Laws requiring all impaired-driving offenders to install alcohol interlocks reduce the number of impaired drivers in fatal crashes by 16%, a new study by the Insurance Institute for Highway Safety shows. Forty-five states require interlocks for at least certain impaired-driving offenders.
Twenty-eight states, the District of Columbia and four California counties have some type of interlock requirement that applies to first-time offenders. If all states without such laws adopted them, more than 500 additional lives could be saved each year.
Alcohol interlocks are in-vehicle breath-testing units that require a blood alcohol concentration (BAC) below a certain level, typically somewhere between 0.02 and 0.04%, before the vehicle can be started.
More than 25% of U.S. crash deaths occur in crashes in which at least one driver has a BAC of 0.08% or higher. The prevalence of impaired driving in fatal crashes has changed little in the past two decades, and interlock laws are one of the few recent policy innovations that have made a difference.
“We looked at the number of alcohol-impaired passenger vehicle drivers involved in fatal crashes over time and compared them with the number of drivers in fatal crashes that didn’t involve impairment,” says Eric Teoh, IIHS senior statistician and the paper’s lead author. “We found that state laws mandating interlocks for all DUI offenders reduced the number of drivers in fatal crashes with BACs of 0.08% or higher by 16% compared with no interlock law.”
In 2016, the latest year for which fatal crash data are available, 10,497 people died in crashes involving drivers with a BAC of 0.08% or higher. Of those, 8,853 involved impaired passenger-vehicle drivers. At that time, the number of states with first-offender laws was 25. Had all states had all-offender interlock requirements in place, 543 of those deaths would have been prevented, Teoh calculated.
For the purposes of the analysis, Teoh and his co-authors grouped together two types of all-offender interlock laws: those that require all offenders, including first-time offenders, to install interlocks to have their license reinstated and those that only require it to drive during a post-conviction suspension. The analysis controlled for factors besides interlock laws that could affect crashes.
Laws that required interlocks for repeat offenders only cut the number of drivers with BACs of 0.08% by 3% compared with no interlock law, and that effect wasn’t statistically significant, the study showed. Laws that required them for both repeat offenders and offenders with high BACs provided an 8% benefit.
