Traffic Calming

Traffic Calming

How Effective?

The following sections present statistics that demonstrate how successful a variety of traffic calming interventions have been.

Speed reduction

  • It has been shown that on average each 1 mph reduction in mean vehicle speed results in an average RTI reduction of 5 per cent. Hence a 10 mph speed reduction may give a 50 per cent RTI saving. This result has been confirmed by a number of studies.

(DfT, 2007)

  • In one area of London, only full-width humps spaced less than 40 metres apart were able to reduce the 85th percentile speed of vehicles to below 20 mph.

(Southwark Living Streets, 2008)

  • Gateway Speeds from nine UK village traffic calming schemes (over 12 months after installation) showed that:

    • Following implementation of the schemes there were reductions in inbound speeds at all the gateways.

    • Speed reductions ranged from 3 mph to 13 mph for mean speeds, and up to 15 mph for 85th percentile speeds.

    • The largest reduction in speed at the various gateways, relative to the magnitude of 'before' speeds, occurred at the narrowed entries (with speed cushions) to the 20 mph zone at Costessey. 

    • Signing (including marking) measures giving a high visual impact can produce quite large speed reductions at village entries. However, these speeds are still likely to be above the speed limit.

  •  Speed data from within the same villages showed that:
    • Reductions in mean speed, over both directions, ranged from 2 mph to 12 mph, with 85th percentile reductions of up to 14 mph.

    • Individually, physical measures yielded speed reductions of between 7 mph and 12 mph.

    • It is important that features are visible at night, particularly as it has been shown that speeds tend to increase at this time. The use of reflective material is therefore imperative.

(DfT, March 2000)

  • In one village based scheme, two way mean speeds fell by 7-8 mph to 31 mph and 85th percentile speeds fell by 8-10 mph to 37 mph. The proportion of vehicles exceeding 40 mph (in a new 30 mph zone) fell from 50 per cent to 10 per cent.

(J. Kennedy et al., 2005)

  • Schemes with physical measures in a village would be expected to reduce mean speeds by almost 8 mph and to reduce the proportion of drivers exceeding the speed limit by more than 30 percentage points.

(DfT, December 2000)

  • Various types of horizontal deflections have been used in calming schemes to reduce traffic speed. Chicanes installed in a large number of schemes on local and trunk roads, resulted in average speed reductions for mean and 85th percentile speeds of 12 mph at the chicanes.

  • Between chicanes, this reduction dropped to 7 mph. Two way chicanes appeared to achieve slightly smaller speed reductions than one way versions.

(I.A. Sayer et al., 1998)

  • In four schemes on urban distributor roads, the 85th percentile speeds were reduced to levels below or near the 30 mph limit. On average speeds were reduced by 11 mph from 38 mph to 27 mph.Mean speeds reduced by 10 mph from 34 mph to 24 mph.

(D.C. Webster, 1995b)

RTI reduction

  • Bunn et al (2004) performed a meta-analysis of area-wide traffic calming projects and estimated a reduction of 37 per cent in fatal RTIs, 11 per cent in injury RTIs and 5 per cent in all RTIs.

(G. Yanis et al., 2008)

  • RTIs were reduced by 71 per cent at 34 sites and by 60 per cent in 20 mph zones, which comprised mainly humps.

  • RTI data supplied by Fife (Fife Council, 2004) for their ‘H’ and ‘S’ hump scheme on South Parks Road, Glenrothes, shows that injury RTIs have been reduced from 10 in 5 years (2 per year) to only one in the five years (0.2 per year) after the scheme was installed.

  • RTI reductions of 86 per cent have been reported for schemes containing speed cushions at sites in Huddersfield and Northampton. An average of 40 per cent reduction in RTIs was seen at 4 sites where thermoplastic ‘thumps’ were used.

(DfT, 2007)

  • Looking at village traffic calming projects from 1992 to 1997, the project leaders found that when vehicle speeds dropped 2 to 7 mph, RTIs were reduced by 47 percent.

  • Installation of village traffic calming greatly reduced injuries to children in the communities involved:

    • Child pedestrian injuries dropped by 40 percent;
    • Child pedestrians killed or seriously injured dropped by 77 percent;
    • Child cyclist injuries dropped by 51 percent; and,
    • Child cyclists killed or seriously injured dropped by 49 percent.


  • Of 17 schemes where before and after RTI data was available, an overall reduction in RTI frequency of 54 per cent (41.2 -19.0 RTIs per annum) was seen following the introduction of traffic calming measures.

(I.A. Sayer et al., 1998)

20 mph and home zones

  • A review of six 20 mph zones found that 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.

(D.C. Webster and A.M. Mackie, 1996)

  • In a series of pilot schemes for home zones, traffic calming reduced the mean speeds on average by about 5 mph to less than 15 mph. The 85th percentile speeds were reduced by about 6 mph to less than 19 mph.

  • The percentage of vehicles exceeding 20 mph was reduced on average from 42 per cent to 12 per cent.

  • RTIs were not a significant problem at any of the sites prior to changing to home zones, but the results indicated that a 0.3 reduction in RTIs per annum had resulted from the implementation of home zones.

(D. Webster et al., 2006)

  • From 1994, there was a widespread introduction of 20 mph zones in Hull, and by 2003, there were 120 zones covering 500 streets. The casualty statistics between 1994 and 2001 showed a drop of 14 per cent in Hull, compared to a rise of 1.5 per cent in the rest of Yorkshire and Humberside.

  • In the 20 mph zones in Hull, there was a decrease in total RTIs of 56 per cent and in KSI casualties of 90 per cent. The biggest reductions were pedestrian casualties, which fell by 54 per cent, child casualties which dropped by 54 per cent and child pedestrian casualties fell by 74 per cent.

  • A 2007 TRL review of half of the 20 mph zones which had been implemented in London (78 zones) found that they reduced injury RTIs by about 42 per cent and KSI RTIs by 53 per cent.

(RoSPA, 2012)

  • The introduction of 20 mph zones was associated with a 41.9 per cent reduction in road casualties, after adjustment for underlying time trends.

  • There was no evidence of casualty migration to areas adjacent to 20 mph zones, where casualties also fell slightly by an average of 8.0 per cent. Using the more conservative risk reduction estimates based on 2000-6, it was estimated that 20 mph zones prevent 203 casualties each year, of whom 27 would be killed and seriously injured and 51 would be pedestrians.

  • The study concluded that 20 mph zones are effective measures for reducing road injuries and deaths.

(C Grundy, et al, 2009)

  • When comparing data before and after implementation of a 20 mph zone, annual RTI frequency was found to drop by about 60 per cent.Child pedestrian RTIs fell by as much as 70 per cent and child cyclist RTIs by 48 per cent. (National Children’s Bureau, 2004)

  • In the Urban Safety Project, overall RTIs were reduced by 13 per cent but there were great variations between schemes. Slight RTIs declined proportionately more than fatal and serious ones.

(NHS Health Development Agency, 2001)

Gaps in the research

  • There appears to be some significant variation in the level of RTI reduction that traffic calming measures can achieve.

  • Therefore, further research to identify the factors that may be causing these variations would be beneficial as it would help to ensure that future schemes are designed to achieve their maximum potential in RTI reduction.



  • Date Added: 03 Apr 2012, 08:08 AM
  • Last Update: 30 Jan 2017, 01:49 PM