As cities grow, what happens to urban form and how does that change traffic conditions? How does growing traffic affect urban structure? These questions have received considerable theoretical and empirical attention over the last 25 years. They relate to the NIMBY debate, which associates most new development with traffic problems. Yet, until recently, substantial evidence tended to show that urban growth did not lead to "traffic doomsday". These findings contradicted the standard urban model and were surprising because roads are mainly unpriced and perceived as a significant market failure. Many researchers explained the rise of suburb-to-suburb commuting (and the dispersion of employment) as a traffic "safety valve". In that case, suburbanization was more a solution than a problem. On the other hand,, recently released findings from the 2000 Census show an increase in average commuting times that is difficult to reconcile with the earlier findings. What had changed in the 1990s? This research attempts a preliminary answer to this question. The key explanation may be income growth, especially in the late 1990s.