We investigated behavioral changes in a large cohort of drivers to identify underlying causes of the decline in road casualties in France. In 2001 and 2004, 11240 participants used self-administered questionnaires to report attitudes toward road safety and driving behaviors. Injury road traffic collisions were recorded from 2001 to 2005 through the cohort's annual questionnaire. Between 2001 and 2004, speeding and cell phone use decreased concomitantly with a decrease in injury road traffic collision rates among participants. Reported driving while sleepy remained unchanged and driving while alcohol intoxicated was reported by a higher proportion in 2004 than in 2001. Decreases in speeding between 2001 and 2004 were strongly linked with positive attitudes toward road safety in 2001. In this cohort, speeding and using a cell phone while driving decreased over the 2001 to 2004 period concomitantly with increases in traffic law enforcement and a dramatic decline in road mortality in France. However, the deterrent effect of traffic enforcement policies may have been reduced by negative attitudes toward traffic safety and having had a history of traffic penalty cancellations.