Persons given a driving probation are ordered by a court to use interlock devices to prevent them from driving a vehicle while illegally intoxicated. Interlock devices protect drivers, passengers and pedestrians by preventing car accidents. Interlock devices are primarily used by persons convicted of driving under the influence or driving while intoxicated. Each state determines the manner in which interlock devices are used during a probationary driving period.
Many interlock devices are equipped with a breath analysis attachment that detects a percentage of blood alcohol after the driver blows into it. If the device detects the driver's blood alcohol content is over the legal driving limit, the car's ignition doesn't start. The legal alcohol limit varies by state, and devices are calibrated to meet state regulations. Interlock devices are calibrated to a specific address, and data is controlled by a service center. Some interlock devices use fingerprint or video technology to ensure the probationary driver is using the device as intended.
Probationary drivers are required to continue using the interlock device periodically while on the road. This is called a rolling retest, and a failed test causes the device to sound the car horn and flash car lights. The driver must then pull off the road. The car is inoperable until the driver does a clean test, indicating blood alcohol content has dropped to a legal level.