The affordability index is a different way to look at housing affordability. Since housing has been going down the tubes lately, its not surprising that the crises is hitting the suburbs of major metropolitan areas the hardest. Why? Because they are out in the middle of nowhere and its getting expensive to move around by car alone. Well folks are now starting to measure the housing + transportation costs of families and individuals showing that true affordability isn't a cheap home in the suburbs, but rather the sum of these two costs.
Take a look at the costs of different neighborhoods. In a transit rich neighborhood with all housing being equal, your cost of would be 41%. But if you had to drive everywhere, your costs would be 57%. That's quite a lot of savings by living near transit. See for yourself if you live in a transit rich or auto dependent neighborhood.
H/T Carless in Seattle.