An alcohol ignition interlock has a breath-testing unit that is connected to a vehicle's ignition. In order to start the vehicle, the driver must blow into the device and register a blood alcohol reading that is below a predetermined level. If the blood alcohol reading exceeds this level, the interlock prevents the vehicle from starting. An estimated 318,714 interlocks were in use in 2014 in the United States.
Studies have shown that alcohol ignition interlocks are effective in reducing recidivism. In a 1999 Institute study, multiple offenders eligible for license reinstatement were randomly assigned interlock-restricted licenses or unrestricted licenses coupled with the conventional post licensing treatment program. The interlock restriction reduced the risk of committing an alcohol-related traffic violation within the first year following conviction by 64 percent.
A systematic review and meta-analysis of studies of ignition interlock programs found the programs seemed to reduce recidivism while the devices were installed in offenders' vehicles, but found no evidence of effectiveness once the devices were removed.
In 25 states, all alcohol-impaired driving offenders, including first-time offenders, must install interlocks to resume driving. An additional 14 states require interlocks for first-time offenders with high BACs (usually 0.15 percent or higher) and for repeat offenders, six states (California, Georgia, Idaho, Massachusetts, Ohio and Pennsylvania) require them only for repeat offenders, and one state (Nevada) requires them only for high-BAC offenders. Four states (Indiana, Montana, North Dakota and South Dakota) and the District of Columbia have no mandatory interlock requirements.
An Institute study showed that mandatory interlock laws for all offenders are effective.
Researchers looked at what happened in Washington when the state expanded its interlock requirement in 2004 to cover everyone convicted of DUI, not just those with multiple offenses, alcohol test refusals or high BACs. They found the recidivism rate fell 12 percent for first-time offenders with BACs under 0.15 percent. The law change was associated with an 8.3 percent reduction in single-vehicle late-night crash risk, suggesting a general deterrent effect of the expanded interlock requirement.
However, even when laws require offenders to have interlocks if they drive, many people don’t go through with the installation. In the Washington study, only about a third of the first-time offenders with BACs under 0.15 percent obtained interlocks. (If all of them had, it is estimated that their recidivism rate would have fallen by nearly half.)
In some states offenders may skip the interlock stage by serving the suspension. In a pilot program in Santa Fe County, N. M., when judges imposed house arrest via electronic monitoring as the alternative to an interlock, 70 percent of offenders installed interlocks, compared with only 17 percent in other counties.
When the program ended, the Santa Fe installation rate declined.