A pair of studies by The Children's Hospital of Philadelphia and State Farm have identified factors that may lead teens to drive with multiple peer passengers and, then, how those passengers may affect their driver's behaviour just before a serious crash.

The US studies have been published in the Journal of Adolescent Health.

Experts have long known that peer passengers increase teen driver crash risk. What hasn't been well understood was how they increase crash risk.

The first study surveyed 198 teen drivers and found that teens who are most likely to drive with multiple passengers shared the following characteristics: they considered themselves "thrill-seekers," perceived their parents as not setting rules or monitoring their whereabouts, and possessed a weak perception of the risks associated with driving in general.

"The good news is that that these teens make up the minority," said Jessica Mirman, PhD, study author and a behavioural researcher. "Teens in this study generally reported strong perceptions of the risks of driving, low frequencies of driving with multiple passengers, and strong beliefs that their parents monitored their behaviour and set rules."

The second study analysed a nationally-representative sample of 677 teen drivers involved in serious crashes to compare the likelihood of driver distraction and risk-taking behaviours just prior to the crash when teens drive with peer passengers and when they drive alone.

According to study author Allison Curry, PhD, director of epidemiology at the Center for Injury Research and Prevention, both male and female teen drivers with peer passengers were more likely to be distracted just before a crash as compared to teens who crashed while driving alone.

"Among the teens who said they were distracted by something inside the vehicle before they crashed, 71% of males and 47% of females said they were distracted directly by the actions of their passengers," she explained.

Additionally, the researchers found males with passengers were almost six times more likely to perform an illegal manoeuvre and more than twice as likely to drive aggressively just before a crash, as compared to males driving alone. Females rarely drove aggressively prior to a crash, regardless of whether they had passengers in the car.

The study authors also emphasised the important role parents play in supporting safe driving among teens and their passengers. They recommend parents set a house rule of no non-sibling teen passengers for the first six months of driving and only one non-sibling passenger for the second six months.