Source: data from R. Tolley and B. Turton (1995) Transport Systems, Policy and Planning, New York: Longman, p. 184.
Urban transportation modes are associated with different performance levels. However, performance is a multidimensional concept as different metrics can be used to assess it. The above figure illustrates two urban transportation performance measures, vehicle speed, and passenger density, that can be combined into a third one; a ratio of speed over space consumption (how much space is required to maintain normal operations involving speed and performance characteristics). Public transit consumes, on average, ten times less space than individual transportation. However, if speed is considered the most important performance criterion, the modal ranking changes in favor of the automobile. Thus, in car-dependent cities, a much larger amount of space must be allocated to transportation than in transit-oriented cities. Lower densities come with the benefit of faster commuting and locational flexibility in terms of residence and workplace. Higher densities are associated with higher performance of public transit systems, but this is usually with lower speeds and locational flexibility.