The spatial importance of each transport mode varies according to a number of
factors, density being the most important. If density is considered as a gradient, rings of
mobility represent variations in the spatial importance of each mode at providing urban
mobility. Further, each transport mode has unique performance and space consumption
characteristics. The most relevant example is the automobile. It requires space to move
around (roads) but it also spends 98 percent of its existence stationary in a parking space.
Consequently, a significant amount of urban space must be allocated to accommodate
the automobile, especially when it does not move and is thus economically and socially
useless. At an aggregate level, measures reveal a significant spatial imprint of road
transportation among developed countries. In the United States , more land is thus used
by the automobile than for housing. In Western Europe, roads account for between 15
and 20 percent of the urban surface while for developing countries, this figure is about
10 percent (6 percent on average for Chinese cities).