WASHINGTON – Laws that set numerous strict conditions before teenagers can get a license can reduce fatal crashes involving 16-year-old drivers by about one-fifth, public health researchers say.

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 driver, 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 Monday 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.

States put limits on new drivers, but the rules vary. Researchers said that by the end of 2004, 41 states and the District of Columbia had programs that included a learner’s permit with supervised training, an intermediate period with a limited amount of unsupervised driving and a final stage without restrictions.