Question

Why do roads have speed limits rather than velocity limits

Answer 1

Speed is the rate at which something goes, and velocity is the average acceleration, which means a car can go slow for a bit, and fast for a bit, and have a velocity of medium. Also, speed is much easier to measure. 

Answer 2

Speed is the rate at which something moves. Velocity is both speed and direction. Basically, having a velocity limit would mean only that you couldn't exceed a speed in a certain direction. Speed limits restrict speed in all directions because no direction is specified.