Ignition Locks Reduce DWIs

The findings help settle a dispute over whether interlocks work as well with first-time offenders as with repeat offenders, providing key new evidence that could influence the decisions of lawmakers and judges on how to keep drunk drivers off the road. Four states now mandate interlocks for first DWI offenses: New Mexico, Louisiana, Arizona and Illinois (effective in 2009).

Researchers compared two groups of first time DWI offenders in New Mexico: those who had installed interlocks as part of their sentence, and those who had not.

"We found that first-time offenders who had interlock devices were 60 percent less likely to have a repeat offense than those who did not use interlock devices," said Paul Marques, PhD, with the Pacific Institute for Research and Evaluation "This study on first-time offenders reinforces prior studies on this issue that show a 65 percent reduction in drunk driving while interlocks are installed."

This study examined the records of 1,461 first time DWI offenders who had interlocks installed in their vehicles, and compared them to 17,562 first offenders who didn't use the devices. The two groups were matched up by age, gender, and blood alcohol concentration (BAC) at the time of arrest.

Two other earlier studies questioned whether interlocks changed the behavior of first-time DWI offenders. But researchers in the New Mexico study noted that in the earlier studies: only a small proportion of those required to install the interlocks actually did so. The New Mexico study included only drivers who installed the devices.

"The idea that there should be any important difference between the risk posed by a first offender and a repeat offender is unsupported," Marques says. "The average first offender has driven drunk many times before he or she was arrested. The big risk difference is between non-offenders and first offenders. The risk difference between first offenders and repeat offenders is small by comparison."

The authors also found an economic benefit to vehicle interlocks. One device costs the offender $2.25 per day, a small price for the public compared to the damage and destruction caused by DWI crashes. The study estimated that for every dollar spent on interlocks for first offenders, the public saves $3. Only about 10 percent of arrested DWIs nationally are ordered a period of interlock-controlled driving, says Marques.

"For so many years we've managed DWIs as criminals, but this is more then just a crime issue since many DWI offenders are alcohol dependent. Interlocks present an opportunity to help change behavior rather than simply punishing or incarcerating the offender," Marques says. "It's not enough to revoke a license - 75 percent of all people with revoked licenses drive anyway - but you don't want to sentence an entire family to poverty if they're dependent on that driver getting to and from his or her job. By installing an interlock, the risk that the DWI offender poses is controlled, and interlocks become a public benefit."

New Mexico has been using interlocks for about a decade. The state recently enacted a law that mandates use of interlocks for all DWI offenders.

This research was recently published in the journal Traffic Injury Prevention. It was funded in part by the Substance Abuse Policy Research Program (SAPRP) of the Robert Wood Johnson Foundation. Other funding agencies for this study include the National Highway Traffic Safety Administration and the National Institute on Alcohol Abuse and Alcoholism. Co-authors include Dr. Robert Voas of the Pacific Institute and Dr. Richard Roth of Impact DWI in New Mexico.