Twelve states in the U.S. have curfew laws, which prohibit young drivers from operating motor vehicles during late evening or early morning hours. The effect of such laws on motor vehicle crashes was studied in four of these states. In each state the laws were found substantially to reduce the crashes of sixteen year olds. Sixteen year old driver crash involvements during curfew hours were reduced by an estimated 69 percent in Pennsylvania, 62 percent in New York, 40 percent in Maryland, and 25 percent in Louisiana. Except in Maryland, the percentage of the sixteen year old population licensed was lower in curfew than comparison states. It is possible that curfew laws reduce early licensure, in which case reductions in crash involvements resulting from curfews are greater than shown above.