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Motor Vehicle Injury – Alcohol-Impaired Driving: Ignition Interlocks

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What the Task Force Found

About The Systematic Review

The CPSTF 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.

The review was conducted on behalf of the CPSTF by scientists from CDC’s Division of Unintentional Injury Prevention with input from a team of specialists in systematic review methods and experts in research, practice and policy related to motor vehicle injury prevention.

Context

The court system may mandate installation of an ignition interlock, or a state licensing agency may offer one to someone convicted of alcohol-impaired driving as an alternative to suspending their driver’s license. Ignition interlocks are usually installed for the same length of time that a driver’s license would be suspended—most often between 6 months and 2 years.

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

More details about study results are available in the published evidence review pdf icon [PDF - 584 kB].

The CPSTF 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, drivers with ignition interlocks had crash rates similar to those of the general driving population. Drivers with 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.

Applicability

Included studies primarily evaluated programs that did the following:

  • Targeted "hardcore" drinking drivers—repeat offenders or first-time offenders who had high BACs at arrest (usually ≥0.15 g/dL)
  • Enrolled a relatively small subset of all offenders

To maximize public health impact, interlock programs will need to do the following:

  • Extend their reach to include a broader cross-section of offenders
  • Ensure a higher proportion of offenders have interlocks installed

Evidence Gaps

Additional research and evaluation are needed to answer the following questions and fill existing gaps in the evidence base. (What are 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 lead to longer term effects than fixed period installations? For example, should participants have to demonstrate that they no longer need the interlock to prevent driving after drinking?
  • 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 technologies? Examples might include 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 offenders who had interlocks installed in their cars, and compared them to 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 non-comparable intervention and comparison groups.
  • Several studies included evidence suggesting at least some degree of non-comparability between groups. In particular, the interlock groups tended to be older, drive more, have higher incomes, and have more offenses or more serious offenses.

Publications