The reason people commute is generally because living in a city is very expensive, and living outside the city means a nice house and maybe even a back yard.
There has been a big push to see housing affordability as intertwined with transportation costs. If you have to drive a car to do everything in your life, that costs a lot more than someone who can walk to the grocery store, movie theater, bar, etc.
I think this would be subject to a kind of selection bias, for example punishing rural areas for employing laborers and so on. In fact, one of the reasons manufacturers set up shop in rural areas is because the areas have lower real costs of living.
If laborers move to Manhattan, they won't suddenly start making a banker's salary. So the fact that housing is affordable somewhere relative to average incomes there, in many cases, is wholly irrelevant.
Similarly, if an urban high-earner could take his earnings into a low-cost suburban area, his purchasing power would be huge.
Transportation costs are a real issue that should be weighed, but that index completely distorts it by tying cost of living to regional incomes.
If I'm wrong, then all the retirees in Florida must be misguided, and really they should move to NYC, where things are "cheaper".