There are a lot of road design features that come from the fact that humans do make certain mistakes consistently. For example, the US's love affair with four-way stops comes from drivers' inability to safely navigate uncontrolled intersections, even when there's no particular reason they couldn't. Stoplights usually have a delay between the red light for one direction and the green light for the next direction because driver's often run fresh reds, or get stuck in the intersection and need to clear out. There are tons of warning signs because drivers can't be trusted to notice something as simple as a sharp curve coming up.

Of course, the road system is designed for humans' foibles, not computers' foibles, and computers will have to deal with that. But the bar is low.

Computers aren't really as consistent as you say, either. Obviously, a deterministic machine will produce the same outputs for the same inputs. But when your inputs are camera data from the real world, you'll never get the same input twice. For example, my car sometimes misreads or fails to read speed limit signs, but it'll usually read the exact same sign perfectly fine the next time I go past.