Self driving cars will eventually be significantly less expensive than current vehicles. They will be small and electric inexpensive to operate and trips will be very cheap.

When the cost of a trip drops, more trips will be made. If self-driving succeeds to the point where I can send my car to pick up my dry cleaning or food without needing to be in the car, then expect the number of vehicles on the road to go up a lot.

We might be able to pack two times as many cars on the road and make intersections way more efficient, but that won't be enough.