Interlocks cut alcohol-related crash deaths

May 24, 2016

A driver blows into a breath-testing unit that checks for the presence of alcohol. If the reading exceeds a preset level, the car won't start.

Laws that require alcohol interlocks for anyone convicted of driving under the influence (DUI) of alcohol or drugs have reduced alcohol-involved crash deaths by 15 percent, a study by researchers at the University of Pennsylvania found.

Interlocks prevent people who have been drinking alcohol from starting their cars. Drivers must blow into a breath-testing unit, and if the reading exceeds a preset level, the vehicle won't start.

Penn researchers mined federal fatal crash data from the 18 states that mandated interlocks for all DUI convictions by 2013 and the 32 states with less-stringent laws. They compared the number of alcohol-involved crash deaths during 1999-2013 for the two groups of states, controlling for such factors as annual vehicle miles traveled, state highway speed limits and traffic law changes.

Compared with states having less stringent laws, those states with mandatory interlock laws saw a decrease of 0.8 deaths for every 100,000 people each year. The researchers note that this is comparable to the estimated number of lives saved by frontal airbags (0.9 lives saved per 100,000 people).

In states with universal interlock laws, 915 lives were saved between 2007-13, the researchers estimate. The authors assumed that the laws had no effect in the first three years after implementation.

The findings further bolster the evidence that mandatory interlocks prevent alcohol-impaired driving. Previous studies by IIHS and other groups have found that offenders who get interlocks are much less likely to be arrested again on DUI charges than those who don't.

For example, an IIHS study of the effects of Washington's interlock requirement found a 12 percent drop in the recidivism rate after the state expanded its interlock requirement to cover everyone convicted of DUI. 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.

Not all offenders covered by interlock laws actually install them. Some risk driving on a suspended license during the interlock period for economic and personal reasons. IIHS research indicates that laws requiring all DUI offenders to drive with an interlock before regaining their full license would result in further reductions in recidivism.

The Penn study authors note that their findings likely underestimate the potential effect of universal interlocks. They cite failure to install interlocks by all drivers required to use them; differences in enforcement among states; local laws that are stricter than state requirements and changes in penalties, monitoring and administration during the study period as factors that could have lowered the effectiveness of interlock requirements in the study states.

Mandatory interlocks may be the key to reigniting stalled progress in reducing the number of alcohol-impaired driving deaths, which plunged during the 1980s and early 1990s. Since then, the proportion of fatally injured drivers with a blood alcohol concentration (BAC) of 0.08 percent or higher has remained at about one-third. In 2014, 6 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 three years on their records. IIHS estimates that 650 of the deaths in 2014 could have been prevented if these drivers had zero BACs.

"Although crashes and crash fatalities decline, we're not seeing a significant reduction in the proportion of those involving alcohol," says the study's senior author, Douglas J. Wiebe. "We're encouraged by the increasing number of states enacting mandatory interlock laws since 2013 and hope these findings advance public health conversations aimed at saving more lives."

In May, Maryland became the 27th state to mandate interlocks for all drivers convicted of DUI. An additional 12 states require interlocks for offenders with high BACs (usually 0.15 percent or higher) and for repeat offenders, five states and certain California counties require them only for repeat offenders, and one state requires them only for high-BAC offenders and offenders convicted of a felony regardless of BAC. Four states and Washington, D.C., have no mandatory interlock requirements.

An estimated 318,714 interlocks were in use during 2014 in the U.S.

The advocacy group Mothers Against Drunk Driving in a report published in February estimates that ignition interlocks have prevented more than 1.77 million would-be alcohol-impaired drivers in the U.S. from starting their vehicles since states first passed ignition interlock laws.

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