20mph zones and speed limits

Introduction of 20mph zones and speed limits

In the past three years there has been one fatality and 257 people injured in speed related crashes in Slough’s residential areas. Many of these were pedestrians, cyclists and children.

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

How were the areas identified?

An initial borough wide cluster analysis of pedestrian, cycle and speed related accidents were carried out to create a shortlist of areas to be considered for 20mph zones. Areas around schools and elderly residential sites were prioritised.

Seven zones were identified from this analysis as being suitable for inclusion in a 20 zone or as needing improvements to existing zones. These are:

  • Langley Road
  • Oatlands Drive / Sheffield Road
  • Northern Road / Granville/ Villiers
  • Wexham Road / The Frithe
  • Trelawney Ave
  • Whitby/ Woodlands
  • Town Centre

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.

Department for Transport guidance

In a recent consultation, the Department for Transport set out plans to "encourage highway authorities to introduce, over time, 20 mph zones or limits into streets which are primarily residential in nature and into town or city streets where pedestrian and cyclist movements are high, such as around schools, shops, markets, playgrounds and other areas, where these are not part of any major through route".

DfT Circular 01/2006 which encourages and supports Local Authorities to implement 20 mph limits and zones in situations where there is a particular risk to vulnerable road users.

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.