Coordinating Transit Transfers in Real Time

Transfers are a major source of travel time variability for transit passengers. Coordinating transfers between transit routes in real time can reduce passenger waiting times and travel time variability, but these benefits need to be contrasted with the delays to on-board and downstream passengers. This paper develops a dynamic holding strategy for transfer coordination where the decision to hold depends on real-time estimates of bus arrivals and passenger numbers and the uncertainty in these estimates. The strategy is then optimized based on the estimates. It is found that all the uncertainty parameters can be ignored with no loss of accuracy provided the bus arrival estimates are unbiased. The optimal control reduces the delays due to transfers considerably. These time savings increase with the ratio of transferring passengers to through passengers, and with reductions in the uncertainty of bus arrival time estimates. Field observations at a multimodal transfer point in Oakland show that the proposed control strategy could reduce transfer delay by 30-39% inclusive of holding cost in a real-world scenario. The paper concludes with a discussion of expected benefits in different scenarios, and some complementary measures, such as the provision of real-time information at transfer points and conditional signal priority, which could enhance the benefits of coordination.