On the role of route choice modeling in transit sketchy design

Sketchy design models are used to examine the fundamental tradeoff in transit systems and to guide high-level decisions. Traditionally, passengers' route choice is greatly simplified in such models. This study aims to understand whether this simplification would compromise qualitatively the results expected from the sketchy models. To this end, three transit systems, which all offer competitive alternative routes, are analyzed using the continuous approximation approach. We test what the impact on transit system performance (e.g. optimal designs and system costs) would be if travelers somehow split between these routes, rather than concentrate on the "best" one. A random utility model is employed to enable a probabilistic assignment of passengers to different routes according to the "perceived" utility. Analytical methods are then developed to estimate the aggregate share of each route in each system, based on which the user cost is obtained. Numerical results show that, while stochastic route choice modestly increases the optimal user cost, it has a negligible effect on the agency cost. Furthermore, the actual system design is largely insensitive to route choice modeling. Thus, while the simplest deterministic route choice assumption may not be valid in all systems, transit planners can safely ignore route choice details in most cases, at least for the purpose of strategic planning.