Via Richard Layman, the University of Minnesota has a broadscale study of accessibility in the Twin Cities region. For those not up on the latest transportation lingo, accessibility is contrasted with mobility as a way of judging the efficiency of a transportation system.
Mobility looks at the cost of travel on a per mile basis (that is, the speed of travel, basically, but you can also include fuel and environmental costs). Most traffic engineering today is within a mobility framework--traffic engineers want to see cars moving smoothly along, congestion-free.
Accessibility, on the other hand, looks at the cost of travel on a per-destination basis. That is, what's the total cost of a trip to your shopping center or job.
The difference between the two can be seen in that a pedestrian-friendly neighborhood can have low mobility--you move around slowly--but is highly accessible, because you can get to a lot of places, even at that low rate of speed. Personally, mobility seems to me to be an easier standard to measure against, but one that misses out on how transportation is experienced. Mobility also encourages sprawl, since you're only paying attention to the speed, it's easier for overall trip distances to grow unchecked.
The U. Minnesota group has put out a short report on their study so far, called "Asking the Right Questions" [PDF]. It's a reasonable introduction to the idea of accessibility, if you don't have much time.
ALSO: The ability, or desire, of Americans to respond to changes in the price of gas has lessened in recent years. I'm inclined to say that this is the result of people choosing less and less accessible locations to live in (and, of course, our regulated land-use market providing fewer and fewer accessible residences), but it could be something else going on.