Parents and driving experts long have known that teen drivers are more likely to crash if they have a bunch of friends in the car.
Now, two studies show what may predispose teens to pile buddies into the car and how those passengers can lead them to drive unsafely.
"Knowing this, we can develop programs that work in tandem with current graduated driver licensing laws that limit the number of passengers for teens during their first year of driving," study author Allison Curry said in a press statement.
Curry is director of epidemiology at the Center for Injury Research and Prevention at the Children's Hospital of Philadelphia, which conducted the research with State Farm.
In a survey of 198 teen drivers, the first study found that teens who were most likely to drive with multiple passengers considered themselves "thrill-seekers," perceived their parents as not setting rules or monitoring their whereabouts, and had a poor grasp on driving risks.
Study researchers noted that most teens reported seldom driving with multiple passengers, and most had a good handle on driving risks and held strong beliefs that their parents monitored their behavior and set rules.
The second study analyzed 677 serious crashes involving teen drivers. It found that teen drivers with teen passengers were more likely to be distracted just before a crash, compared to teens who crashed while driving alone.
Among the teens who said they were distracted by something inside the vehicle before it crashed, 71 percent of males and 47 percent of females said they were distracted by their passengers.
Teen boys were six times more likely to break a driving law and more than twice as likely to drive aggressively before a crash, compared to boys driving alone. Girls rarely drove aggressively before a crash, whether or not they had passengers.
Researchers recommend parents set a house rule of no teen passengers, other than siblings, during the first six months of driving and no more than one friend in the car for the second six months.
Teen drivers have notoriously high car insurance rates because they are more likely to get into accidents than adults. The crash rate per mile driven for 16- to 19-year-olds is four times the risk for older drivers, according to the Insurance Institute for Highway Safety.