congestion reduction is such a goal only because people are wasting time in traffic :). If cars are empty or people inside are engaged in a dopaminergic activity (working, texting, having sex, etc...) it will become less of an issue. Especially with electric cars as idling of the engine becomes non-issue. Add to that that currently amount of time my car on the road is limited by my time (and thus i plan/optimize when/where to go, like getting into store on the way from work, etc...), the self-driving car can go to store for pickup or deliver some papers, etc... while i'm doing something else. I.e. the total sum of time of me getting quickly home from work and the car going to store on its own will be larger than one optimized trip - i.e. my time in the car would be lesser while car's time on the road would be higher if everything else was equal. Though it willn't be and thus- i'm looking forward for many productive hours stuck in "smart & autonomous" traffic trying to get home as quickly as possible and sending the car on whatever errands before and after - hope it will enjoy being stuck in traffic among the sames.