20mph speed limits and zones for better public health: Meta-narrative evidence synthesis

Background: 

Recently, twenty miles per hour (20mph/30kph) speed limit and zone interventions have been increasingly adopted in various European cities. These 20mph speed limits and zones do not only contribute to road safety but they may also be beneficial for public health outcomes such as active travel, physical activity, and air pollution. We aimed to provide an updated assessment of the evidence for potential health effects of 20mph speed limits and zones. 

Methods: 

We searched four electronic databases MEDLINE, EMBASE, Web of Science and Transport Research Information Service and assessed the grey literature through keywords related to ‘20mph’ or ‘30kph’ and health outcomes. 

Results: 

A total of 23 publications (13 academic and ten grey literature publications) were identified. Overall, 20mph speed limits and zones have clear effects on injuries, collisions, and casualties. The wider effects of 20mph speed limits and zones on public health outcomes such as active travel, physical activity, and air pollution need further evaluation. Several subgroup analyses suggested differential effects for age groups, gender, neighbourhood deprivation level, ethnic background, employment status, health status, and type of road user (e.g., cyclist). Additionally, methodological limitations of the evidence base were highlighted, including challenges regarding exposure and outcome measurement and description, and the difficulty of evaluating changes in public health variables over long follow-up periods when using natural experimental methods. 

Conclusions: 

20mph speed limits and zones have the potential to improve road safety. Whilst the broader public health effects of 20mph remain poorly understood given difficulties in evaluating, the logic path through which 20mph speed limits and zones can benefit public health is clear. Future research should address this gap and consider the broader role that lower speed limits and zones can play in creating healthier cities.