Designing urban transit systems : an approach to the route-technology selection problem

Abstract

The service specification model is a tool for generating and screening public transportation systems during the initial planning stages. It is based on the concept of a service specification or supply function that integrates hardware system attributes and operating policy. A service specification is an integrated set of statements that defines which hardware-headway combination is to be used for any level of flow across a link. Walk mode may be included in the specification. The model defines a transit system within a network which includes all potential and existing transit links. The current model assumes that transit demand is known. The mechanism of the model is an iterative assignment procedure that is similar to the capacity restraint model. The template network is started at the best hardware-headway service level. Link service levels are iteratively adjusted to correspond to link flow level as specified by the service specification. The iterative process ends when no further changes in link service level are required. Empirical tests show that the model is sensitive to the policy decisions and hardware mix incorporated in the service specification and to the size and orientation of the transit demand. The attainment of an equilibrium flow distribution appears to be influenced by the form of the service specification, the percentage of nonplanar links, and the presence of fixed transit-time links in the template network. The model appears to be a useful tool for generating alternative transport system configurations based on different technology mixes and operating policies in any transportation context for which a service specification can be formulated.

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