Earlier quoted context omitted.
What exactly do you propose? A zipcar that doesn't allow allow you to drive outside of the city?
Sure. Shut down the engine when it loses cell signal, with some warning so you could turn around. It'd be inconvenient, but not more than the alternative.
Shut down the engine when someone is still driving? That's not just inconvenient, that's suicidal. What if you're not able to get back to a place with cell coverage in time? Is the car just going to stop in the middle of the highway or the middle of a tunnel?