Background Road traffic casualties show some of the widest socioeconomic differentials of any cause of morbidity or mortality, and as yet there is little evidence on what works to reduce them. This study quantified the current and potential future impact of the introduction of 20 mph zones on socioeconomic inequalities in road casualties in London.

Methods An observational study based on analysis of geographically coded police road casualties data, 1987–2006. Changes in counts of casualties from road collisions, those killed and seriously injured and pedestrian injuries by quintile of deprivation were calculated.

Results The effect of 20 mph zones was similar across quintiles of socioeconomic deprivation, being associated with a 41.8% (95% CI 21.0% to 62.6%) decline in casualties in areas in the least deprived quintile versus 38.3% (31.5% to 45.0%) in the most deprived quintile. Because of the greater number of road casualties in deprived areas and the targeting of zones to such areas, the number of casualties prevented by zones was substantially larger in areas of greater socioeconomic deprivation. However, the underlying decline in road casualties on all roads was appreciably greater in less deprived areas (p<0.001 for trend) so that despite the targeting of 20 mph zones, socioeconomic inequalities in road injuries in London have widened over time. Extending 20 mph schemes has only limited the potential to reduce differentials further.

Conclusions The implementation of 20 mph zones targeted at deprived areas has mitigated widening socioeconomic differentials in road injury in London and to some degree narrowed them, but there is limited potential for further gain.