Evaluation of Alcohol Ignition Interlock Laws in the United States

Alcohol-impaired driving results in thousands of deaths annually. Alcohol ignition interlocks require a negative breath test to start a vehicle's engine, and 45 states have mandated some form of interlock law for drivers convicted of driving while intoxicated (DWI). Funded by the Insurance Institute for Highway Safety (IIHS), NORC used differences by three interlock laws to evaluate their effects on alcohol-impaired passenger vehicle drivers involved in fatal crashes between 2001-2014 in the United States across state and time. State/time differences unrelated to interlock laws were controlled for by fitting a Poisson model. The exposure measure was number of passenger vehicle drivers in fatal crashes that did not involve impaired drivers. Laws requiring interlocks for drivers convicted of DWI covered: repeat offenders, repeat offenders and high-BAC offenders, all offenders, or none.

The number of states with all-offender interlock laws during the study period went from 3 in 2001 to 20 in 2014, and the number of states with any of the three laws increased from 19 to 45. All-offender laws were associated with 16 percent fewer drivers with 0.08+ BAC involved in fatal crashes, compared to no law. Repeat-offender laws were associated with a nonsignificant 3 percent reduction in impaired drivers, compared to no law. Repeat and high-BAC laws were associated with an 8 percent reduction in impaired drivers in fatal crashes, compared to no law. Laws mandating alcohol ignition interlocks, especially those covering all offenders, are an effective impaired driving countermeasure associated with significant reductions in the number of impaired drivers in fatal crashes.

General deterrence was measured by examining the effects on passenger vehicle drivers in FARS that did not have prior (within 3 years of the crash) DWI convictions. These drivers form the vast majority of the total study sample. Specific deterrence included drivers with prior DWI convictions involved in fatal crashes. The sample includes drivers who were convicted of DWI who did and did not receive interlocks, so it was not interpreted as a direct measure of specific deterrence. Both impaired and unimpaired drivers in fatal crashes were restricted to either those with prior DWI or those without prior DWI.

This study demonstrates several important associations. First, we found that interlock laws, especially those covering all offenders, are an effective impaired driving countermeasure for fatal crashes. Second, the current study shows that the laws are especially effective at preventing fatal crashes among drivers with a history of DWI - a population of drivers at high risk for recidivism and crash involvement. As such, jurisdictions that do not currently have alcohol ignition interlock laws, and have the goal of reducing alcohol-related fatal crashes among their population, should consider adopting these laws.