Dr. Azofeifa offered three likely explanations: an overall drop in drinking among young people, aggressive law enforcement steps like roadside testing and “a lot of prevention efforts” at schools across the country.

Image Police officers conducted a roadside "sobriety check point" in Washington in 2012. Credit... Mark Wilson/Getty Images

Even with the declines, the numbers suggest there still were a lot of dangerous young motorists on the road last year. Nearly one in five people between the ages of 21 and 25 acknowledged drinking and driving. Among 16- to 20-year olds, they were far fewer, about one in 15.

The study relied on data from more than 380,000 respondents to the federal government’s annual National Survey on Drug Use and Health. Young people were asked during in-person interviews to report their use of alcohol and other drugs over the past 12 months.

As the study notes, vehicle accidents are the leading cause of death among young people. In 2013, more than 2,000 people between the ages of 16 and 19 were killed on American roads — or about six a day, according to the C.D.C.

J. T. Griffin, a lobbyist with Mothers Against Drunk Driving, welcomed the latest findings, attributing at least part of the improvement to education efforts.