Actually I think road speeds for various curves are pretty standardized, so it shouldn't be that hard for the car to estimate the correct speed based on the curve.
It's a tad more complicated than that - it needs to consider also driving conditions, any hazards on the road. Probably can't be done on computer vision alone, you need at least some sensors to get a "feel" of how good is the road grip.
My Audi tries to account for curves in the road when running on cruise control through the onboard nav. It’s horrendously conservative in its opinion of the maximum safe speed to the point I find it dangerous to rely upon. If I left it to its own devices, other cars would be passing me as if I were standing still. Otherwise the driver assist features are fairly slick.