Review of traffic calming schemes in 20 mph zones

  • Published: TRL, 1996
  • Authors: D.C. Webster and A.M. Mackie
  • Date Added: 05 Feb 2013
  • Last Update: 04 Jul 2014
  • Format: html


To review the effect of traffic calming using 20 mph zones.


A literature review was undertaken to assess the impact of six 20 mph zones around the UK.

Key Findings:

  • The most quoted reason for applying for an authorisation of a 20 mph zone was RTI reduction indicating that safety is the main justification for implementing the schemes.

  • Most zones were in predominantly residential areas, with about 10 per cent being in town or city centres.

  • The average size of the zones was 0.28

  • The most common traffic calming measures used in 20 mph zones were round topped and flat topped humps and speed tables. Speed cushions were used in some more recent schemes.

  • The 20 mph zones have been successful in substantially reducing speeds and RTIs in the areas where they have been applied.

  • Child pedestrian and child cyclist RTIs fell by 70 and 48 per cent respectively after scheme installation giving an overall reduction of 67 per cent for all child RTIs.

  • The reduction in RTIs for all cyclists was 29 per cent.

  • Speed results show that the average speed at a calming measure was 13.2 mph while between measures it rose to 17.8 mph.

  • This indicates that the calming measures are effective at enforcing the 20 mph limit.

  • There was a 6.2 per cent reduction in RTIs for every 1 mph reduction in vehicle speed.

  • RTI migration into surrounding areas was not found to be a problem but care should still be taken in design of schemes to prevent traffic transferring to unsuitable routes.


Speed reduction, RTI reduction, 20 mph zones, traffic calming


Good example of reductions in RTIs numbers resulting from speed reductions.