Skip to main content
You can also sort pages by filters.
Table of Contents
Download the Full Book

Effectiveness: Proven for reducing recidivism while on the vehicle 5 Star Proven for reducing recidivism while on the vehicle Cost: $$
Use: Medium
Time: Medium

An alcohol ignition interlock prevents a vehicle from starting 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 vehicles to be started by sober drivers, but not by alcohol-impaired drivers. A post-start retest requires the driver to remain sober while driving. A data recorder logs driver BrAC at each test and can be used by probation officers to monitor offender drinking and driving behavior. Marques and Voas (2010) provide an overview of interlock use, effectiveness, operational considerations, and program management issues. Marques (2005), Beirness and Robertson (2005), and Robertson et al. (2006) summarize interlock programs in the United States and other countries and discuss typical problems and solutions. See also Brunson and Knighten (2005, Practice #5), Neuman et al. (2003, Strategy C2), 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) and an Evaluation of State Ignition Interlock Programs: Interlock Use Analysis From 28 States 2006-2011 (Casanova-Powell et al., 2015). Finally, 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, 2013b).

Use: All 50 States and the District of Columbia allow interlocks to be used for some DWI offenders (NHTSA, 2013a). In 30 States, the District of Columbia, and 4 California counties interlocks are mandatory for all convicted offenders, including first offenders (IIHS, 2017). Indiana, Montana, North Dakota, and South Dakota have no mandatory interlock requirements.

Despite widespread laws, a relatively small percentage of eligible offenders have an interlock installed. However, interlock use has increased substantially over the past 10 years, from 146,000 in 2008 to 348,476 in 2017 (based on information supplied by interlock manufacturers; Robertson et al., 2018). Given the roughly 1.4 million arrests in the United States each year for DWI, the ratio of installed interlocks to arrests is approximately 1 in 5. 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, an examination of effects of the incremental expansion of interlock laws in Washington State to cover all DUI offences found corresponding improvements in installation rates and recidivism with the implementation of each legislative change (McCartt et al., 2018). Use of interlocks is also higher when interlocks are offered as alternatives to home confinement via electronic monitoring (Roth et al., 2009). Through a combination of these measures, New Mexico installed interlocks in the vehicles of half of all convicted DWI offenders in 2007 – the highest level of penetration of any State (Marques et al., 2010). Finally, use of interlocks in a pilot program in California 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). The authors concluded that the main reason for this significant increase was due to the fact that interlock installation was mandatory in pilot counties, while interlock installation was optional in non-pilot counties.

Effectiveness: 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 GAO, 2014). Findings were similar for first offenders and repeat offenders. After interlocks were removed, however, the effects largely disappeared, with interlock and comparison drivers having similar recidivism rates. Similarly, a study conducted in California found that the strong and reliable reduction in recidivism diminished over time for DWI offenders that had one or two prior DWI convictions (California DMV, 2018).

Studies that have examined the effects of interlocks on crashes, indicate that alcohol-related crashes decrease while interlocks are installed in vehicles (Elder et al., 2011; Kaufman & Wiebe, 2016; McGinty et al., 2017; Vanlaar, Hing & Robertson, 2017). One study that evaluated the effects of State interlock laws on alcohol-involved fatal crashes found that laws requiring interlocks for all offenders were associated with a 7% reduction in crashes with BACs > .08 g/dL and an 8% reduction in crashes with BACs ≥ .15 g/dL (McGinty et al., 2016; see also Kaufman & Wiebe, 2016). The benefits were greater in States with mandatory laws than those with partial laws.

One limitation of interlock research is that study participants often are not randomly assigned to interlock or no-interlock groups, so there may be important pre-existing differences between groups (GAO, 2014). However, research suggests that interlocks are a highly effective method for preventing alcohol-impaired driving – and possibly crashes – while they are installed.

While they are installed, interlocks stop impaired motorists from driving, but unless motorists change their attitudes and behaviors, they may simply continue driving impaired once the interlocks are removed (Voas et al., 2016). 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 individualized treatment plans involving individual or group therapy. One study examined the effectiveness of combining mandated treatment with interlocks on recidivism among interlock offenders with 3 or more interlock violations. Compared to a control group that had interlocks but only 1 or 2 interlock violations, those with 3 violations that received treatment showed a significant 32% reduction in recidivism after the interlocks were removed. This improvement was not significantly different for women than for men, or for Hispanics and Blacks than for Whites. However, the additional treatment was much less effective for drivers under 25.

Costs: A 2012 estimate collected from States lists interlock device rental costs from $12 to $90 per month. Device installation and removal fees range from $70 to $150. In addition, other costs such as device recalibration and monitoring fees can add $30 to $150 each month (Fieldler, Brittle, & Stafford, 2012). Offenders usually pay these costs; however, some States such as Illinois and New Mexico have indigent funds and unaffordability criteria to reduce the costs for low-income offenders.

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 issues:

  • Barriers to use: Interlocks have demonstrated their effectiveness in controlling impaired driving while they are installed. In light of 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 an effort 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 (Marques et al., 2010; Voas et al., 2001). Other States allow offenders to shorten (or eliminate) the license suspension period if they are willing to operate an interlock-installed vehicle. For example, Colorado reduced the license suspension period from 1 year to 1 month for offenders who apply for interlocks (NCSL, 2014). Arkansas, Maine, Mississippi, and Nebraska passed similar laws. An evaluation of a similar 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, Strategy C2). 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 interlock programs (Vanlaar et al., 2013; Vanlaar et al., 2010). Offenders become familiar with how the equipment operates, and in some cases, may seek ways to circumvent the interlocks. Presently, few jurisdictions use the compliance data collected by interlocks to identify offenders who may be at high risk for recidivism. 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). To improve compliance with interlocks, it is important to closely monitor offenders during their participation in interlock programs. 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 three State interlock programs found non-compliance was highest in the State with less consistent monitoring practices (California) than in the two States (Florida and Texas) with stronger monitoring practices (Vanlaar et al., 2013). Monitoring the number of miles driven on interlock vehicles can prevent offenders from circumventing the devices by driving other vehicles. 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 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., 2012; 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 interlock service providers 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 interlocks installed or serviced. To improve the availability of interlocks, States can require vendors to provide service to rural areas as prerequisites for obtaining contracts with the State (Mayer, 2014).
  • Public support: There is strong support among the general public for ignition interlocks. In two national surveys, approximately 80% of respondents approved of requiring interlocks in the vehicles of convicted DWI offenders, including first offenders (AAAFTS, 2014; McCartt, Wells, & Teoh, 2010; see also Bishop et al., 2017; Downs et al., 2017). 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 general deterrence effects among the broader driving population. In particular, 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.