Washington ? Laws that set numerous strict conditions before teenagers can get a license can reduce fatal crashes involving 16-year-old drivers by up 21 percent, public health researchers say.

The more restrictions imposed, the greater the reduction. Examples include a waiting period before a young driver is eligible to move from a learner’s permit to an intermediate license, restrictions on driving at night, required hours of supervision by an adult and limits on the number of passengers a teenage driver can have.

States with such restrictions as part of strong graduated driver’s licensing programs showed declines in fatal crashes involving 16-year-olds, according to a study being released today by Johns Hopkins Bloomberg School of Public Health in Baltimore.

“We already knew that the programs reduced crash rates of young drivers, but we didn’t know which programs were most effective in reducing risk,” said Susan P. Baker, a professor at the school. From the study, “it is clear that more comprehensive programs have the greatest effect,” she said.

Traffic accidents are the leading cause of death for teenagers. Federal figures show that 16-year-old drivers were involved in 957 fatal crashes that killed 1,111 people in 2004. Those crashes resulted in the deaths of 399 16-year-old drivers and 385 16-year-old passengers.

The study based its analysis on programs requiring:

¢ a minimum age of 15 1/2 for earning a learner’s permit.

¢ a waiting period of at least three months after getting a learner’s permit before applying for an intermediate license.

¢ a minimum of 30 hours of supervised driving.

¢ a minimum age of 16 for an intermediate state license.

¢ a minimum age of 17 for full licensing.

¢ driving restrictions at night.

¢ a restriction on carrying passengers.

The study found that such programs reduced fatal crashes for 16-year-old drivers by an average of 11 percent.

When the researchers compared states with five program components to states without a program, they found an 18 percent reduction in fatal crashes involving 16-year-old drivers.

Programs with six or seven components were linked to a 21 percent reduction, showing the added benefit.