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 California, the use of an alcohol ignition interlock device (IID) is regulated under the Vehicle Code and is often required for drivers convicted of DUI offenses. The state mandates the installation of IIDs for repeat offenders and may require them for first-time offenders, especially if their blood alcohol concentration (BAC) was particularly high or if the offense involved injury. The duration for which an IID must be installed varies based on the number of offenses; for a first DUI, it may be required for up to 6 months, while multiple offenses can lead to a requirement of several years. The installation must be carried out by a state-approved technician, and the device must be calibrated and inspected regularly. The IID prevents the vehicle from starting if the driver's BAC is above a preset limit, typically .02 BAC. Data collected by the IID, including breath test results and any attempts to tamper with the device, is reported to the monitoring company and the court. Despite the effectiveness of IIDs in reducing repeat DUI offenses, compliance rates are not as high as the law intends, with a significant number of those arrested for DUI not having an IID installed as required.