Ignition interlock devices are installed on the dashboard of a convicted drunk driver. The driver must breathe into the device in order to start the car and the blood alcohol level must usually be between 0.02 percent and 0.04 percent. The driver must also periodically breathe into the device while driving, to ensure that another person doesn't breathe into the device to start the car and then let the intoxicated person drive away. If the driver fails to provide a sober breath sample, a car alarm will go off and will not stop until the car is turned off or a clean breath sample is recorded. Ignition interlock devices must be installed at the offender's expense and cost between $70 and $150. They are usually mandated for two to three years after the conviction.

In Los Angeles interlock devices are highly advocated by groups such as Mothers Against Drunk Driving (MADD). According to MADD, simply suspending driving is ineffective, as driving is necessary for most people to get to work, or go to the grocery store, or doctors office. The inconvenience of not being able to drive generally outweighs the offender's fear of breaking the law, and 50 to 70 percent of drunk drivers with suspended licences are reported to drive anyway. With the use of an ignition interlock Los Angeles motor vehicle operators who have had drunk driving offenses are allowed to drive, so long as they prove they are sober enough to do so. While it is possible for a drunk driver to have someone else breathe into the device, or to borrow a car from someone without an ignition device installed, most states have strict penalties for anyone who helps a DUI offender in this way.