There's no way self-driving cars will reduce traffic volume. If anything, it will increase since not only you don't need to be able to drive in order to travel in a self-driving car, the car might be completely empty and driving on its own for some sort of reason. And of course people who currently drive a car will still be driving a car (which might be self-driving or not).

There is a way: A lot of traffic these days is people looking for a parking spot. If the car leaves the area right away after dropping you off, it will be less congested there.

They may decrease congestion (backups) due to improved flow, but that does not necessarily mean there will be fewer automobiles, if anything, better flow/throughput likely would encourage new or more motorists to take to the road.

I doubt they'll turn major arteries into quiet streets, especially at rush hour.

Unless it becomes cheaper and more convenient to share rides in robotic vans booked on a smartphone and scheduled via a robotic taxi platforms that can match the bids (vans with spare seats going in the right direction) with asks (potential passengers requesting a cheap trip on their smartphone).