Everybody loves the idea of new technology that allows them to continue doing what they are doing, in this case avoiding biking, walking, and public transit. Basically, people want to believe self driving cars will reduce congestion. Ergo, it's pretty easy to convince them it will.
http://www.sfchronicle.com/bayarea/nevius/article/More-ride-...
That article provides no evidence to support this claim other than the fact that 37k people have driven for Uber/Lyft/et al in SF. The author then extrapolates that adding 37k drivers to SF is a huge number. As if all those drivers appeared in SF ex nihilo.
Even if the cars are privately owned and people just have their car come in to drop them off at work before sending them out of the way to park somewhere (since inner city parking is scarce, so there's no space to park every car at the worker's building), that doubles the number of trips -- once to drop them off, and once to go somewhere and park.
And the remote parking may double the number of trips, but the parking is not going to be far away and will take much less space than a normal parking lot.
They may bring some efficiency to traffic patterns leading to better road utilization, but that only goes so far to make up for the much higher level of traffic that will result when it's much cheaper and easier to call a car when you want one.
San Francisco is already feeling the "Uber effect" of greater traffic from car-share services:
http://www.sfgate.com/bayarea/article/San-Francisco-traffic-...