Objective: To identify the most appropriate metric to determine the effectiveness of mobile speed cameras in reducing road traffic related injuries.
Design: Controlled before and after study which compares two methods for examining the local effectiveness of mobile speed cameras-a circular zone around the camera and a route based method to define exposure at various distances from sites.
Setting: South Wales, UK.
Subjects: Persons injured by road traffic before and after intervention.
Intervention: Use of mobile speed cameras at 101 sites.
Main outcome measures: Rate ratio of injurious crashes at intervention and control sites.
Results: Camera sites had lower than expected numbers of injurious crashes up to 300 metres using circles and up to 500 metres using routes. Routes methods indicated a larger effect than the circles method except in the 100 metres nearest sites. A 500 metre route method was used to investigate the effect within strata of time after intervention, time of day, speed limit, and type of road user injured. The number of injurious crashes after intervention was substantially reduced (rate ratio 0.49, 95% confidence interval 0.42 to 0.57) and sustained throughout two years after intervention. Significant decreases occurred in daytime and night time, on roads with speed limits of 30 and 60-70 miles/hour and for crashes that injured pedestrians, motorcycle users, and car occupants.
Conclusions: The route based method is the better method of measure effectiveness at distances up to 500 metres. This method demonstrates a 51% reduction in injurious crashes.