AAA:

Graduated driver licensing (GDL) systems are designed to protect new drivers by limiting their exposure to risk initially and gradually phasing in additional driving privileges as they gain experience. Although numerous studies have shown that GDL has significantly reduced the numbers of 16- and 17-year-old drivers involved, injured, and killed in motor vehicle crashes, the few studies of its effects on 18- and 19-year-olds have produced conflicting results. Some researchers suspect that because most states’ GDL systems only apply to new drivers younger than 18, GDL might encourage young people to wait until age 18 to obtain a license to avoid the requirements and restrictions associated with GDL, resulting in older teenagers having less driving experience and higher crash risk than they would without GDL.

Via Syeven Sinofsky