Urban Boundaries

Using cities for urban analysis is a joke. Travel behavior (commuting, shopping, traffic) doesn't stop at a city boundary. Cities are surrounded by suburbs, and suburbs are tied together by commuting flows. That's been recognized since the '70s, with the advent of Metropolitan Planning Organizations. Not to say that MPO boundaries are perfect.  Metropolitan Areas are made up of county boundaries, and inclusion within an MPO is based on the urban commuter flows.  Counties are large, and contain rural and urban parts--many analyses (notably of density) naively use county area totals. The best unit of analysis I know is the Federal Highway Urbanized Area boundary files. Sadly, each state prepares its own, and there is no central repository.