A human would drive too fast on in icy conditions... if the computer knows there are icy conditions and it doesn't slow down before it even senses trouble then the car was programmed to be going too fast. If it is so dangerous there is no way to remove these scenarios (like a blizzard) then a human should be forced to override the system in which case the human is at fault.

I trust sensors to detect icy conditions better than a trust myself.

[edit] Most bridges where I am have signs that explicitly warn that bridges freeze over. And people intuitively know... bridge + recent precipitation + cold whether = slow down. I don't know why a computer wouldn't know that.