Comparative Analysis of Unconditional and Conditional Priority for Use at Isolated Signalized Intersections

Conditional priority is recognized as having the potential to improve transit service reliability at small cost to general traffic. The popular preference for conditional priority over unconditional priority encounters the challenges of various and challenging test scenarios. This paper makes a comparative analysis of unconditional and conditional priority in terms of intersection performance, based on the logic rule-based bus rapid transit signal priority (BRTSP), an active transit signal priority (TSP) for use at isolated signalized intersections with median bus-only lanes. Compared to previous study, the design of phase scheme, including two BRT phases, eight vehicle phases and four pedestrian phases, and the technical framework of BRTSP, centered on four categories of logic rules, have been improved to incorporate a mechanism for providing phase insertion, a special signal priority treatment in addition to green extension and early green. Phase insertion detector, if phase insertion is desired, is installed on the bus-only lane between check-in and check-out detector to trigger phase insertion request. A new category of logic rules, i.e., rules for early green response, is introduced into as a component of BRTSP to dynamically alter the response of the vehicle phases conflicting with the BRT phases to early green request. Inductive loop detector placed on each lane of the vehicle phases is used to monitor time headway and occupancy time on a lane by lane basis. The simulation experimental results indicated that: (1) phase insertion contributed substantially to late schedule deviations correction, but was recommended to be embedded in conditional priority rather than unconditional priority; (2) there was lack of convincing evidence that conditional priority had an inherent effectiveness of managing the intersection performance by varying the lateness criterion; (3) at TSP-enabled isolated intersections with green extension and early green, compared to having conditional priority, having an integration of unconditional priority and holding early BRT vehicles at control points may be a cost-effective solution not only to improve schedule adherence but also to accommodate the benefits of general traffic; and (4) at TSP-enabled isolated intersections with green extension, early green and phase insertion, there seemed to be a pressing need to move from unconditional priority to conditional priority.