An alcohol ignition interlock device (IID) is a breath-test device that is connected to a vehicle’s ignition. A vehicle with an IID installed will not start unless the driver blows into the interlock and has a blood alcohol concentration (BAC) below a limit set by the court—usually .02 BAC. IIDs must be installed by a certified technician approved by the state’s department of motor vehicles (DMV). The IID will collect data (engine starts and stops, breath test results, and tampering attempts) and report it to the monitoring company, which will report it to the court.

IIDs reduce repeat offenses for DUI/DWI by approximately 70% when installed. Laws vary from state to state and courts have significant discretion, but a judge may order a person convicted of a first DUI/DWI offense to install an IID and will order a person convicted of a second or third DUI/DWI to install an IID. Courts will often order an IID installed on the defendant’s vehicle as a condition of bond (to be released from jail) and of probation (following jail or prison time, or in lieu of any or more jail or prison time). The court may order the defendant’s IID to remain on the vehicle for 1-3 years or more.

Despite these laws and programs, only about one-fifth of those arrested for DUI/DWI have an IID installed.

In Montana (MT), an alcohol ignition interlock device (IID) is a mechanism used as a preventive measure for individuals convicted of DUI/DWI offenses. The state law mandates the installation of an IID for all second and subsequent DUI offenses. For a first DUI offense, the court has the discretion to order the installation of an IID. The device prevents the vehicle from starting if the driver's BAC is above a preset limit, typically .02 BAC. Installation must be performed by a certified technician recognized by Montana's Motor Vehicle Division (MVD). The IID records data such as engine starts and stops, breath test results, and any tampering attempts, which is then reported to the monitoring company and, subsequently, to the court. The duration for which an IID must be installed varies, but it can be ordered for periods ranging from 1 to 3 years or more, depending on the severity of the offense and the court's decision. The use of IIDs is intended to significantly reduce the likelihood of repeat DUI/DWI offenses. Despite the effectiveness of IIDs, there is a noted gap between the number of individuals arrested for DUI/DWI and those who actually have an IID installed.