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 Tennessee, an alcohol ignition interlock device (IID) is mandated for all DUI convictions, including first-time offenders, under certain conditions. The state law requires that the IID must be installed by a certified technician and the device must be approved by the Tennessee Department of Safety. The BAC limit for the IID is typically set at .02. For first-time offenders, the court may order the installation of an IID as part of their sentence, especially if their BAC was .08 or higher, if there was a child in the vehicle, or if they were involved in an accident. Repeat offenders are required to have an IID installed. The duration for which the IID must be installed varies, but it can be ordered for periods ranging from one year to several years, depending on the severity of the offense and the offender's DUI history. The IID program aims to reduce repeat DUI offenses and enhance public safety by ensuring that individuals with a history of DUI are unable to operate their vehicle while impaired.