State laws requiring ignition interlocks for all convicted drunk drivers — including first-time offenders — appear to reduce the number of fatal drunk driving crashes, according to newly published research.

The study, conducted by researchers at Johns Hopkins Bloomberg School of Public Health and Colorado School of Public Health, was published Jan. 5 in the American Journal of Preventive Medicine. Researchers found that mandatory interlock laws were associated with a 7% decrease in the rate of fatal crashes with at least one driver having a blood alcohol content over the legal limit. The drop translates into an estimated 1,250 prevented fatal crashes in states with mandatory interlock laws since states first started passing such laws in 1993.

An ignition interlock is an alcohol-sensing device that’s connected to the ignition of a vehicle. The device detects alcohol in the driver’s breath. If the sensor detects alcohol in excess of a preset limit, the vehicle will not start. (To learn more about individual state ignition interlock laws, click here.) 

The study’s researchers found that interlock laws that are mandatory for all DUI offenders were much more effective than those applicable to only some offenders, such as repeat offenders or those arrested with an exceptionally high blood alcohol content.

In the U.S. in 2015, alcohol-impaired driving fatalities climbed 3.2%, increasing from 9,943 the previous year to 10,265, according to the National Highway Traffic Safety Administration.

“Our study demonstrates the value of mandatory ignition interlock laws across the United States,” said study leader Emma E. McGinty, deputy director at the Johns Hopkins Center for Mental Health and Addiction Policy Research at the Bloomberg School. “We already know that alcohol plays a tragic role in the number of motor vehicle crash fatalities each year. Interlock laws which are mandatory for all DUI offenders save lives.”

To estimate the effects of existing ignition interlock laws, the researchers studied the effects of interlock laws on trends in alcohol-involved fatal crashes over a 32-year period — 1982 to 2013 — and controlled for other motor vehicle safety laws and trends in crashes over time. The team assessed changes in pre- and post-interlock law rates of alcohol-involved fatal crashes with crash data obtained from the National Highway Traffic Safety Administration’s Fatality Analysis Reporting System (FARS). These rate changes were measured against the different categories of interlock laws: permissive (at the discretion of a judge), partial (applicable to only some DUI offenders), and mandatory for all.

The researchers used two measures based on FARS data: alcohol-involved fatal crashes with a driver having a blood alcohol level of 0.08 — the legal limit — and a second data set with a driver with a blood alcohol level greater than 0.15.