Stricter driver's license laws for U..S teenagers have coincided with fewer fatal crashes among 16-year-olds but more among 18-year-olds, a nationwide finds.

The study suggests the restrictions have not produced the desired effects. The well-intentioned measures, which release driving privileges in stages, have only shifted the status of inexperienced drivers from younger to older teens, some critics say.

The combination of immaturity and inexperience makes teenage drivers especially susceptible to motor vehicle accidents.

The rules vary considerably by state, but they characteristically have a minimum age to earn a driver's permit and a driver's license. There is also the requirement that a certain number of hours behind the wheel are completed while being supervised by an adult.

For assessing the value of such programs, a team led by Scott V. Masten of the California Department of Motor Vehicles Research and Development Branch in Sacramento examined data on more than 131,000 fatal crashes which involved teen drivers from all 50 states and the District of Columbia between 1986 and 2007.

The study checked states with the most restrictive graduated licensing programs involving supervised driving time as well as night-driving restrictions and passenger limitations.

The study found a 26 percent reduction in the rate of fatal crashes involving 16-year-old drivers compared with states without any restrictions. But the rate of fatal crashes among 18-year-old drivers in those states jumped 12 percent compared with the states without restrictions.

The study also compared drivers in states with strong graduated licensing programs with those in states with weak programs. The rate of fatal crashes among 16-year-old drivers was 16 percent lower but was 10 percent higher among 18-year-old drivers.

The study was published in the Journal of the American Medical Association.