Program Profile: Impact of Statewide Ignition Interlock Laws on Alcohol-Involved Crash Fatalities in the United States

This is a drunk driving prevention initiative that seeks to reduce alcohol-involved crash fatalities by requiring the installation of ignition interlock devices on vehicles of all drivers convicted of driving under the influence. The program is rated Promising. Intervention states with a statewide ignition interlock requirement had a 15 percent decrease in alcohol-involved vehicle crash fatalities, compared with control states. This was a statistically significant difference.

A Promising rating implies that implementing the program may result in the intended outcome(s).

This program's rating is based on evidence that includes either 1) one study conducted in multiple sites; or 2) two or three studies, each conducted at a different site. Learn about how we make the multisite determination.

Program Goals

In the United States, 30 percent of fatal crashes (roughly 11, 000 deaths per year) are caused by drunk drivers (Kaufman and Wiebe 2016). While the United States uses a variety of strategies to prevent and deter drunk driving, ranging from public education campaigns and designated-driver programs to enforcement initiatives (such as sobriety checkpoints, and license suspension and revocation), these policies rely on the intoxicated individuals’ deciding not to drive and on the police to identify these impaired drivers before they become involved in a vehicular accident.

While license suspension is a primary enforcement strategy to deter drunk driving, more than half of drivers convicted of driving under the influence (DUI) of alcohol continue to drive with a suspended license (Beck et al. 1999; McCartt, Geary, and Berning 2003; Voas, Tippets, and Lange 1999). Additionally, license suspension affects compliant individuals convicted of a DUI from driving when sober, by potentially limiting their travel for work and other daily activities. Thus, beginning in 2004, some states began to implement a statewide (or universal) ignition interlock policy that is designed to target the risky behavior of drunk drivers convicted of a DUI offense. The goal of the ignition interlock policy is to prevent drunk driving and reduce alcohol-involved crash fatalities.

Target Population

Some states use ignition interlock programs to target behaviors of individuals who have repeatedly been arrested for a DUI, those driving with very high blood–alcohol content, and those identified as appropriate at the discretion of a judge. Universal ignition interlock programs target drunk drivers who have been convicted of an alcohol DUI offense (including drivers convicted of a first-time offense).

Program Components

Statewide (or universal) ignition interlock laws require all individuals convicted of a DUI of alcohol to install and connect an ignition interlock device to their vehicle’s ignition system. The installed interlock device requires the driver to provide a breath sample (using breath analysis technology) each time to start the engine of the vehicle, with the device calibrated to “lock” the ignition if the breath alcohol level exceeds a preprogrammed level (usually 0.02 g/dL). If the driver’s blood–alcohol content is above the programmed limit, then the vehicle will not start, and the driver must resubmit a breath sample.

In addition, interlock devices are programmed for “rolling retests” at designated intervals while the individual is driving (i.e., every 20 to 30 minutes). If the individual convicted of a DUI fails one of these tests, the device triggers the horn and flashing lights, which will continue to attract notice until the car is turned off. The purpose of the retest is to 1) prevent the individual convicted of a DUI from having another person start the vehicle for them and 2) prevent them from drinking once the vehicle is started.

To ensure the device is not tampered with, interlock devices are equipped with a tamper-proof system for mounting the engine part of the unit (which is inspected every 30 to 60 days to prohibit circumvention), along with a system to detect hot-wiring or other means that bypass the interlock. There is also a data-recording system that logs the blood–alcohol content results, tests compliance, and tests engine operation to determine whether the individual convicted of a DUI is actually using the vehicle as expected and not simply parking it while driving another vehicle.

Study 1

Kaufman and Wiebe (2016) conducted a quasi-experimental study and 50-state difference-in-differences analysis to evaluate the effect of statewide (or universal) ignition interlock requirements on reducing alcohol-involved crash fatalities in the United States, during a 15-year intervention period (1999 to 2013). For the evaluation, the study authors grouped states into treatment conditions (i.e., the intervention group or the control group) based on state policies collected from the Trust for America’s Health (a health policy organization); the years of implementation of ignition interlock polices were corroborated with state legislative records and state highway offices. Between 2004 (when the first state adopted the universal ignition interlock requirements) and 2013, 18 states had adopted universal ignition interlock requirements, which mandated that all individuals convicted of driving under the influence of alcohol have an ignition interlock device installed into their vehicle. These 18 states were included in the intervention group. States that had not adopted the policy initiative by 2013 were included in the control group (n = 32).

The 18 intervention states were Alaska, Arizona, Arkansas, Colorado, Connecticut, Hawaii, Illinois, Kansas, Louisiana, Maine, Nebraska, New Mexico, New York, Oregon, Tennessee, Utah, Virginia, and Washington. Of these states, 44.4 percent were located in the western region of the United States, 22.2 percent in the Southeast, 16.7 percent in the Northeast, and 16.7 percent in the central region of the United States. More than half of the population in these intervention states were White (69.3 percent), with 22.9 percent of the population living in rural areas. Of the 32 states in the control group, 28.1 percent of states were located in the central United States, 18.8 percent were in the Northeast, 15.6 percent were in the Southeast, and 15.6 percent were in the West. The region for the other 21.9 percent of states was mistakenly omitted due to a typographical error; nevertheless, all 50 states were included in the study analysis. (This information was provided in correspondence with the study authors).

Approximately 73 percent of the population in control states were White, with 28.4 percent of the population living in rural areas. There were no statistically significant differences between intervention and control states’ basic demographic characteristics (such as age, region, political orientation, and total vehicle miles driven) during the study period; however, intervention states were statistically significantly more often located in the West and were less rural, and people in intervention states drove fewer total miles on average, compared with the control states.

The outcome of interest was alcohol-involved crash death rates. Alcohol-involved crash deaths were defined as a fatal crash in which at least one of the drivers had a blood–alcohol concentration level greater than zero (zero was selected because interlock devices were typically set to a low blood–alcohol content threshold of 0.02 percent). Data for fatal death rates were collected from the National Highway Traffic Safety Administration’s Fatality Analysis Reporting System. Population denominators were pulled from the U.S. Census. The effect of universal ignition interlock devices on alcohol-involved crash deaths was estimated using difference-in-differences analysis, comparing state rates of alcohol-involved crash deaths before and after policy implementation with the change over the same period in states without the policy. An ordinary least squares (OLS) regression was used to determine differences in alcohol-involved crash deaths between intervention states and control states, with standards errors adjusted for clustering at the state level across years.

In addition, the study authors conducted subgroup analyses, using regression analysis to evaluate differences in the total vehicle crash rates between alcohol-involved and non–alcohol-involved crashes.

Kaufman and Weibe (2016) conducted subgroup analyses to evaluate differences in the total vehicle crash rates between alcohol-involved and noninvolved crashes, using a regression analysis. Because the National Highway Traffic Safety Administration’s Fatality Analysis Reporting System blood–alcohol content test results are unavailable, this subgroup analysis accounted for possible misattribution of alcohol-involved and noninvolved crashes. Based on results from 10 states that had test results for at least 50 percent of both surviving and nonsurviving drivers (based on an average over the period from 1999 to 2009), there was no statistically significant difference in the crash fatality rate between alcohol-involved and noninvolved crashes. Additional findings on policy effects can be found in the reviewed study.

These sources were used in the development of the program profile:

These sources were used in the development of the program profile: