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Effect of 20 mph traffic speed zones on road injuries in London, 1986-2006: controlled interrupted time series analysis

Chris Grundy, lecturer in geographical information systems1, Rebecca Steinbach, research fellow1, Phil Edwards, senior lecturer in statistics2, Judith Green, reader in sociology of health1, Ben Armstrong, professor of epidemiological statistics1, Paul Wilkinson, reader in environmental epidemiology1.

1Department of Public Health and Policy, London School of Hygiene and Tropical Medicine, London WC1E 7HT; 2Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London.

Correspondence to: C Grundy

Objective: To quantify the effect of the introduction of 20 mph (32 km an hour) traffic speed zones on road collisions, injuries, and fatalities in London.

Design: Observational study based on analysis of geographically coded police data on road casualties, 1986-2006. Analyses were made of longitudinal changes in counts of road injuries within each of 119 029 road segments with at least one casualty with conditional fixed effects Poisson models. Estimates of the effect of introducing 20 mph zones on casualties within those zones and in adjacent areas were adjusted for the underlying downward trend in traffic casualties.

Setting: London.

Main outcome measures: All casualties from road collisions; those killed and seriously injured (KSI).

Results: The introduction of 20 mph zones was associated with a 41.9% (95% confidence interval 36.0% to 47.8%) reduction in road casualties, after adjustment for underlying time trends. The percentage reduction was greatest in younger children and greater for the category of killed or seriously injured casualties than for minor injuries. 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% (4.4% to 11.5%).

Conclusions: 20 mph zones are effective measures for reducing road injuries and deaths.

Published 10 December 2009, doi: 10.1136/bmj.b4469. Cite this as: BMJ 2009; 339:b4469