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Use: Medium
Time: Medium

An alcohol ignition interlock prevents a vehicle from starting or being operated unless the driver provides a breath sample with a BrAC lower than a pre-set level, usually .02. Interlocks typically are used as a condition of probation for DWI offenders, to prevent them from driving while impaired by alcohol after their driver’s licenses have been reinstated.

Interlocks are highly effective in allowing a vehicle to be operated by sober drivers, but not by alcohol-impaired drivers. A post-start retest is meant to prevent an offender from having someone else who has not been drinking start the car for them or from drinking while driving. A data recorder logs the driver’s BrAC at each test and can be used by probation officers to monitor the offender’s drinking and driving behavior. Marques and Voas (2010) provided an overview of interlock use, effectiveness, operational considerations, and program management issues. Marques (2005), Beirness and Robertson (2005), and Robertson et al. (2006) summarized interlock programs in the United States and other countries and discussed typical problems and solutions. See also Brunson and Knighten (2005), Neuman et al. (2003), and proceedings from the 11th Annual International Alcohol Interlock Symposium (Robertson et al., 2011).

NHTSA offers an ignition interlock toolkit to assist policymakers, highway safety professionals, and advocates (Mayer, 2014). In addition, NHTSA has published a report, Case Studies of Ignition Interlock Programs, featuring State ignition interlock programs (Fieldler et al., 2012); an Evaluation of State Ignition Interlock Programs: Interlock Use Analysis From 28 States 2006-2011 (Casanova-Powell et al., 2015); and Interlock Data Utilization (Taylor et al., 2017). NHTSA has created model guidelines to assist States in developing and implementing highly effective interlock programs based on successful practices in the United States and other countries (NHTSA, 2013).  Information on States’ legislation, program funding, data collection and management, interlock technology, and driver compliance can be found at Alcohol Interlock Curriculum for Practitioners


All 50 States and the District of Columbia allow interlocks to be used for some DWI offenders. In 34 States and the District of Columbia, interlocks are mandatory for all convicted offenders, including first offenders (Robertson et al., 2022). In 14 States interlocks are mandatory for certain offenders, such as high BAC and repeat offenders. Presently, interlocks are discretionary in North Dakota and South Dakota.

Despite widespread laws a relatively small percentage of eligible offenders have an interlock installed. In a 2019 survey of ignition interlock providers, Robertson et al. (2022) found that 15% of those arrested for DWI had an interlock installed on their vehicle, and 42% of those convicted had an interlock. Although these percentages have increased somewhat over time, the installation rate among eligible offenders remains relatively low.

Use of interlocks is higher when they are required rather than optional. California conducted a pilot program in which interlocks were required in four counties. Interlock use was higher in the four pilot counties that required interlocks for DWI offenders (42.4%) than in non-pilot counties (4.3%) (Chapman et al., 2015). Additionally, use of interlocks is substantially higher when they are required as a prerequisite to license reinstatement. For example, among DWI offenders in Florida who were subject to the State’s interlock requirement, 93% installed interlocks once they qualified for reinstatement (Voas, Tippetts, Fisher, & Grosz, 2010). Similarly, a study in Washington State found interlock installations increased and recidivism decreased when interlocks were required for all offenders and offenders were allowed to install an interlock in lieu of license suspension (McCartt et al., 2018). Finally, use of interlocks is higher when interlocks are offered as an alternative to more restrictive sanctions, such as home confinement via electronic monitoring (Roth et al., 2009).


A review of 15 studies of interlock effectiveness found that offenders who had interlocks installed in their vehicles had arrest recidivism rates that were 75% lower than drivers who did not have interlocks installed (Elder et al., 2011; see also Government Accountability Office [GAO], 2014). Findings were similar for first offenders and repeat offenders. After interlocks were removed, however, the effects largely disappear, with interlock and comparison drivers having similar recidivism rates.

Studies have also demonstrated that interlocks reduce alcohol-related crashes and fatalities while they are installed in vehicles (Elder et al., 2011; Kaufman & Wiebe, 2016; McGinty et al., 2017; Teoh et al., 2021; Vanlaar et al., 2017). For example, Teoh et al. (2021) found that States that require interlocks for all DWI offenders had 26% fewer alcohol-involved fatal crashes than States with no interlock laws. Similarly, States that require interlocks for repeat offenders and high-BAC offenders had 20% fewer alcohol-involved fatal crashes. The authors concluded that ignition interlock laws are effective at reducing the number of impaired drivers in fatal crashes, especially when those laws cover all DWI offenders. Studies estimate that 2,600 lives could be saved each year if every State required interlocks for all DWI offenders (Kaufman & Wiebe, 2016).

Interlocks stop motorists impaired by alcohol from driving while they are installed, but unless motorists change their attitudes and behaviors, they may simply continue driving impaired once the interlock is removed. Florida passed legislation in 2008 to address this problem by mandating treatment for DUI offenders in interlock programs who commit 4 or more interlock violations. These offenders are required to attend 8 to 12 weeks of treatment from certified substance abuse counselors/programs, which includes an individualized treatment plan involving individual or group therapy. A study of Florida’s law found that offenders who received treatment in addition to the interlock had one-third lower DWI recidivism once the interlock was removed compared to offenders who had the interlock only (Voas et al., 2016).


Cost is frequently cited as a barrier to greater use of interlocks. Offenders are typically required to pay the costs for installing, calibrating, monitoring, and removing interlocks. However, a growing number of States have indigent funds to reduce the costs for low-income offenders. A recent study found two-thirds of States offer financial support to interlock program participants who need it (Robertson et al., 2017). The study also found that less than 10% of offenders typically use these indigent funds. Alabama, Delaware, and Connecticut enacted legislation to expand their ignition interlock indigent programs in 2018 (NCSL, 2019).

Time to implement:

Interlock programs may require enabling legislation. Once authorized, interlock programs require 4 to 6 months to implement a network of interlock providers.

Other considerations:

  • Barriers to use: Interlocks have demonstrated their effectiveness in controlling impaired driving while they are installed. Considering this success, their limited use may be due to several factors, such as lengthy license suspension periods, offenders who delay license reinstatement, judges who lack confidence in the interlock technology or who fail to enforce “mandatory” interlock requirements, interlock costs, and localities that lack enough interlock providers. In a survey of DWI offenders who chose not to use an interlock (when it was optional), the main reasons cited were interlock costs and stigma from being perceived as a problem drinker (Forsman & Wallhagan, 2019). To increase the number of offenders who drive interlock-equipped vehicles, some States have made the alternatives to interlocks more undesirable. For example, pilot programs in Indiana and New Mexico found that roughly two-thirds of offenders chose to have interlocks installed when the alternative was house arrest with electronic alcohol monitoring or jail (Marques et al., 2010; Voas et al., 2001). Other States such as Arkansas, Colorado, Maine, Mississippi and Nebraska allow offenders to shorten (or eliminate) the license suspension period if they are willing to operate an interlock-installed vehicle. An evaluation of such a law in Ontario found that a reduced suspension program increased installation rates from 45% to 70% among eligible first-time offenders (Ma et al., 2016). For a discussion of barriers to interlock use, see Beirness and Marques (2004), Beirness et al. (2008), Beirness and Robertson (2005), and Neuman et al. (2003). For a discussion on how States have successfully overcome obstacles encountered with interlock programs, see Casanova-Powell et al. (2015).
  • Compliance with interlocks: Some offenders have relatively high rates of breath test failures and other violations, typically near the beginning of their participation in an interlock program (Vanlaar et al., 2013; Vanlaar et al., 2010). Research shows that offenders with breath test failures have higher rates of reoffending once the interlock is removed compared to those with a “clean” record (Bailey et al., 2018; Le et al., 2019). Presently, few jurisdictions use the compliance data collected by interlocks to identify offenders who may be at high risk for recidivism (Taylor et al., 2017). The data could also be used to require an extension of the interlock period for those with poor compliance, or even to inform treatment options (Marques et al., 2010). Better coordination between interlock programs and treatment providers might help reduce recidivism once interlocks are removed (Taylor et al., 2017).
  • To improve compliance with interlocks, it is important to closely monitor offenders during their participation in an interlock program. One study found that offenders who were closely monitored (e.g., their data were reviewed weekly, and they received letters documenting their progress) had fewer initial breath test failures and other indicators of non-compliance than offenders who received standard monitoring through the State licensing office (Zador et al., 2011). Similarly, an in-depth study of 3 State interlock programs found non-compliance was highest in the State with less consistent monitoring practices (California) than in the 2 States (Florida and Texas) with stronger monitoring practices (Vanlaar et al., 2013). As of May 2019 22 States require interlock devices that are equipped with cameras to ensure that the people using the interlocks are the drivers (NCSL, 2019). Additionally, monitoring the number of miles driven on an interlock vehicle can prevent an offender from circumventing the device by driving another vehicle. Some States set vehicle usage criteria for the number of miles the offender will likely be driving per week while the interlock is installed. If the mileage on the interlock-equipped vehicle is unexpectedly low, further sanctions can be put in place (Mayer, 2014).
  • First-time offenders: There are special issues concerning interlocks and first-time offenders. Historically, interlock programs were mandatory for repeat offenders and voluntary for first-time offenders (Robertson et al., 2010). In many jurisdictions first-time offenders are not monitored by the court system. Consequently, it can be difficult to respond to violations and to ensure that first-time offenders participate, install the devices, and complete the interlock program. Despite challenges in closely monitoring first-time offenders, evidence suggests interlocks effectively reduce recidivism among this group while the interlock is installed (Marques et al., 2010; McCartt et al., 2013; McCartt et al., 2018). For more information about issues in implementing interlock programs with first-time offenders, see Robertson et al. (2010).
  • Rural areas: For offenders living in rural areas, access to an interlock service provider may be problematic (Cheesman et al., 2014). Interlock service providers may be limited or non-existent in rural jurisdictions, requiring offenders to drive long distances to get an interlock installed or serviced. To improve the availability of interlocks, States can require vendors to provide service to rural areas as a prerequisite for obtaining a contract with the State (Mayer, 2014).
  • Public support:  There is strong support among the general public for ignition interlocks. In national surveys, approximately 80% of respondents support requiring interlocks for drivers convicted of DWI, including first offenders (AAAFTS, 2018; McCartt, Wells, & Teoh, 2010). Moreover, about 65% of respondents favored having alcohol detection technology in all new vehicles. The general public also believes strongly that interlocks work. In a NHTSA survey, respondents were asked about the effectiveness of eight strategies to reduce or prevent impaired driving. Interlocks ranked highest in the percentage who rated the strategy “very effective” (63%) (Moulton et al., 2010).
  • General deterrence: The implementation of ignition interlock programs targeting DWI offenders does not seem to produce a general deterrence effect among the broader driving population. An evaluation of general deterrence was conducted in California by comparing recidivism rates in four counties that participated in a pilot program involving mandatory interlock installation to recidivism rates in all other California counties (Chapman et al., 2015). The study found that mandatory interlock installation was ineffective at reducing county-wide DWI recidivism below those of the comparison counties. This lack of difference in conviction rates held for drivers with one, two, or three or more prior DWI convictions. Note that this study did not track local advertising of the program in the four pilot counties, so it is unknown if the absence of a general deterrence effect was affected by the level of outreach effort.