Traffic is a function of how many cars go through a road/intersection per unit time, which is reduced if cars keep shorter following distances, which self-driving cars can do because of faster reaction times.

In the extreme, imagine if a light turns green and all cars in line accelerate at the same time. Intersections would let through a lot more cars.

Or think of freeway speeds where the length of a car is near-negligible. A one-second following distance means almost 60 cars pass a given point in a minute, versus almost 30 cars per minute for a two-second following distance.