I could see something like an augmented reality setup for a driver where fake pedestrians wander into the road ahead of you and if you don't slow down in time you are warned / flagged as impaired / ignition shuts down. Passes the specificity test, for sure - which something like a reflex test to turn the ignition switch does not.

The problem is that it's way too invasive for people to sign up for it voluntarily - not to mention you'd cause a bunch of sudden stops for no reason and maybe create more accidents than you prevent.