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.

这项研究建立了公交通道和城市形态的理论解释和实证检验。它说明了从城市到中央商务区（CBD）的距离，随着工作距离的增加，对工作的中转访问会如何下降，并介绍了城市的时空结构。在美国45个人口最多的城市中，对此进行了经验检验。分析发现，不管城市在哪里，随着人们从市中心迁出，过境通道都会减少，并且在大多数城市中，过境通道会从就业过多到相对稀缺而下降。在这种过渡中，过境网络充当“催化剂”，促使人们获得对《生物多样性公约》就业集中化的机会。分析还宣布，由公交通道定义的城市结构是一个不稳定的概念。这改变了传统的城市结构定义，因为它可以使以CBD为中心的城市分散而矛盾。距中心的网络距离与公交路线之间的反比关系强度是旅行时间阈值的函数，并遵循收益递减的定律。函数的顶点指示绝对最大的CBD中心度。实际上，城市结构已从中央商务区分散到中央商务区，并由于过境旅行时间阈值的增加而开始向中央商务区转移。这是流动性和位置的乘积，并认为交通网络已经通过允许CBD集中的政策而发展，至少在短期内如此。虽然尚不清楚，但长期的公平考虑可能会将这种增长改变为集中的CBD权力下放之一。