In some cases, policies and procedures designed to limit the availability of a substance have had the unintended consequence of promoting substance substitution—that is, reducing use of a substance to which access is significantly limited in favor of another substance that may provide similar benefits. This is of particular interest when we consider individuals who have been convicted of driving under the influence (DUI) of alcohol. A common intervention for such individuals is the mandated installation of an alcohol ignition interlock device (IID). These devices prevent the use of a vehicle when the driver’s breath alcohol content (BrAC) surpasses a preestablished threshold (commonly about 0.02). These interventions require the driver to adapt their alcohol use to the IID.
Research examining the effectiveness of these devices has historically been focused on individual rates of alcohol consumption or driving related outcomes such as the number of times the driver attempted to start the car with a BrAC above the threshold, whether they were involved in fatal or nonfatal crashes, etc. What has largely been overlooked, however, is the potential for substance substitution while the IID is installed. That is, when the IID is installed, the driver must adapt their alcohol-related behaviors to accommodate this new barrier. Certainly, some drivers adapt by regulating their drinking to nondriving times, but for certain subgroups of drivers, there is the potential that installing an IID on a vehicle unintentionally creates a transfer of risk—from drunk driving to drugged driving—each of which has a significant contribution to crash risk.
Our studies using biomarkers of substance use (hair, blood, and oral fluid) allowed us to accurately assess substance-related behaviors without the need to rely on self-report data. We found that when the IID is installed about two-thirds of individuals seem to adapt their existing alcohol-related behaviors to avoid drinking and driving, while a third of the individuals respond by significantly reducing their alcohol use. Among those who reduced their alcohol use, we found a statistically significant increase in cannabis use when the device was installed compared to before the device was installed, which suggests the potential for substance substitution. Equally important, however, is the finding that six months after the IID is removed from the vehicle, not only do rates of alcohol consumption revert to what they were before the IID was installed, but rates of cannabis use continued to significantly increase. This could suggest a transfer of risk from alcohol-impaired driving prior to the IID to polysubstance impaired driving after the device is removed for this subgroup of IID users.
This research outlines the need for targeted clinical interventions designed to reduce rates of substance substitution among individuals who have problems with alcohol use, as well as a careful reexamination of how to ensure better outcomes when using IIDs as a tool to reduce rates of impaired driving.