1) Bus riders have cars
In SF, many people don't own cars because the city isn't particularly car friendly. If buses get too slow, I can see some riders using alternate transportation, but I doubt very many would choose to use a car (and most probably don't have one to use).
Note: I don't live in SF, but was born, raises and continue to live an hour North. My primary mode of travel there is in a car, and it's not ever pleasant.
2) Bus riders ride buss primarily because it's quicker
What about being cheaper? What about convenience (it's a different type of convenience, but it's liberating to not have to worry your vehicle is safe and whether you've forgotten keys somewhere).
I believe that was the study. I'll have to go look at the source to be sure, though.
Speaking of convenience factor, I can't wait for self-driving cars in this regard. I know it's a topic change, but widespread autonomous transport will clear up much of our traffic problems. Indeed, humans are the weak link when it comes to traffic. Not roads, signs, capacity, mass-transit, etc. but humans. We're just not capable of coordinating our behavior in traffic.
> I believe that was the study. I'll have to go look at the source to be sure, though.
That makes more sense, although I have to imagine some prioritization of public transit would alleviate this somewhat.
> Speaking of convenience factor, I can't wait for self-driving cars in this regard
Imagine self driving mixed with public transportation. Instead of large buses (or in addition to) we could have more, smaller vans, seating 6-10, depending. We could have multiple per prior bus serviced route if replacing a bus, and they could be more accurately dispatched base on load (having a bus service a route when there's few to no riders is a waste). Additionally, these could be used to add less commonly used, but still beneficial routes between farther points.
To really make it next gen, you could allow people to reserve seats online for a small fee, which would give useful information on route usage and upcoming demand, to allow reserve units to be dispatched accordingly.
If the cost of the drivers is removed, and the vehicle cost and repair can be brought down, a lot of really interesting things could be done with public transit.
...congestion pricing can help reverse a long-standing
vicious cycle of traffic, one that removes the incentives
to take public transportation. The more people who choose
to drive to work, the worse the traffic. This raises the
time the buses must spend in traffic, which raises the
cost for bus companies, who raise the fares for bus
commuters -- who are being penalized despite their own
efforts to reduce total traffic. As the bus becomes less
of a good deal, more people defect to cars, making things
worse for the bus riders, who have even less incentive to
ride the bus. (p. 167)
And this brings us full-circle to the argument that those buses should be free for riders. If a city charged cars to drive during congested times and subsidized bus fares with that money, this might be a solution to ease up congestion.