If robot taxis happen, they won't be just like regular taxis without a driver. The mix between investment, hourly cost and per mile cost would change a lot, making entirely new business models possible and shifting the optimization peaks of existing ones.
Personally, if I would somehow happen upon a dozen robot taxis to turn into a business, I would select elderly people in a rural area as my target audience. Not your typical early adopter and quite some natural churn (short age window between should not drive anymore but still able to take care of themselves), but you get awesome ad targeting if you put out flyers in the doctor's waiting room. Oh, and you need to hire the one taxi driver who currently serves the doctor's appointment market in that area as your support guy who chats all the nice old ladies through the introductory phase over the car's hands free.
Even if calling a driverless car cost more for remote areas, it would (or at least could) still cost a fraction of the cost of a taxi. $30 instead of $120 for a long distance drive is totally reasonable.
Nor do driverless cars have to make up for "idle time". The only legitimate counter argument as to why a taxi should earn as much as they do is that they sit around not making any money for 10-60 minutes at a time. However, that goes away with driverless cars, and any excuse to charge exorbitant prices the way taxis do completely vanishes.
[0]http://news.walmart.com/news-archive/2016/01/20/more-than-on...
Also I suspect another issue is the cliffs for government benefits. If you work between 45 - 60 hours a week for $12 - $13 an hour like I did a few years ago you aren't exactly well off yet programs like Obamacare will fine you for not buying health insurance that you can't really afford. Similarly free clinics will turn you away and I wasn't able to find a single aid program that you would qualify for except about $20 a month in food stamps. I figured out the costs once and there are a great many situations where a rational actor would simply work less or for less money and do better economically.
People bring up legal recourse for these situations, but realistically, it's very scary when you're in that scenario and have to actually deal with getting underpaid or not getting paid for a while but having your dignity.
About the tipped wages mentioned a couple comments up, from my experience working delivery, that's a great way to get yourself fired. Because I guarantee you all the employees won't go in together on saying it, you're basically telling the management "I don't get tipped as much as everyone else" which is great grounds for poor performance, it's a catch-22.
IMO that's the strongest argument for replacing welfare programs by a negative income tax, or universal basic income.
Population densities in the US suck, and everything is oriented around personal cars. Totally different things.
That's the reason there are no cabs and I don't blame cab drivers. They don't get paid to sit around 40 minutes to find a passenger. I'm surprised that finding an uber in one of those places is not difficult either.
Outside of the US, a lot of the population is very condensed (well the US is to some degree) making cabs and public transportation much more efficient/effective IMO.