Chicago ? Parents beware: Giving in to teens’ demands for their own cars can have dangerous consequences, new research suggests.

Teenagers with their own cars or free use of one are much more likely to get in crashes than those who share a car. And crashes are much less common among teens whose parents set clear driving safety rules.

The findings are in two studies by researchers at Children’s Hospital of Philadelphia and funded by State Farm Insurance Co. They were released Friday and are in the October issue of Pediatrics.

The researchers say the findings can help parents keep their kids from becoming a grim statistic: Traffic crashes are the leading cause of death for U.S. teens, killing more than 5,000 each year.

Getting a driver’s license and car are often viewed as a rite of passage for U.S. teens, and many parents underestimate the risks.

More than 7,000 people nationwide were killed in crashes involving teen drivers in 2007, government data show. More than 3,000 of these deaths were teen drivers, and more than 250,000 teen drivers were injured.

“With teen drivers, you have to recognize that it’s a public health issue,” said Dr. Jeffrey Weiss, a Phoenix pediatrician who co-wrote an American Academy of Pediatrics report on teen drivers.

The 2006 report encourages parents to highlight the seriousness of driving privileges by requiring teens to sign driving contracts promising to abide by safety rules.