An ignition interlock device is a piece of equipment installed in a vehicle into which the driver must breathe before he can start the vehicle. If the device detects a blood alcohol level above a certain limit on the driver's breath, the car does not start.
Courts or state laws may permit a person whose license has been suspended due to a DUI conviction to obtain a restricted driver's license so that he can drive to work and other specified places. As a condition of receiving a restricted license, the driver may have to use an ignition interlock device. Some states require a driver convicted of multiple DUIs to install an ignition interlock system in his vehicle as a condition of his driver's license reinstatement.