Traffic is a compression wave. The car in front of you accelerates, then you accelerate, then the person behind you, etc. Same for slowing down. Self-driving cars would presumably synchronize, allowing all of the cars to start moving at almost the same time.

Also, people don't feel safe driving at 60mph in a densely packed road, so they don't. This should be no problem for a computer, so you could have a densely packed highway but traffic still moving at very high speeds. Basically, flow velocity could remain constant as the highway becomes more congested.

Another optimization that comes to mind is that on city streets, your car would route around traffic light timing.

Jitneys today carry 8 people + driver, maybe more with a wider vehicle (safe for self driving). Self-driving had all the efficiencies the other reply talked about.