It would be interesting to relate this to how stimulating the environment is. I walk fast in cities because doing so lets my brain "compress" the information from the never ending shop windows, people yelling, people walking, cars honking, people asking for money, ambient music, street names, landmarks, camera flashes etc. I also walk very fast in malls for this reason. (A correlate is --yes-- I actually walk slower at night, because there is more to pay attention to at night.) It's possible that fast walking is an adaptation to decrease the neurological load from the stimuli economic activity generates (or even a consequence of the fact that exciting things prime the brain towards moving and make physical action more desirable), and not a consequence of "how much your time is worth".