Transit access and urban space-time structure of American cities

This study builds theoretical explanations and empirical examinations of transit access and urban form. It explains how transit access to jobs drops with network distance from the Central Business District (CBD), ceteris paribus, and introduces the urban space-time structure. This is empirically examined in the 45 most populated American cities. The analysis finds, regardless of the city, transit access declines as one moves out from the center, and in most cities, transit access decays from a surfeit of employment to relative scarcity. In this transition, the transit network acts as a “catalyst” to induce access to CBD employments centralization. The analysis also declares that urban structure defined by transit access is a fluid concept. This changes the traditional urban structure definition as it can make a CBD centric city dispersed and contrariwise. The strength of inverse relationship between the network distance from the center and transit access is a function of the travel-time threshold and follows the law of diminishing returns. The vertex point of the function indicates the absolute maximum CBD centricity. The urban structure, indeed, shifts from CBD decentralized to CBD centralized and begins shifting to CBD decentralized by an increase in the transit travel-time threshold. This is the product of mobility and place, and argues that the transport network has grown by a policy that permits CBD concentration, at least in the short run. While it is not clear, long-term considerations of equity may modify this growth to one of concentrated CBD decentralization. The concentration is typical of the take-off stage of the transport network, and that equalization takes place as the network matures.