Actually, I think it's often the case. But even when it isn't the guy usually doesn't press the gas until the brake lights on the car ahead of him turn off. There's a significant amount of delay introduced by people, especially if someone is reading a text message at the light. Also, humans will slow down more than they have to (as they have no way of knowing precisely) when they see traffic slowing ahead, which leads to the accordion effect. Self-driving cars will only slow down as much as necessary. They also won't gawk. :) No, but I think there is a lot of slack to be found in human driving behaviors, especially when the system can be optimized as a whole.It's not as if the guy driving the car 10 cars back doesn't see the light turn green at the same moment that the person driving the first car in line does.
You'll note that conservation of mass is a law of physics, and it can't be violated by computers, even if they're really, really, really smart.