SEVERAL authors-among them Fisher and Viton [8], Keeler et al. [i2], Pozdena [I 7], and Small [22, 23] have pointed out the apparent superiority of bus transit over other forms of mass transit. The conclusions of these analyses rest on the following framework: in a given transportation market structure, consider the full costs of providing a trip of certain characteristics. 'Full costs' are defined to include out-of-pocket costs of the providers of the service, as well as the time costs incurred by the users of the transit system and the external costs that the users impose on each other and the rest of the economy. That is, 'full cost' is the full resource cost of providing trips. If we compare, for different traffic volumes, the minimum full cost of providing trips on different modes, then bus transit is less costly than modern rapid-rail transit; and where travel volumes exceed about I OOO persons per hour, it is less costly than auto travel as well.