Traffic Safety:

Alcohol Ignition Interlocks Are Effective While Installed; Less Is Known about How to Increase Installation Rates

GAO-14-559: Published: Jun 20, 2014. Publicly Released: Jun 25, 2014.

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Susan Fleming
(202) 512-2834


Office of Public Affairs
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What GAO Found

Research GAO reviewed consistently indicated that when installed ignition “interlocks”—devices that prevent drivers from starting their cars if they have been drinking alcohol—effectively reduce the rate of re-arrest for driving while intoxicated (DWI) when installed. But once the devices are removed, DWI re-arrest rates return to pre-interlock rates. (Most studies use DWI arrest as a proxy for alcohol-impaired driving.) Further, the National Highway Traffic Safety Administration (NHTSA) estimated that between 15 and 20 percent of offenders arrested for DWI actually install ignition interlocks. Many factors contribute to low installation rates. For example, some states lack the resources to monitor offenders to ensure they install ignition interlocks; other states require that offenders pay fees and penalties to be eligible to install ignition interlocks and return to driving with interlocks. State ignition interlock programs vary in terms of how they are designed, but little research exists on which specific interlock program characteristics—such as monitoring or length of installation—could improve the effectiveness of interlock programs. NHTSA is currently conducting studies on factors that could help states improve installation rates or otherwise improve the effectiveness of their interlock programs. NHTSA expects these studies to be completed by 2015.

NHTSA has offered a variety of technical assistance, research, and education to help states establish and improve their ignition-interlock programs, as well as implement the ignition interlock grant program established by the Moving Ahead for Progress in the 21st Century Act (MAP-21). While state officials confirmed that NHTSA's overall ignition-interlock-related activities have been useful, some questioned NHTSA's implementation of the ignition interlock grant program. Specifically, NHTSA's implementation was based on the plain meaning of the authorizing language in MAP-21, which did not include any reference to exemptions. As a result, states with “employer exemptions”—programs that require offenders to drive only vehicles equipped with ignition interlocks for personal use but allow them to drive employer-owned vehicles for work purposes—were disqualified. Some state officials told us these exemptions are seldom used in practice, but are important to maintain because they facilitate the ability of offenders to work. According to NHTSA officials, they recognized that to qualify for the grant, many states would have to modify their ignition-interlock laws to make them applicable to first time offenders and eliminate exemptions; therefore, few states were expected to qualify in the grant's first years because it would be difficult for state legislatures to change their ignition-interlock laws in that time frame. In fiscal year 2013, 2 states qualified for the grant; most of the additional 12 states that applied for the grant were disqualified at least in part due to employer exemptions, but the legislatures in 2 of those states later removed such exemptions from their laws, resulting in 4 states qualifying for the grant in fiscal year 2014. Because the ignition interlock grant is relatively new, the extent to which additional state legislatures may be willing or able to modify their laws to qualify for the grant is unclear. A 2012 NHTSA review of states' impaired-driving laws found that at least 5 states' ignition-interlock laws included employer vehicle exemptions, but additional states had other factors that would prevent them from qualifying for the ignition-interlock grant.

Why GAO Did This Study

Motor vehicle crashes involving alcohol-impaired drivers killed 10,322 people in 2012 and account for almost one third of all traffic fatalities annually. Ignition interlocks are one strategy states use to combat DWI. In 2012, MAP-21 established a grant program for states that adopt and implement mandatory alcohol ignition-interlock laws for all convicted DWI offenders. Funding authorization for this program expires at the end of fiscal year 2014.

GAO was asked to review the effectiveness of ignition interlocks and NHTSA's implementation of the new grant program. This report discusses (1) what is known about ignition interlock effectiveness and (2) the extent to which NHTSA has assisted states in implementing ignition-interlock programs, including the grant program. GAO reviewed 25 studies that analyzed relationships between ignition interlocks and DWI arrests and fatalities; interviewed NHTSA officials and reviewed reports about NHTSA's assistance to states; and interviewed representatives from safety-advocacy and research organizations, and officials involved with ignition-interlock programs from 10 states. The states were selected based on grant program qualification and the number of alcohol-impaired fatalities, among other factors. The information from these states is not generalizable. DOT officials reviewed a draft of this report and generally agreed with the findings. DOT offered technical corrections, which we incorporated as appropriate.

For more information, contact Susan Fleming at (202) 512-2834 or

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