In reply to radddogg:
> The reason I asked was on behalf of a friend driving Car A. I left some info out on purpose as it was info the insurance company could not rely on. According to my friend, Car B was excessively speeding, estimated to be around 100 mph and came out of nowhere. So I said the same as you lot, he'll be found at fault, to which he was surprised as everyone else had said the speeding car was at fault. My reasoning was that obviously with no witnesses this won't be considered.
Even if established, it wouldn't necessarily shift liability. You are expected to be able to judge the speed of other vehicles, regardless of whether they are within the speed limit or not. For example, if you pull out on a car doing 60mph in a 30 zone you will generally be held responsible. The exception would be if the speed meant that you couldn't reasonably be expected to see them in time to avoid a collision.