"There are only so many minutes and miles in a day that people will travel," Schipper explained in an interview on the Stanford campus. Schipper, former director of research for the World Resources Institute's Center for Sustainable Transport (known as EMBARQ), believes higher fuel prices play only a part in what is happening. Time constraints, road congestion, aging populations, and taxes on vehicle ownership all appear to be combining to force a major shift in travel patterns.