It's complicated to say. It makes getting stuck in traffic less bad as you can do other things, so that tend to increase it. But it also means less traffic co

It's complicated to say. It makes getting stuck in traffic less bad as you can do other things, so that tend to increase it. But it also means less traffic consisting of cars cruising around looking for a parking spot in places where that's an important source of traffic. So I expect it to make traffic in LA worse but traffic in New York better based on those. Ideally you'd also have more multi-modal trips, taking the train most of the way to your destination and being picked up by a self driving car when you get there. That requires infrastructure investment, though, so won't happen quickly. The biggest change, I think, would be less need for so much parking in so many places. I don't think you could get cities all the way down to just 1 parking spot per car but we could get it down quite a ways and get it out of central business districts. EDIT: Oh, and with regards to that study, the fraction of the time a rideshare car is going to spend driving between customers as opposed to with customers is going to scale with the fraction of people using rideshare. The more trips are rideshare the more likely it is that on completing a ride any given car will find a new rider nearby as opposed ot far away. You would have issues like daily commutes everybody is going the same direction. But in that case adding more traffic in the other direction isn't as much of a problem as the vehicle kilometers would suggest.