Effects of 20 mph interventions on a range of public health outcomes using the meta-narrative method

Background: Road traffic injuries are a leading cause of preventable death globally, but can be reduced by introducing speed lowering interventions such as 20 mph or 30 km/h speed ‘zones’ and ‘limits’. ‘Zones’ utilise physical traffic calming measures and ‘limits’ only utilise signage and lines. Transport is a social determinant of health and therefore such interventions may in/directly also impact on other health outcomes. Aim: To investigate the effect of 20 mph speed ‘zones’ and ‘limits’ on a range of health outcomes, and to establish if there are differences in the effectiveness of 20 mph zones and 20 mph limits. Methods: MEDLINE, EMBASE, Web of Science and Transport Research Information Service (TRIS) databases were searched [1983-January 2019) to identify relevant studies. Reference lists, relevant systematic reviews and the grey literature were also searched. Inclusion criteria: 20 mph ‘zone’ or ‘limit’ interventions: and public health outcomes (collisions, casualties, mode of transport, noise pollution, air quality, inequalities and liveability (e.g. physical activity and perceptions of safety)) and including a control/comparison group. Results: Eleven studies were identified reporting nine 20 mph ‘zone’ and two 20 mph ‘limit’ interventions. 20 mph ‘zones’ were associated with a reduction in the number and severity of collisions and casualties; have less robust evidence of the effect on air pollution; and have the potential to indirectly impact physical activity and liveability through various mechanisms for change (although currently the evidence is lacking and requires further work). No significant associations were reported between 20 mph ‘limits’ and any public health outcome. Conclusion: This review suggests 20 mph ‘zones’ are effective in reducing collisions and casualties. However, it provides insufficient evidence to draw conclusions on the effect of 20 mph ‘zones’ on pollution, inequalities or liveability. For 20 mph ‘limits’ more rigorous evaluations are required in order to draw robust conclusions