I know I know..you're going to say "it's the speed!"...yes, to a degree, but it is more the difference in speed that is the problem. This is why most major studies have pretty much proven if you increase the limit to around the 85th percentile, suddenly accidents drop off because drivers are paying more attention, and there is less speed disparity on the the road. The same logic can be applied elsewhere -- Say someone was driving in at 10km/h in an 60km/h...what do you think will happen when all the drivers are trying to avoid them?
Personally, I'm a fairly firm believer that if the limits make sense, then most drivers will abide by them. The problem is so many of our major arteries are set to speeds that do not make sense, and where in fact the vast majority are breaking the law every time they are on those roads. Try driving on the 401 at exactly 100km/h...and watch how many cars are going faster than you, and how many near misses occur when people are not expecting a car to be going that slow on the road. Also, note that the 401 (or at least sections of it) used to be 110km/h until the fuel scare/crisis in the 70s. The speed was lowered not to increase safety but to lower fuel usage...yet now the scare is gone, cars are safer and more fuel efficient, yet the lower speed remains.