They have a system like this in Canada (and other parts of North America?) for severe or repeat DUI offenders. Its a unit thats installed under the dash and wired into the ignition system by the police. They solve the 'sober person breathing for car starting" by having the machine perform random tests while the car is driving. If the machine doesn't receive a new breath sample within thirty seconds or so of the notification, either the car will not start or it shuts itself off (I've got conflicting memory regarding this).
British article on system
System Manufacturer
Sounds like a 90% reduction in reoffending rates in Canada, so it must be working