Driving the speed limit is frequently unsafe, e.g. in adverse weather conditions or unusual street conditions (like when passing an adjacent block party with lots of adults and children milling about.

The speed limit is the maximum speed limit in ideal conditions.

Driving the speed limit is frequently unsafe. I drive on highway 401 in Ontario frequently, where the speed limit is 100kmh, but traffic flows at 120kmh or above, with faster traffic going more like 130. Going the speed limit means you are going 20kmh slower than the speed of traffic, and 30kmh slower than faster moving cars, which can definitely be dangerous since accidents are more often caused by speed differential, rather than absolute speed.