Earlier quoted context omitted.
The traffic reduction will come from two sources -- fewer cars and the fact that the cars can drive at optimal speed, reducing unnecessary slowdowns. That being said, while some cars would need to be stationary, there are plenty of people moving around all day long.
Do we know there will be fewer cars? Self Driving cars allow all non-drivers to use a car.
On the other hand, the cost of car ownership could be reduced with self driving cars.