits.

Cost to society

To add the financial context, the cost to society of injuring different types of casualties is summarised in the table below. As well as the council’s commitment to make Slough’s roads safer for all, this valuation also lends weight to the importance of addressing locations where there is a clear need to protect vulnerable road users.

Casualty type Average value of prevention
Pedestrian £76,880
Cyclists £48,430
Motorcycle £91,880
Cars £36,870

Department for Transport, Transport Analysis Guidance (TAG)
Accident data is publicly available at www.crashmap.co.uk.