Road safety: time to rethink how we assess interventions?

How should the effectiveness of road safety interventions be measured? The author of a new report says a fundamental rethink is needed because the standard method – comparing data from three years before and after an intervention – is flawed if the before period casualty data influenced the selection of sites for treatment

Ever since the first speed cameras were installed on Britain’s roads in the 1990s, a debate has raged about how effective they are in cutting collisions and casualties. From time to time reports have been published showing a significant drop in accidents and casualties at camera sites, typically based on comparing three years of data before and after cameras were installed.

But such comparisons overlook two vital points. Firstly, the number of collisions can change due to general...

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