Long-term data from cities around the world appear to show that there is a fundamental threshold of urban intensity (residents and jobs) of around 35 per hectare where automobile dependence is significantly reduced. This article seeks to determine a theoretical base for what the data show. It suggests that below the threshold intensity of urban activity, the physical constraints of distance and time enforce car use as the norm. The basis of these physical constraints is outlined and the link between density and access to services that provide amenity is established, including the service levels of public transport. A design technique for viability of centers is suggested as well as how a city can restructure itself to overcome automobile dependence.