Why are cars changing velocity all the time? Not enough input and output on the conditions outside the car. Self driving cars don't have to drive like humans,

Why are cars changing velocity all the time? Not enough input and output on the conditions outside the car. Self driving cars don't have to drive like humans, they have to drive like 90% of the other cars. Remove the humans and self driving cars can adopt far more sophisticated traffic patterns than the simple stop/go of most city traffic.