  We present a flexible public transit network design model which optimizes a
social access objective while guaranteeing that the system's costs and transit
times remain within a preset margin of their current levels. The purpose of the
model is to find a set of minor, immediate modifications to an existing bus
network that can give more communities access to the chosen services while
having a minimal impact on the current network's operator costs and user costs.
Design decisions consist of reallocation of existing resources in order to
adjust line frequencies and capacities. We present a hybrid tabu
search/simulated annealing algorithm for the solution of this
optimization-based model. As a case study we apply the model to the problem of
improving equity of access to primary health care facilities in the Chicago
metropolitan area. The results of the model suggest that it is possible to
achieve better primary care access equity through reassignment of existing
buses and implementation of express runs, while leaving overall service levels
relatively unaffected.
