Investigating the effect of ambient light conditions on road traffic crashes: The case of Cape Town, South Africa

Investigating the risk of road crashes associated with ambient light is often challenged by bias resulting from confounding effects of other risk factors. The study used the odds ratio and the risk ratio methods to isolate the effect of ambient light from other risk factors. This was possible in this research using the ‘whole-year’ approach adopted in previous research and utilising seasonal variations in ambient light in Cape Town. The analysis was based on crash and ambient light data collected between 2014 and 2018 in Cape Town. A total of 361 452 crashes were reported during this period. With only minor exceptions, the results confirm a higher crash risk in both twilight and dark conditions. The increase in the risk was different depending on the ambient light level, the crash type (total and pedestrian crashes) and the time of the day (morning and evening times). The results suggest that more efforts need to be made to assist road users to travel safely under twilight and dark conditions.