The tech already exists. It's basically a breathalyzer/ignition interlock. You have to blow into it and if your BAC is below the set limit the ignition switch will unlock so you can start it. Or something like that. The problem is, if you have a sober friend that's a goof, they can start your car for you.
Except that at intervals the car will demand a new sample and not move until the right mixture hits the computer.