Dive into the data: The Year Interlock Law Adopted by State and Impaired Driving Fatal Crashes by State by Year.

Legal research was conducted to determine the kind of ignition interlock law that was adopted and the year the law went into effect. The Fatality Analysis Reporting System was used in the analysis of impaired driving fatal crashes in the state. Differences in three interlock laws were evaluated by comparing alcohol-impaired passenger vehicle drivers involved in fatal crashes between 2001 and 2019 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 the 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.

Interlock laws have a general deterrent effect on impaired driving.

The number of states with all-offender interlock laws during the study period went from 3 in 2001 to 29 in 2019, and the number of states with any of the three laws increased from 16 to 44. States with all-offender laws were associated with 26% fewer drivers with 0.08 BACs involved in fatal crashes, compared to States with no law. States with repeat-offender laws were associated with a 9% reduction in impaired drivers, compared to states with no law. States with repeat and high-BAC laws were associated with a 20% reduction in impaired drivers in fatal crashes, compared with no law.