State laws that limit driving privileges for teens have reduced the incidence of drinking and driving among the nation's youngest licensees, according to a new study from researchers at Washington University School of Medicine in St. Louis.

The vast majority of states now have laws that limit teen driving privileges and impose stiff penalties for driving under the influence. Graduated driving licensing laws limit the number of passengers young drivers may transport and how late at night they're allowed to drive, among other restrictions.

The researchers evaluated the effects of those laws on alcohol use and risky driving behaviors among teens in 45 states. The study is now available online and will be published September 2012 in the journal Alcoholism: Clinical & Experimental Research.

"Teens in states with the strongest laws were less likely to drive after drinking or to ride in a car with a driver who had been drinking," says first author Patricia A. Cavazos-Rehg, PhD, research assistant professor of psychiatry. "In states with the toughest laws, teens were half as likely to engage in those risky behaviors."

Teens comprise less than 5 percent of licensed drivers in the country, but they account for roughly 20 percent of motor vehicle crashes.

"In the last decade, most states have strengthened their graduated licensing laws," Cavazos-Rehg explains. "States are learning that a benefit of enacting more restrictive laws is a reduction in risky behavior by young drivers. That can lead to fewer accidents and save lives."

Earlier research has demonstrated that graduated driving laws reduce both fatal and non-fatal motor vehicle accidents among teens. Other studies have found that states with more restrictive licensing laws also had experienced 30 percent reductions in fatal crashes involving teen drivers.

"We were interested in determining whether these laws were making it less likely for teens to engage in drinking and driving behaviors," Cavazos-Rehg says.

The researchers gathered drinking and driving information from more than 220,000 16-17-year olds who were surveyed between 1999 and 2009, when state laws that restrict teen driving were strengthened.

They then compared the anonymous surveys with state laws and found that teens living in states with the strictest laws also were least likely to drink and drive or to ride in a car with a driver who had been drinking.

Cavazos-Rehg can't say precisely what is causing the decrease in risky drinking and driving behaviors, but she believes nighttime restrictions that limit how late a young driver can be behind the wheel may be key.

"That's getting teens to go home earlier, which means they may not have the opportunity to go to parties where there are opportunities to drink," she says. "We haven't looked at that closely yet, but that's the next step."

The other aspects that may influence teens' behavior include stiff penalties, like suspension of a driver's license, for violations, Cavazos-Rehg says. Many laws also require more experience and education behind the wheel before a license is granted than was required in past years. And in most states that have enacted these laws, teens don't receive full driving privileges until they are 18 or even older.