If you turn the whole thing around it will make sense.
First, disable any starting unit in the car.
Second, have the drive blow into the tester.
Third, free motor starting or freeze it depending on the alcohol concentration in the breathing air.
That of course won't be 100% of control. If the driver stops on the way, empties a bottle of whiskey with the engine running, and continues driving that system fails.
You could of course use a timer circuit to turn off the engine after a certain time running at idle and call for a new test before restarting.
To do it the described way you just require a normal breath analyzer as used by the police.
Boncuk
BTW, I never drink while driving. I stop first.