There are fewer drunk drivers on the road, but their place has been taken by people high on marijuana and prescription drugs, according to two reports from the National Highway Traffic Safety Administration.

The number of inebriated drivers has declined by almost a third since 2007, but a 2014 survey found that nearly one in four on the road tested positive for a drug that endangered them or others, NHTSA said.

“The latest roadside survey raises significant questions about drug use and highway safety,” NHTSA Administrator Mark Rosekind said. “The rising prevalence of marijuana and other drugs is a challenge to everyone who is dedicated to saving lives and reducing crashes.”

The survey is a voluntary, anonymous effort to collect information from regions across the country. Road signs tell drivers that a data-collection site is ahead of them, and those who wish to participate pull over. The survey has been taken five times in the past 40 years.

The 2014 survey found that about 8 percent of people on the road on weekend nights had alcohol in their system, and slightly more than 1 percent were above the legal limit. That was 30 percent below the 2007 figure, NHTSA said, and an 80 percent drop since the first survey was taken in 1973.

But the number of people with drugs in their system was found to be on the rise. It jumped from 16.3 percent in 2007 to 20 percent of weekend nighttime drivers in 2014. Drivers with marijuana in their system soared by almost 50 percent.

NHTSA conducted a second study to determine whether smoking marijuana increased the risk of crashes. They found that it did but, adding a caveat, said that pot smoking is most common among a group already at high risk for crashes: young men.

“We know that marijuana impairs judgment, reaction times and awareness,” said Jeff Michael, NHTSA’s associate administrator. “These findings highlight the importance of research to better understand how marijuana use affects drivers, so states and communities can craft the best safety policies.”

The study was done in Virginia Beach, where researchers gathered data from more than 3,000 crashes over a 20-month period.

“The combined message of these two surveys is that our work to understand and combat drunk driving is paying off,” Rosekind said, “but that we have much to learn about how illegal drugs and prescription medicines affect highway safety — and that developing that knowledge is urgent, because more and more drivers have these drugs in their systems.”