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Status Report, Vol. 47, No. 2 | March 6, 2012 Subscribe

Alcohol ignition interlocksStudy shows devices reduce DUI recidivism

Demonstration of an Intoxalock interlock device

To stem the tide of alcohol-related crashes, no single remedy is likely to be enough. However, one tool that can help is the alcohol interlock, a device that many people with impaired driving convictions are required to install to prevent them from starting their vehicles if they have been drinking.

A new study from the Institute shows that requiring interlocks for everyone convicted of alcohol-impaired driving, not just repeat offenders and those with high blood alcohol concentrations (BACs), reduces the likelihood that people will reoffend. Researchers studied driver records in Washington for people with convictions related to alcohol-impaired driving. They found that after the state expanded its interlock requirement to everyone convicted of driving under the influence (DUI), the recidivism rate for people affected by the expansion fell 12 percent. However, only about a third of those offenders actually went through with interlock installations. If all of them had, their recidivism rate would have fallen by nearly half, the researchers estimate.

"Drivers with previous impaired-driving convictions are overrepresented in alcohol-related fatal crashes, so deterring people from reoffending is a good first step to reduce the death toll," says Anne McCartt, the Institute's senior vice president for research and the study's main author. "This study shows strong interlock laws can help."

Alcohol-impaired driving deaths fell sharply in the U.S. during the 1980s and early 1990s, but there has been little progress since then. The proportion of fatally injured drivers with BACs of 0.08 percent or higher has remained at about one-third. In 2010, about 7 percent of drivers with BACs of 0.08 percent or higher who were involved in fatal crashes had previous alcohol-impaired driving convictions within the past 3 years on their records. Using interlocks to cut the ranks of repeat offenders can help chip away at the overall impaired-driving problem.

An alcohol interlock is a breath-testing unit connected to a vehicle's ignition. The driver blows into it and must register a BAC below a preset level. If the reading exceeds that level, the vehicle won't start.

Washington is one of 15 states that require everyone convicted of DUI to install an interlock for a certain period of time if they want to drive. An additional 22 states apply the restriction to drivers with high BACs and/or to repeat offenders. In the remaining states, interlock requirements aren't mandatory but may be imposed. An estimated 249,000 interlocks were in use throughout the nation during 2011.

Legislation pending in Congress would encourage states to require interlocks for all impaired-driving convictions by linking highway funds to the issue. Opponents of the measure argue that interlocks should be mandated only for people with multiple convictions or BACs of more than 0.15 percent. However, nearly a third of legally impaired drivers involved in fatal crashes have BACs below 0.15 percent. The new study shows that an interlock requirement for first offenders in that range reduces the likelihood they will be caught doing it again.

Previous interlock studies have found that offenders who get interlocks are much less likely to be arrested again on DUI charges than those who don't. One Institute-sponsored study randomly assigned offenders with multiple DUIs to get interlock-restricted licenses and found that recidivism dropped 64 percent with the restrictions (see "Ignition interlocks reduce re-arrest rates of alcohol offenders," Jan. 15, 2000).

That earlier study looked at the effect of interlock restrictions on all offenders subject to them, not just those who obtained interlocks. Similarly, the latest study looks at the effect of the interlock law on all offenders. Unlike the random-assignment study, it focuses on first offenders.

Judges in Washington have had the authority to order the installation of interlocks since 1987. In 1999, interlocks became mandatory in the state for repeat offenders, first-time offenders with BACs of 0.15 or higher, and offenders who refused an alcohol test. In 2004, Washington extended the interlock requirement to all drivers convicted of DUI. That includes first-time offenders with BACs of less than 0.15 percent, referred to in the study as simple DUIs.

To examine the effect of the 2004 expansion, Institute researchers obtained driver records from Washington's Department of Licensing. The records, current as of July 22, 2010, included information on convictions for DUI and negligent driving convictions stemming from a DUI arrest. The researchers gathered data on prior and subsequent offenses to identify first offenders and calculate recidivism. They also tracked interlock orders and installations associated with the convictions. Trends were analyzed for people whose initial conviction stemmed from an arrest during January 1999-June 2006.

About three-quarters of convictions from DUI arrests during the period were first offenses. Of those, about 30-40 percent were simple DUI convictions. Another 30-40 percent were alcohol-related negligent driving convictions, which occur when drivers arrested on DUI charges are allowed to plead guilty to the lesser charge. Such a conviction doesn't carry an interlock requirement, but it does count as a prior offense if the person is arrested again on a DUI charge.

Analyzing recidivism rates over time, the researchers concluded that expanding the interlock requirement reduced the 2-year recidivism rate for first-time simple DUI offenders by 1.3 percentage points. For example, in the second quarter of 2006, the last part of the study period, the recidivism rate was 9.3 percent. That is a 12 percent reduction from the 10.6 percent that would have been expected without the change. There also was an 11 percent reduction in recidivism (from 10.2 percent to 9.1 percent) among all first offenders, including those convicted of negligent driving, those with high BACs, and other types.

Trend in 2-year recidivism rates for simple DUI offenders
arrested during January 1999-June 2006

graph image

The law change didn't result in all offenders getting interlocks, but the number who did rose sharply. Among simple DUI offenders, about one-third got the devices, compared with less than 5 percent before. The researchers estimated what would have happened if everyone had gotten interlocks and concluded that recidivism would have fallen more steeply. For instance, recidivism for simple DUI offenders arrested in the second quarter of 2006 would have gone from 9.3 percent to 5.3 percent. If all first offenders arrested in that quarter, including those convicted of negligent driving, had gotten interlocks, the rate would have dropped from 9.1 percent to 3.2 percent.

The ability to plead to negligent driving allows impaired-driving offenders to avoid interlocks. Not surprisingly, the number of such convictions went up somewhat after Washington's law was changed to include simple DUI convictions. So one obvious way to increase the proportion of first offenders who get interlocks is to require them for alcohol-related negligent driving convictions.

Beyond that, it is unclear whether all the offenders covered by the law can be compelled to install interlocks, but Washington is trying to increase the number who do. After the study period, in 2011, the law was changed again so people with DUI convictions now can't simply wait out the interlock period with a suspension. If they ever want to regain their license, they need to first drive with an interlock.

"The latest change could be expected to increase the number of people convicted of impaired driving who go through with installing an interlock, which should further cut the number of convicted drivers who go out and reoffend," McCartt says.

As part of the study, the Institute also looked at single-vehicle nighttime crash trends, used as a surrogate for alcohol-related crashes, during 2001-07 in Washington. Comparing them with crash trends in California and Oregon, which didn't have any big changes in impaired-driving laws during those years, the researchers found a small reduction in crashes resulting from the change in the interlock law, but the effect wasn't significant.

"The increase in interlock installations resulting from the 2004 law change was still too small to have a substantial effect on crashes," McCartt says. "If the 2011 change is effective, then crash rates could be expected to go down. However, large effects on crashes will require more impaired drivers who have never had a DUI arrest to change their behavior as well. Interlock programs can help here, too, because the more they are publicized, the more of a deterrent effect they will have on the general population."

Of course, a more effective deterrent would be technology to prevent impaired drivers from starting any vehicle. The Driver Alcohol Detection System for Safety, a joint project of the government and the auto industry, is developing such technology. The goal is to come up with a reliable and unobtrusive system (see "Alcohol-detection device project is now in development phase," Nov. 17, 2011). Current interlock technology, although reliable, is too intrusive for this purpose because it requires a driver to blow into a device and takes at least 30 seconds to compute a BAC.

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