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Status Report, Vol. 53, No. 2 | March 29, 2018 Subscribe

Locking out impaired drivingLaws that require interlocks for all DUI offenders save lives

Laws requiring all impaired-driving offenders to install alcohol interlocks reduce the number of impaired drivers in fatal crashes by 16 percent, a new IIHS study shows. If all states without such laws adopted them, more than 500 additional lives could be saved each year.

A separate study shows that those laws could be made even more effective. In a detailed examination of Washington's interlock policies, Institute researchers found that, as the state's interlock laws were strengthened, interlock installations went up and recidivism declined. At the same time, more DUI charges were reduced to lesser offenses that don't require interlocks. That suggests states could increase the impact of their interlock laws by closing such loopholes.

The two studies are the latest to support the expansion of alcohol interlocks — in-vehicle breath-testing units that require a blood alcohol concentration (BAC) below a certain level, typically somewhere between 0.02 and 0.04 percent, before the vehicle can be started.

More than a quarter of U.S. crash deaths occur in crashes in which at least one driver has a BAC of 0.08 percent or higher. The prevalence of impaired driving in fatal crashes has changed little in the past two decades, and interlock laws are one of the few recent policy innovations that have made a difference.

Forty-five states require interlocks for at least certain impaired-driving offenders. Twenty-eight states, the District of Columbia and four California counties have some type of interlock requirement that applies to first-time offenders.

Even when they are mandated for first offenders, interlocks come into play only after a DUI arrest, so their direct purpose is to reduce recidivism. Like other types of sanctions, however, they may act as a deterrent for those who haven't yet committed a first offense if they are well-publicized.

Interlock laws cut deaths

Whether interlocks keep those convicted of DUI from reoffending or deter people generally from driving while impaired, the overarching goal is to reduce alcohol-impaired driving and the deaths and injuries that result. The national study shows they have succeeded.

"We looked at the number of impaired passenger vehicle drivers involved in fatal crashes over time and compared them with the number of drivers in fatal crashes that didn't involve impairment," says Eric Teoh, IIHS senior statistician and the paper's lead author. "We found that state laws mandating interlocks for all DUI offenders reduced the number of drivers in fatal crashes with BACs of 0.08 percent or higher by 16 percent compared with no interlock law."


Interlock laws decrease impaired-driver fatal crashes

In 2016, 10,497 people died in crashes involving drivers with a BAC of 0.08 percent or higher. Of those, 8,853 involved impaired passenger-vehicle drivers. At that time, the number of states with first-offender laws was 25. Had all states had all-offender interlock requirements in place, 543 of those deaths would have been prevented, Teoh calculated.

For the analysis, the authors grouped together two types of all-offender interlock laws: those that require all offenders, including first-time offenders, to install interlocks in order to have their license reinstated and those that only require it to drive during a post-conviction suspension. The analysis controlled for factors besides interlocks that could affect crashes.

Laws that required interlocks for repeat offenders only cut the number of drivers with BACs of 0.08 percent by 3 percent compared with no interlock law, and that effect wasn't statistically significant, the study showed. Laws that required them for both repeat offenders and offenders with high BACs provided an 8 percent benefit.


U.S. alcohol interlock requirements

Laws that apply to all impaired-driving offenders, March 2018

U.S. interlock laws map

The 28 states highlighted on this map, along with the District of Columbia and four California counties, have some type of interlock requirement that applies to all offenders, not just repeat offenders or those with high blood alcohol concentrations (BACs). Specifically, in 23 states, D.C. and four California counties, all offenders must install interlocks to drive during a post-conviction license suspension. In 15 states and four California counties, all offenders must spend some period of time with an interlock installed on their vehicle before having their license reinstated. More on alcohol interlock policies.


One state's experience

The examination of Washington's laws updates an earlier study that found that recidivism declined after the state expanded its interlock requirement, which previously targeted only repeat and high-BAC offenders and offenders who refused the alcohol test, to all offenders in 2004 (see "Alcohol ignition interlocks: Study shows devices reduce DUI recidivism," March 6, 2012). The update looks at trends in Washington over a longer period during which the interlock law was further bolstered.

The additional changes included allowing an interlock in lieu of an administrative driver's license suspension before conviction, a change that went into effect in 2009. Then, beginning in 2011, convicted drivers had to prove they had driven with an interlock for the last four months of their interlock requirement without any interlock violations before getting their full driving privileges restored.

As it became harder for impaired drivers in Washington to avoid interlocks, there were fewer repeat offenses. For first offenders arrested during the last quarter of 2012, the recidivism rate declined from an expected 7.7 percent without the law changes to 5.6 percent. The interlock installation rate was only 38 percent in that quarter. If all of these DUI offenders had installed interlocks, the recidivism rate could have shrunk to 2 percent.

Why did only 38 percent of impaired-driving offenders install interlocks even though doing so was a requirement for license reinstatement? Some people may have continued to drive without a valid license. Others may have given up driving.

But one key reason is that, after the changes to the law, an increased number of first offenders had their charges reduced to alcohol-related negligent or reckless driving. Those offenses don't require interlock orders, but they do count as prior offenses if a person is arrested again for impaired driving. When such convictions are excluded from the analysis, the installation rate increases to 54 percent.

"Washington's experience shows that more robust interlock laws can cut down on repeat offenses," says Charles Farmer, IIHS vice president for research and statistical services and a co-author of the study. "It also suggests that such changes could have an even greater effect if loopholes that allow people to avoid interlocks by pleading to lesser offenses were closed. It's a perfect example of why legislation, enforcement and adjudication need to work together for highway safety policies to achieve the desired result."


After Washington's interlock law
was strengthened, fewer people reoffended

Washington state interlock laws

Washington's experience shows that more robust interlock laws can cut down on repeat offenses. However, despite the requirements, many of the state's impaired drivers continued to avoid interlocks, thereby shrinking the law's benefits.

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