For unusual fixed features like this, perhaps car manufacturers should start putting exceptions into their navigation databases, rather than tweaking their automatic detection. You generally don't want to be driving over cattle grids, with their sudden gradient changes, at high speed anyway. So tell the car to slow down as it approaches (or warn the driver loudly to slow down), and briefly disable automatic braking. If it's a rare event, it'll have the drivers' attention.

I see the future of self-driving cars to be one where highway authorities map and sign features in a way that is designed for self-driving cars to be able to parse, rather than requiring them to heuristically sense as much as they do today. This would be a step down the path that's inevitable anyway.