Evaluating the safety and speed impacts of the 20mph speed limit in the UK: Evidence and insights

Studies on the effectiveness of 20mph speed limits in the UK vary due to differences in context, methods, data, environments, and schemes. Many of them do not account for confounding factors like regression-to-the-mean, long-term trends, or changes in traffic volume, making it difficult to generalise their overall impact. Moreover, there is a need to account for the different effects of physical measures (speed humps, cameras) and sign-only measures, when examining the benefits of 20mph limits. Hence, the present paper attempts to add to current knowledge in the field and address these issues by developing a methodology to assess and classify the quality and validity of existing studies on 20mph speed limits in the UK. In this research we reviewed 21 studies and more than 260 effect estimates in total, applying fixed-effects and random-effects analyses to evaluate the impact on crashes, personal injuries as well as speed changes. Our analyses show that 20mph speed limits reduced traffic crashes by an average of 26.45%, with a 21.64% reduction for schemes with signs only. Casualties decreased by 22.92% for all schemes, compared to 10.91% for sign-only schemes. Furthermore, the introduction of 20mph speed limits with physical measures resulted in greater reductions in crashes and casualties than sign-only schemes. Additionally, sign-only schemes reduced mean speed by 1.63 mph. These findings are expected to assist policymakers to make informed decisions on implementing 20mph speed limits.