Driving after a DUI conviction can be a dicey experience. Many states require drivers, if they want to keep using their cars, to install ignition interlock devices that measure alcohol levels before allowing the vehicle to start.

One of the most common is from Des Moines, Iowa-based Intoxalock, which takes the form of a small box with a plastic tube into which the driver blows. The box then measures the level of alcohol in the breath. You must be below your state’s legal limit to start the vehicle. (In some states, the system will also log your location using GPS and/or take a photo of you every time you blow in the tube.)

The interlock device can only be leased, and it costs around $70–$120 per month.

Even if you are allowed to start a vehicle after this test, you aren’t done with the interlock. The device generates “random retests” during your drive, and you had better not miss one. Intoxalock recommends that you “keep your radio volume low and windows up” to hear its device beep.

Once the beep goes off, you have 3–15 minutes to clear the test again, giving you enough time to pull over and blow again. If you ignore or fail the random retest, your car won’t stop, but “your horn and lights may activate, signaling you to pull over and turn off the car,” says Intoxalock. And “depending on your state’s regulations, you may enter a temporary lockout.”