I think a distinction should be made between going
slightly below the speed limit and gong
greatly below the speed limit. If you're going slightly below the speed limit (say 5-10 mph under) and you "cause" an accident, then I'm totally blaming the other guy. If you're going to go
greatly under the speed limit you gotta use some common sense and watch out for other drivers (not that you shouldn't do that all the time).
The thing that you have to remember is the way the human mind and senses work. It's amazing how much of the world we don't actually "sense" because most of the mundane details of our surroundings our mind just "fills in" based on assumptions and past experiences. It's basically a techniques our minds use to conserve brain power. That's how many magic tricks work--we don't actually see everything--we mostly "see" what we expect to see. That's also the reason why people get so mentally tires when they're in a new environment--they're mind doesen't know what to expect so it has to work harder.
This applies to driving, when you're doing something vastly different from what is expected ("normal") you always run the risk of somebody not seeing you becuase they expected something different. This is also why there's a lot of accidents involving motorcycles and cyclists. People don't "see" them because their minds get lazy. They look but they look for a car. When nothing in their mind triggers "that's a car", the mind moves on.
Just the way the human mind works.
Here's an interesting example: