Comment on Williams and Farmer's evaluation of daytime running lights.

In ‘Comments on Theeuwes and Riemersma’s Revisit of Daytime Running Lights’ Williams and Farmer ( 1996) comment on a paper we published in Accident Analysis and Prevention. In this paper we show that the original claim by Andersson and Nilsson ( 198 1) that the nationwide implementation of Daytime Running Lights (DRL) in Sweden resulted in a reduction of multiple daytime accidents was not warranted. We showed that when the correct statistical procedures are applied, the data show no effect of DRL. Without commenting or doubting any of our statistical procedures, Williams and Farmer come to the conclusion that the implications of our study ‘DRLs are ineffective in reducing crashes’ is not warranted. This conclusion by Williams and Farmer is unexpected. Even though the most detailed and comprehensive study on DRL shows no effect, Williams and Farmer want to believe otherwise. The Swedish study is considered the most comprehensive study in the world since it is the only study that provides accident data of two years before and two years after the nationwide mandatory implementation of DRL. When these data were analyzed appropriately, we showed that there was no effect of DRL. Williams and Farmer come up with a whole set of different arguments why-in spite of the fact that there was no effect in Sweden-we have to believe that DRLs are effective. First, there is supposedly a theoretical basis for a positive effect of DRL. After conducting an extensive literature review (Theeuwes and Riemersma 1990), we concluded that there was no theoretical basis for an effect of DRL during normal daylight condition. Obviously, in dusk (ambient luminance level of 1000 lux or less), DRL, as any