Graduated licence programs seem to help prevent fatal crashes among 16-year-olds but not for older teens, a U.S. study suggests.

Graduated driver licensing programs are designed to teach teens the rules of the road while preventing them from getting behind the wheel under more dangerous conditions such as driving at night or with several passengers in the car. New Jersey is one of the few states where graduated driver's licensing restrictions apply to all first-time applicants younger than 21. Doug Hood/The Asbury Park Press/Associated Press

In Wednesday's issue of the Journal of the American Medical Association, researchers looked at the association of graduated licence programs and fatal crashes for 16- to 19-year-olds from 1986 to 2007.

Over that time, fatal crash incidence fell among 16 to 17-year-old drivers but hit a plateau at ages 18 to 19, which suggests teens are just putting off getting a licence until 18.

"There's an incentive right now to skip out and just wait until you're 18," said Scott Masten, the study's lead author and a researcher with California's Department of Motor Vehicles.

"I was actually bummed by my own findings — to find out we're offsetting the benefits" in young drivers so much, he said. "It was quite unexpected."

All U.S. states have a graduated licence program but the restrictions vary, such as how much time teens must drive supervised by an adult before making decisions on their own.

The Insurance Institute of Highway Safety evaluated the strength of state licensing systems as good, fair, marginal or poor depending on criteria such as entry age and length of the learner and intermediate phases of the licence, and how strict the restrictions were on nighttime driving and allowable passengers.

New Jersey is one of the few states where graduated driver's licensing restrictions apply to all first-time applicants younger than 21.

That approach has been associated with significant reductions in crash rates for 17- and 18-year-old, virtually eliminated crashes among 16-year-olds without adversely affecting crash rates for 19-year-olds, Anne McCartt and Eric Teoh of the Insurance Institute for Highway Safety said in a journal commentary accompanying the study.

"Additional crash reductions in many states likely could be achieved through strengthening current graduated driver licensing programs," the commentators wrote. "Whether and how graduated driver licensing has affected older teenagers and whether these programs should be extended to include older teenagers merit further study.

With files from The Associated Press