Objective: To determine how the number of passengers, their age and their sex influence the risk of different types of Spanish drivers causing a collision between two or more cars.
Methods: We selected, from the Spanish database of traffic crashes resulting in personal injuries or death, those collisions between two or more cars that occurred between 1990 and 1999 in which only one of the involved drivers committed a driving infraction. These drivers were considered the cases; non-infractor drivers were considered their matched controls. We collected information on the number, age and sex of the passengers in each vehicle, along with some potential confounding variables of the drivers and the vehicles involved. Crude and adjusted odds ratios were calculated for the main categories of driver and passenger.
Results: A protective effect for the presence of passengers was detected (adjusted odds ratio: 0.69; 95% CI: 0.67-0.70). The protective effect was higher for drivers aged more than 45 years and lower for the youngest drivers (<24 years old). The strongest association was observed for female passengers who accompanied male drivers. The protective effect was lower for passengers older than 64 years.
Conclusions: Our results suggest that drivers are less likely to cause a car collision between two or more cars that results in personal injuries or death when they are accompanied by passengers, regardless of driver or passenger characteristics.