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Motor Vehicle Injury – Alcohol-Impaired Driving: Ignition Interlocks


What the Task Force Found

About The Systematic Review

The Task Force finding is based on evidence from two systematic reviews that considered a total of 15 studies (search period through December 2007). The first review, conducted by the Cochrane Collaboration (Willis et al., 2004), identified 11 studies evaluating the effect of interlock installation on re-arrest rates for alcohol-impaired driving. The evidence from this review was supplemented by a follow-up review that covered a period through December 2007. This follow-up review included four additional studies and also evaluated evidence from the Cochrane Collaboration review to examine the effects of interlocks on crash outcomes.

Scientists from Centers for Disease Control and Prevention’s (CDC) Division of Unintentional Injury Prevention led this follow-up review. They received input from a team of specialists in systematic review methods and experts in research, practice and policy related to reducing alcohol-impaired driving.


The court system may mandate installation of ignition interlocks or state licensing agencies may offer them as an alternative to a suspended driver’s license for persons convicted of alcohol-impaired driving. The amount of time they are installed typically matches the period for which the license would otherwise be suspended. This most often ranges from 6 to 24 months. Typically, only a small percentage of eligible people participate in ignition interlock programs because many offenders prefer license suspension. Rates of usage, however, vary substantially based on how programs are administered.

Summary of Results

The Task Force recommendation was based on results from two systematic reviews that considered a total of 15 studies.

  • While interlocks were installed, re-arrest rates decreased by a median of 67% relative to comparison groups (13 studies). This estimate is based on all of the available studies that reported separate results for re-arrests during the interlock installation period.
  • When interlocks were removed, re-arrest rates reverted to rates similar to those of persons convicted of alcohol-impaired driving who had not used interlocks (11 studies).
  • Drivers with interlocks installed had fewer alcohol-related crashes than those who had licenses suspended for an alcohol-impaired driving conviction (1 study).
  • Overall crash rates for drivers with interlocks were similar to those for the general driving population. Drivers with ignition interlocks, however, had a substantially greater number of crashes overall than did drivers with suspended licenses. This is likely because those with ignition interlocks drove more than those with suspended licenses (2 studies).


Summary of Economic Evidence

An economic review of this intervention did not find any relevant studies.


The studies included in this review primarily evaluated interlock programs that (1) were directed to "hardcore" drinking drivers, either repeat offenders or first-time offenders who had high BACs at arrest (usually ≥0.15 g/dL), and (2) enrolled a relatively small subset of all DWI offenders.

In contrast, to maximize public health impact, interlock programs will need to extend their reach to include a broader cross-section of offenders, and will need to find ways to ensure that a higher proportion of offenders actually have interlocks installed.

Evidence Gaps

Each Community Preventive Services Task Force (Task Force) review identifies critical evidence gaps—areas where information is lacking. Evidence gaps can exist whether or not a recommendation is made. In cases when the Task Force finds insufficient evidence to determine whether an intervention strategy works, evidence gaps encourage researchers and program evaluators to conduct more effectiveness studies. When the Task Force recommends an intervention, evidence gaps highlight missing information that would help users determine if the intervention could meet their particular needs. For example, evidence may be needed to determine where the intervention will work, with which populations, how much it will cost to implement, whether it will provide adequate return on investment, or how users should structure or deliver the intervention to ensure effectiveness. Finally, evidence may be missing for outcomes different from those on which the Task Force recommendation is based.

Identified Evidence Gaps

  • How should programs be implemented and operated to be most effective?
  • What is the ideal length of interlock program participation?
  • Would performance-based criteria for ignition interlock removal (i.e., participants would have to demonstrate that they no longer need the interlock to prevent driving after drinking) lead to longer term effects than fixed period installations?
  • What are the most effective strategies to increase the number of offenders who participate in ignition interlock interventions?
  • What are the potential roles of newer technology such as interlock hardware that is more resistant to circumvention attempts, or driver identification systems that ensure the driver actually provides a breath sample?

Study Characteristics

  • Reported participation rates varied from less than 1% of offenders to 64% of offenders (median: 13%).
  • The majority of reviewed studies prospectively followed cohorts of offenders who had interlocks installed in their cars, and compared them to cohorts of offenders who did not have interlocks and whose licenses were suspended instead. Because several nonrandom factors can influence whether a given offender has an interlock installed, such studies have a substantial risk of producing biased effect estimates resulting from noncomparable intervention and comparison groups.
  • Several studies included evidence suggesting at least some degree of noncomparability between groups. In particular, the interlock groups tended to be older, drive more, have higher incomes, and have more offenses or more serious offenses.