I think the engineers will come up with the obvious answers- a Dead Man switch would turn off the vehicle if the driver falls asleep, is no longer conscious, no? That could mean 1) turn off the engine or 2) disconnect it from the wheels. In a standard shift car, taking your foot off the gas pedal will usually result in the engine turning off, since it can't handle the load at idle, especially if it's an old engine- so the gas pedal could be considered a dead man switch. The clutch pedal could be re-configured to be a dead man switch if you pushed it down, to engage the clutch- but then you'd have to force your foot down for as long as the car is operating, which would get tiresome.

But for a historical perspective, the earliest cars ( before Kettering invented electric starters) had hand-cranked starting. If the engine died, you had to get out- maybe in the rain or snow- and give the thing a crank to start it back up. Having an engine that would easily accidentally shut off would have struck most customers as a bad thing.

Vehicles suddenly and unexpectedly stopping in traffic is not a good thing. Look to GM's current issue with bad ignition switches, which behave in much the same way a dead man switch would though I suppose a signaling system would accompany a designed dead man system.