An interlock device is a mechanism like a breathalyser, installed on a motor vehicle's dashboard. Before the vehicle's engine can be started, the driver first must exhale into the device; if the result of the test is greater than the programmed blood alcohol content allowed, the device prevents the engine from being started. At random times after the engine has been started, the device will require another breath sample.
The purpose of this is to prevent someone other than the driver from providing a breath sample. If the breath sample isn't provided, or the sample exceeds the ignition interlock's preset blood alcohol level, the device will log the event, warn the driver and then start up an alarm (e.g., lights flashing, horn honking) until the ignition is turned off, or a clean breath sample has been provided. A common misconception is that interlock devices will simply turn off the engine if alcohol is detected; this would, however, create an unsafe driving situation and expose interlock manufacturers to considerable liability.