On the relationship between congestion and road safety in cities

We empirically examine the relationship between traffic congestion and deaths in road accidents at the city level. We use panel data from 129 large cities in Europe for the period 2008–2017. We find strong evidence of a quadratic relationship between congestion and deaths in accidents, using both parametric and non-parametric econometric techniques. The threshold point at which the relationship between congestion and deaths in accidents is reversed and becomes positive occurs when congestion results in about a 30 per cent increase in travel time compared to a free flow situation. For most congested cities, any effective measure to contain congestion may also lead to better safety outcomes.