For example, drivers may be offered the option of slower, free lanes or faster lanes with tolls. Each pricing system has evolved out of a different uiban context and philosophy. The spatial structure of European cities tends to be monocentric and sometimes radial, with a “natural” boundaiy, and are more likely to introduce a cordon, or corridor around the urban area, while typical US cities are polycentric and are based on a grid structure. Another reason for the difference in pricing systems lies in different perspectives on congestion fees.