Road traffic casualties: understanding the night-time death toll

A disproportionate number of fatal injuries occur after dark. The paper presents some statistics of road traffic injuries in a novel way which suggests that low luminance plays a major role in this effect. A sound physiological explanation for this is advanced based on the poor temporal characteristics of rod photoreceptors. It is argued that processing information based on low luminance, low contrast targets is much slower than that for high contrast bright targets. To test the idea, simple visual reaction times were measured under typical low visibility conditions encountered on non-lit roads and were found to be substantially longer than under optimal conditions. It is shown that longer reaction times translate into significantly increased stopping distances. This important point has received insufficient attention in the road safety literature, by the Highways Agency, the police, injury prevention officials, and the UK Highway Code.