Active Signal Priority for Light Rail Transit at Grade Crossings

Document Type

Journal Article

Publication Date


Subject Area

operations - traffic, infrastructure - bus/tram priority, infrastructure - traffic signals, mode - rail, mode - tram/light rail


Traffic signal timing, Traffic signal settings, Traffic signal priority systems, Traffic signal preemption, Traffic signal controllers, Traffic delay, Settings (Traffic signals), San Diego Trolley, Preemption (Traffic signals), Optimization, Optimisation, Light rail transit grade crossings, Electronic traffic controls, Controllers (Traffic signal control equipment)


The basic concept and implementation of an active priority system of light rail transit (LRT) for highway–rail grade crossings are presented. The proposed priority system consists of four major components: train detectors, train travel and dwell time predictors, priority request generator (PRG), and traffic signal controllers. As the brain of the system, the PRG adopts a three-scheme conditional priority control strategy. Scheme I, which applies to late LRT trains, features a timing optimization model. The objective of this optimization model is to minimize intersection delays for late trains and meanwhile to minimize impacts on other traffic. Unlike existing signal priority studies, which typically focus on isolated intersections, the proposed model deals with signal timings at multiple intersections and provides an optimized green band for an incoming train. The optimized green band would start at the right time to cover the predicted train arrival time and would be wide enough to accommodate prediction errors. A case study, based on the San Diego Trolley system, in California, demonstrates enormous intersection delay savings of 89.5% and 25.3 s/train for late trains after application of the proposed Scheme I strategy. At the same time, the impact on other traffic in the priority cycle is only 4.1 s/veh. For Schemes II and III when no priority is needed, traffic delay savings are 32.5% and 52.0%, respectively. A simulation study confirms the system benefits and validates the practicality of the active priority system for future field testing.