Now researchers say they're trying to figure out why the graduated driver's license programs are so successful.
The nonprofit National Safety Council has collected 12 studies in which researchers examined similar programs in the U.S., Canada and New Zealand.
In each case, the teen crash rate was reduced after the implementation of learner's permits that let teens drive only with adult supervision, and intermediate licenses that let teens drive alone, but only during daylight hours.
Researchers say the most likely reason for fewer accidents is that provisional licenses keep teens off the road, particularly at night. But researchers say it's also possible that the restrictions encourage more careful driving by teens or more parental supervision.
Opponents who include some state lawmakers say the programs infringe on parental rights.