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 Pennsylvania, the use of an alcohol ignition interlock device (IID) is mandated for certain DUI offenders. Under Pennsylvania's Ignition Interlock Law, first-time DUI offenders with high blood alcohol levels (BAC of 0.10 or greater), as well as repeat DUI offenders, are required to install an IID in their vehicles. The device prevents the vehicle from starting if the driver's BAC is above the preset limit, typically .02 BAC. Installation must be performed by a state-approved technician. The IID records data such as breath test results and attempts to tamper with the device, which is reported to the monitoring agency and potentially to the court. The duration for which an IID must be installed varies, but it can be required for 1 year for first-time offenders and longer for repeat offenders. The court may also impose the installation of an IID as a condition of bail or probation. Despite the effectiveness of IIDs in reducing repeat DUI offenses, compliance rates are not as high as the law intends.