Yeah, that was an attempt at brevity. I live in Michigan in the country. My roads are not flat ( both the dirt and the paved) and if the car stopped for sticks, it would never make it up the driveway.I don't think it is quite that simple. The drivable space is not always going to be a smooth flat surface like a road. In some cases, the drivable space might be a dirt road or even a grassy area that are definitely not smooth flat surfaces.
Point being, safe to drive on surfaces can be classified.
It's a corollary to a failure probability problem. Instead of adding all the different ways to succeed, just subtract the probability of failure from 1.