Efforts are underway to encourage alcohol interlocks for all impaired driving offenders [driving under the influence (DUI)] in the USA. Interlocks require breath-alcohol concentration (BrAC) tests and prevent engine startups when BrAC is above preset levels. Interlock programs have grown rapidly; however, the devices are only effective while installed. Clearly, we need to strengthen effectiveness while installed, but also to improve monitoring and extend the installed duration when there is evidence of persistent excessive drinking. Perhaps most importantly, we need to improve the safety benefits achieved while installed for long after interlocks have been removed.
Currently, 279,000 interlocks are in use, equaling about 20% of annual DUI arrests in the USA. Since 2006, installation has grown about 15% per year. The popularity of interlock laws is bolstered by 20 years of research studies showing offenders on interlocks have two-thirds lower recidivism rates than comparable controls . Policy initiatives by Mothers Against Drunk Driving seek to expand interlocks to all DUI offenders. This push is reflected in the recently passed 2012 U.S. Transportation Bill, which includes incentives for states to pass and enforce mandatory interlock laws. Unfortunately, receiving less attention than full enrollment is the return to control levels of impaired driving once the devices are removed [2, 3]. Overcoming that problem may yield a greater safety benefit than adding additional interlock users. The average interlock sentence for first-time DUI offenders is less than one year, whereas the average drinking driver will be licensed for decades. Once the current wave of legislation to extend interlocks to first-time DUI offenders has run its course, extending the use of alcohol technologies to improve long-range alcohol safety will assume paramount importance.
When given a choice to enroll in programs that substitute interlocks for some portion of the usual license suspension period, only 10–20% of DUI offenders select it. Presumably, many prefer the option of driving while unlicensed with its low probability of detection. Policymakers often believe that license suspension is the more severe sentence and most states retain it. As a result, a large portion of the time DUI offenders are subject to sanctions, they are suspended, rather than using interlocks, even though recidivism rates with an interlock are lower . As New Mexico successfully implemented in 2002, provisions of the 2012 Transportation Law create an incentive for states to remove most of the mandatory suspension required for interlock eligibility. This is important because recidivism risk is highest in the period immediately following the initial arrest .
Some offenders successfully argue they have no access to a car so cannot enter an interlock program. But, studies show that when the court imposes a more burdensome alternative sanction, such as home confinement, offenders find cars and install interlocks, rather than face the more severe penalty . This goading of offenders into interlock programs, however, is rarely practised.
Research from 1999 to 2010 showed a direct relationship between breath test lockout rates and subsequent re-arrest for DUI . Based on this research pedigree, a few states have provisions in their laws extending the interlock program, or additional monitoring, of offenders with repeated BrAC lockouts.
Improved monitoring can take several forms. The basic interlock is a transparent low-level sanction system—blocking engine ignition—requiring no action other than periodic maintenance, data downloads, and action by the authority to discourage circumvention. Supplemental monitoring with targeted consequences can add a second level of supervision. A random assignment study in Maryland  evaluated regular postal letters that threaten sanctions, or applaud interlock offenders, based on program performance. This approach reduced recidivism for up to two years during the interlock program. States' monitoring can invoke a range of actions, including removal from the interlock program, extension of time on the interlock and a system of escalating supervision, as in Florida.
New alcohol technologies provide monitoring alternatives. Two are transdermal alcohol detection and photo identification systems for interlocks. Transdermal units lock onto the ankle and yield nearly continuous readings of alcohol vapor at the skin. Positive photo identification of interlock test-takers makes it difficult for offenders to blame lockouts on other vehicle users. For those without cars, non-vehicle variants of such monitoring require BrAC tests several times during the day. Alcohol biomarkers—longer time horizon indicators of drinking—are widely used in Europe to monitor DUI offenders. Measurement of alcohol markers in blood, hair or urine sources can objectively improve our understanding of drinking status among DUI offenders.
These advanced systems for monitoring drinking are now getting occasional use in the criminal justice system to reduce reliance on costly jail. As costs for monitoring drinking decline, someday this may replace interlocks as a method of controlling DUI offenders .
We have no evidence on how offenders adjust their drinking and driving to accommodate the interlock. Clearly, offenders ‘learn’ to avoid lockouts while on the interlock [2, 3, 10, 11]. This safety benefit occurs even while total driving is unchanged. Reduction in rates of failed BrAC tests led to initial enthusiasm that interlocks really might be reducing drinking. This inference was challenged by a 2010 study  in which six direct and indirect blood biomarkers were collected at both installation and interlock removal many months later. The pre- and post-comparison of biomarkers showed little to no change in alcohol consumption.
Two approaches can counter the resumption of impaired driving upon interlock removal: active treatment during interlock or harm reduction strategies. An interlock program is not a therapeutic behavioral change program. DUI offenders may drink more strategically to avoid interlock lockouts, but they do not drink less. Accordingly, monitoring authorities that want abstinence could make better use of the interlock period, while drinking is held somewhat in check. A Texan study showed interlock participants under court order were amenable to a motivational intervention in which counselors use and discuss the interlock BrAC data .
Harm reduction is also an option, preserving the behavioral accommodation to the interlock (reduced drinking and driving) without targeting drinking directly. What do interlock users do differently to avoid BrAC lockouts? Little is known about their coping methods but with drinking and driving both unchanged, offenders are making changes that avoid limiting either. A study of those adjustments might help us develop interventions.
In sum, more interlocks are going on more offender cars, and there is growing recognition of the need to install soon after detection and arrest, rather than after long periods of license suspension. But, little effort is underway to actively monitor and mount supplemental interventions on higher risk interlock DUI offenders. Progress on the latter, and most difficult, element will be needed to break the cycle of catch and release.
Declarations of interest