Daytime running lights as a vehicle collision countermeasure: the Swedish evidence reconsidered.

In Sweden the use of daytime running lights (DRL) was made mandatory on 1 October 1977 for all motor vehicles at once, during all seasons and for all areas. According to a study conducted by Andersson and Nilsson (1981) [Andersson and Nilsson. VTI Swedish Road and Transport Research Institute, Report No. 208A; 1981] the introduction of DRL resulted in a reduction of 11% of multiple accidents during daytime. In many discussions on the effectiveness of DRL, these findings have been considered as the strongest evidence that the use of DRL is an effective vehicle collision countermeasure. The present study reexamines this evidence and shows that the reported 11% effect of DRL in the Swedish study is spurious. The effect is mainly the result of the application of a model that shows selective effects of DRL through modelling of unexplained changes in the number of single accidents. It is concluded that the Swedish data fail to show a clear effect of DRL.