- This study provides quantitative data supporting a policy of reducing vehicle speed limits to 20 miles per hour (mph).
- Reducing the speed limit causes drivers to slow down.
- Reducing the speed limit does not motivate drivers to change their mode of transportation from driving to active travel. Because there is not a decrease in vehicle volume, the authors infer that the reduction in speed did not increase feelings of safety enough to shift modal choice (i.e., choosing to bike rather than drive).
- Public attitudes toward a 20-mph speed limit policy improved after the intervention while public perceptions of safety did not change.
- Reducing the speed limit to 20 mph causes drivers to slow down and adds to the growing evidence of the effectiveness of speed interventions. This is one relatively lower-cost strategy to improve safety for people biking and walking. While the study did not find this would directly impact mode choice, a slower speed does benefit public health. There will be fewer crashes and when crashes do occur, injuries will be less severe, and fatalities will be reduced.
- Lowering the speed alone is not enough to encourage people to bike or walk instead of drive. Policies that reduce speed limits can work well in tandem with policies like Complete Streets which allow for roadway improvements that slow vehicles and increase safety and comfort for people walking and biking. This would maximize its benefits and motivate people to use active transportation.
- Further research is needed regarding the impact of speed limit interventions on mode choice. For example, a study that collects comparison data (using a control group) can more easily identify independent variables impacting speed reduction.
Nightengale, G.F., Williams, A.J., Hunter, R.F., Woodstock, J., Turner, K., and Cleland, C.L. (2021). Evaluating the citywide Edinburgh 20mph speed limit intervention effects on traffic speed and volume: A pre-post observational evaluation. Public Library of Science, 16, 12.