If you are going from one non hub city to another non hub city, the overall air travel time goes up substantially (as well as the cost). It will still be a good deal faster than driving cross country, but if driving is relatively stress free and I can do it through the night, I'm going to be far more likely to skip the stressful lines, running between gates, delays and high prices of air travel. I don't think I'll be alone.
In areas of the US with high population density, bus service is pretty okay (more or less the Northeast Corridor).
The bus service from Houston to Miami is a little less okay, but it's because it's 1900 kM and won't have a lot of users (Google suggests public transport would take 34 hours, a train and a couple of buses).
