Drunk drivers are less likely to get behind the wheel if they know a conviction will require them to blow into a breath-testing device every time they want to start their car, research shows.

Traffic fatalities have declined by 7 percent in states that mandate ignition interlocks for first-time drunken-driving offenders, according to a report released Thursday by Johns Hopkins Bloomberg School of Public Health.

“Mandatory interlock laws are laws that require everybody convicted of a DUI to drive with an interlock,” said Beth McGinty, who co-authored the study at the Hopkins’ Center for Injury Research and Policy. “Until this study, we had no evidence on which type of interlock laws worked in terms of preventing alcohol-involved fatal crashes and whether one worked better than the other.”

The study tracked fatalities for about five years before states began passing interlock laws in the late 1980s through 2013, when all states required them under some circumstances. Twenty-two states mandate them for first-time offenders, while other states require them for repeat offenders or those with a particularly high blood alcohol content. Some states let the judge decide whether an interlock is appropriate.

The Hopkins study suggested that even those with no previous DUI convictions would think twice about driving under the influence if faced with the prospect that a first-time offense would require them to use an interlock. It says partial laws that don’t mandate the devices for all offenders are less effective.

“This is the first solid research we have for advocates to say ‘no’ when a policymaker says, ‘Oh, my constituents aren’t going to like that mandatory law. Can’t we just do a partial law for repeat offenders?’ ” McGinty said. “The partial law is not nearly as effective.”

More than a third of the 35,092 fatal car crashes in 2015 involved a driver who had been drinking; 29 percent of them were legally drunk and 20 percent had a blood alcohol content almost twice the legal limit or higher.

Interlock devices are installed in vehicles and require drivers to blow into them before the car’s ignition will start the engine. The advocacy group Mothers Against Drunk Driving (MADD) has estimated that installation of the device costs between $70-$150 and that monitoring by a commercial service costs between $60 and $80. In 2013, MADD said, almost 305,000 ignition-interlock devices had been mandated nationwide for drivers convicted of drunken driving.

“Requiring ignition interlocks for all convicted offenders is a proven tool to help reduce drunken driving, but legislation is just the first step,” said Jonathan Adkins, executive director of the Governors Highway Safety Association. “Implementation is critical to a successful program, and all players must be educated and engaged, including state licensing and highway-safety agencies, law enforcement, prosecutors and judges.”

McGinty says that’s an area for future research.

“We don’t know very much about how these interlock laws are enforced,” she said. “Is someone following up to make sure that the interlock is actually installed and actually being used?”

Virginia and Maryland require interlocks for all DUI offenders. In the District, the decision is left to judges.