25 September 2002: Traffic accidents are more frequent on motorways when traffic flows are lighter, under 400 vehicles per hour, according to research carried out on French autoroutes over a two-year period. They also tend to be more severe.

The incidence rates are at their lowest when traffic flows are between 1,000-1,500 vehicles/h. After that, they begin to rise steadily.

The research, carried out by Jean-Louis Martin, Institut National de Recherche sur les Transports et leur Securite, Bron, also suggested that crash incidence rates are higher on 3-lane than on 2-lane motorways and higher at weekends (when truck traffic is restricted) than on weekdays. In heavy traffic, the number of crashes is higher on weekdays.

Dr Martin found no significant difference between the number of day time and night time crashes, whatever the traffic. However, the severity of crashes was greater at night and when hourly traffic is light.