I dont beleive for a second that someone has sat for a week on all stretches of all roads to decide what the limit should be. If that were the case there would be many roads near me that would have much different limits as some of them are just plain dangerous. Limits are derrived largely from the classification and size of the road and then altered when people complain enough. I have seen plenty of places on my commutes to work where speed limits have been changed and have made no difference what so ever to accident rates. The reason - speed wasnt the cause - it was either poor visibility or just stupid drivers. If you always have the limit set for the lowest common denominator of driver skill, we may as well go back to having a man infront of the car with a flag.
I do like to think I have a more balanced view that most as I regularly cycle on the roads (~100 miles a week commuting), ride a motorbike, drive a car, and have for a period driven light goods vehicles. We dont need better speed limit classifications at all, we need more driver training and punish those that dont pay attention and cause accidents. Speeding in itself doesnt do anyone any harm, its driving too quickly for the surroundings that causes problems. However its far easier to set a limit and then blame anything that should go wrong on someone going "too quick" where more than likely it was becuase they didnt assess the situation properly.