Research at TRL has shown that a reduction in vehicle speeds generally leads to a reduction in the number and severity of accidents, (Finch et al, 1994) and (Taylor et al, 2000). Broadly, each 1 mph reduction in speed is expected to cut injury accident frequency by 5%. There are a number of ways in which substantial speed reductions can be achieved and one of these is the '20 mph zone' in which the 20 mph limit is indicated by boundary signing and physical traffic calming measures are used to provide a self enforcing speed reducing element (DfT, 1999). The use of 20 mph speed limit signs alone, without supporting traffic calming features is likely to lead to only small speed reductions of about 1 mph (Mackie, 1998). The review of urban speed management methods by Mackie also indicated that there was little reduction in injury accidents in 20 mph zones and (30kph zones) which used signs alone apart from the city of Graz in Austria (13% reduction) where the signs-only installations were accompanied by a comprehensive publicity and enforcement campaign. A national study of two hundred 20 mph zones in Great Britain was carried out by TRL for the DfT, which showed that 20 mph zones were beneficial in reducing speeds and accidents (Webster and Mackie, 1996). The main results of the report were that: • Speeds were reduced by 9 mph; • Flows were reduced by 27%; • Injury accidents (all seventies) were reduced by 61%; • Fatal and serious accidents (KSI accidents) were reduced by 70%; • Pedestrian injury accidents were reduced by 63%; • Pedal cyclist injury accidents were reduced by 29%; • Motor cyclist injury accidents were reduced by 73%; • Child (pedestrian and cyclist) injury accidents were reduced by 67%; • The ratio of fatal and serious accidents (KSI accidents) as a proportion of the total was reduced from 0.21 to 0.16.; • Injury accidents were reduced by 6.2% for each 1mph speed reduction; • There was no apparent overall accident migration on the surrounding roads. This level of reduction in accidents and casualties is supported by more recent data from the City of Hull where 26% (191 km) of the city's roads are subject to 20 mph speed restrictions. One hundred and twenty 20 mph zones have been installed since 1994 covering 500 streets. Injury accidents in the zones have decreased by 56%; KSI accidents by 90%; pedestrian casualties by 54%; child casualties by 64%: and child pedestrian casualties by 74% (Brightwell, 2003). In October 2002, TRL were commissioned by the London Accident Analysis Unit (LAAU) to review the performance of 20 mph zones in London. Only five of the 20mph zones in London were included in the earlier DfT national study as, at that time (1996), many had not been installed long enough to obtain the required minimum of 12 months 'after' accident data. This present study applies a similar methodology used for the Dif project to the many 20mph zones in London, allowing (among other things) a comparison of the changes in accident frequency for the London schemes against 20 mph zones in other areas. Section 2 of this report provides information about the 20 mph zones in London, Section 3 considers their impact on injury accidents, Section 4 considers their impact on casualties, Section 5 considers the changes in changes in traffic speeds and flows within the 20 mph zones and Section 6 contains the summary of results and conclusions. (Author/publisher) For the summary report see http://www.tfl.gov.uk/assets/downloads/corporate/ResearchSummaryNo2_20m…
Abstract