I'm in a town in a ~2,000 square mile county, where the county population is less than 40,000 and the town is a bit more than 10,000. There is a county transit authority (which is subsidized) that runs a couple of buses and a few taxis (I don't think they are subsidized, but labor here is cheap and it isn't a hard place to find your way around).

There's lots of places with lower population density than that, but the viability of something that costs 60 or 70 percent of what a taxi costs kicks in at some pretty low point.