Like a breathalyzer, an ignition interlock measures the alcohol in a person’s system. If that amount exceeds a pre-programmed level, then the interlock temporarily locks the vehicle’s ignition.
Most states require random re-testing while driving. At times on the road the device will beep, signaling the driver to deliver a breath sample. If alcohol is detected, the ignition interlock will not turn the engine off while the vehicle is in motion. Usually it will signal the driver to stop by blowing the horn and flashing the lights.