The Deterrent Effect of Ignition Interlock Devices — Before They’re Even Installed

Adding a car breathalyzer to existing penalties could substantially discourage drunken driving

Alcohol contributes to about a third of all motor vehicle fatalities, a rate largely unchanged over the past two decades. (Alcohol-related traffic deaths totaled 13,524 in 2022, according to the National Highway Traffic Safety Administration.) Roughly a million drunken driving arrests are made annually in the U.S., according to the Federal Bureau of Investigation, and that, of course, represents a small subset of instances in which people drive impaired.

A paper in the American Journal of Preventive Medicine suggests that one penalty — ignition interlock devices — could serve as a powerful deterrent even for drivers who have never been arrested or convicted of a DUI offense. 

Rather than focusing on the deterrent effect of a convicted drunken driver suffering the inconvenience of blowing into the device before starting his vehicle, researchers asked whether the prospect of a breathalyzer might help head off drunken driving in the first place. 

The paper by UCLA Anderson’s Robert Zeithammer, UCLA Fielding School’s James Macinko and NYU’s Diana Silver indicates that the threat of a one-year IID penalty provides a similar deterrent to  a $2,200 increase in fines, a 10-day increase in jail time or 20fold increase in the likelihood of being pulled over.

Already Reducing Recidivism 

“Having to install an interlock device even for a short period of time may have a significant deterrent effect,” the authors write. 

Ignition locks, or breathalyzers, have been shown effective at reducing recidivism among those convicted of a DUI, but their application isn’t uniform: 31 states and the District of Columbia require all DUI offenders to install an interlock device, according to the National Conference of State Legislatures. Another eight states — including Michigan, Minnesota and Pennsylvania — require the devices for only repeat offenders or those with a high blood alcohol reading at the time of the arrest. And another five require the devices for only repeat offenders. Six states, including California, Indiana and North Dakota, leave the decision to order IIDs to the discretion of judges.

(In 2021, as part of an infrastructure bill, the Biden administration would have required automakers to install new technology that would detect and prevent impaired driving in all new cars sold. The proposal is now in a lengthy rulemaking phase at the NHTSA and far from assured of implementation.)

To study the deterrent effects of IID penalties on drivers who have never been convicted of a DUI, the authors used a “discrete choice experiment,” which asks participants to choose from a set of hypothetical scenarios. In this case, would they drive while drunk if they faced a fine, jail time, license suspension, mandatory treatment or the installation of an IID? They also were asked how their choices would change if they faced a 20% chance of being caught instead of a 1% chance. 

The results are based on the responses of 583 adults recruited through the online research platform Connect whom the platform identified as having at least one alcoholic drink a week. 

Combining Deterrents

About 39% of participants said they would never drive impaired, regardless of the penalty, not allowing the survey to measure how important different penalties are to them.

Among the remaining 61% of participants, the survey responses were used to calibrate a behavioral model that allows the researchers to measure the deterrence potential of adding an IID to an existing monetary penalty, i.e., the increase in the predicted share of participants who choose not to drive under the influence. Even one month with an IID added to a standard fine made participants less willing to drink and drive, and longer IID penalties had a stronger effect. 

One way to quantify the deterrence effect is to express it in terms of increases in the other penalty dimensions that would generate the same increase in deterrence. This calculation reveals that a one-year IID penalty provides a similar deterrent to a $2,200 increase in fines, a 10-day increase in jail time or 20fold increase in the likelihood of being pulled over.

The deterrent effect of interlock devices seems to be broadly applicable in that it does not vary much as a function of the participant’s dependence on alcohol, prior DUI experience or other demographic factors such as age, sex or race. “This study finds that adding an ignition interlock device to a state’s DUI penalty regime could have a substantial general psychological deterrent effect even before any such devices are installed,” the authors write. 

Of course, any type of penalty can only be an effective deterrent if potential law breakers know about it. “States already authorizing interlock penalties may wish to publicize this fact to the public to benefit from the general deterrence effect documented here,” the authors write.

The authors also note that widespread use of interlock devices could also produce social benefits by serving as an alternative to fines and imprisonment. This is especially meaningful given the disproportionate rates of DUI arrests and convictions among minority populations.