A quote from Ed Glaeser:

"The internal combustion engine was more than just another, faster commuting technology. The car is a point-to-point transit method while all of the older technologies were essentially hub-and-spoke, where people walked to a stop and then took the bus or train. The car made it possible to live at lower densities since it was unnecessary to walk to any bus stop or to walk to get groceries or perform any other function. The second impact of the internal combustion engine was to break down the traditional monocentric city. These cities were generally built around a port or a rail yard but that transportation infrastructure became increasingly irrelevant over the twentieth century. Trucks don't require central stops and as a result, factories could be decentralized. The decentralization of employment has led to much flatter cities that look and feel completely different from the monocentric cities of the past." (Glaeser, `Cities, Agglomeration & Spatial Equilibrium' 2008 p.46)