When teenagers drive with peers in their vehicles, they drive faster than other motorists and leave less distance between themselves and the vehicles in front of them. They more frequently engage in other risky behaviors like speeding. These are the findings of a study by researchers at the National Institutes of Health and Westat. Lead author of the study, Bruce Simons-Morton, notes that “peers in vehicles, especially males, not only can distract teen drivers but also may encourage them to take more risks than they ordinarily would — risks like driving faster and closer to the cars in front of them.” Simons-Morton said these findings support the idea of establishing passenger restrictions as part of states’ graduated licensing systems. Thirty-nine states and the District of Columbia have passenger restriction laws for teen drivers.
("Observed effects of teenage passengers on the risky driving behavior of teenage drivers” by B. Simons-Morton et al. was published in Accident Analysis & Prevention 37 (2005).)