WASHINGTON - For generations, teenagers have been the most dangerous drivers on the road, crashing almost four times more often than older drivers. A study released Tuesday quantifies for the first time in a decade how their risk of a fatal crash multiplies when they have other teenagers in the car.

It increases by almost half when a 16- or 17-year-old driver has one teenage passenger; it doubles with two teen passengers, and it quadruples with three or more young passengers.

Using federal fatality statistics, the American Automobile Association Foundation for Traffic Safety provided the data, which support parents who have forbidden their teenage children from driving with other teenagers.

"We know that carrying young passengers is a huge risk, but it's also a preventable one," said AAA Foundation President Peter Kissinger. "These findings should send a clear message to families that parents can make their teens safer immediately by refusing to allow them to get in the car with other young people, whether they're behind the wheel or in the passenger seat."

In order to increase teen safety, many states already have graduated licensing laws and requirements for driver education or parental supervision.

The AAA study is the latest of three recent reports to raise concerns about teenage drivers. The Governors Highway Safety Association reviewed preliminary data from the first six months of last year, finding a slight increase in the number of fatal crashes involving 16- and 17-year-old drivers. The governors' group said if the trend continued, 2011 would reverse a recent trend of declining teenage fatalities.

In another study, National Highway Traffic Safety Administration last month said its research found that drivers under the age of 24 were much more likely than more mature drivers to send and receive text messages while driving.

Overall, highway fatalities declined for the sixth consecutive year in 2011, according to preliminary data NHTSA released Monday. The agency said 32,310 people died in crashes last year, a decrease of 375 deaths from 2010. NHTSA will break down the statistics by age group when final data are released later this year.