Transit signal priority (TSP)

A bus signal in Orlando, FL in 2007. Photo by Flickr user joeventures.

Introduction

Transit Signal Priority (TSP) is a general term for a set of operational improvements that use technology to reduce dwell time at traffic signals for transit vehicles by holding green lights longer or shortening red lights. TSP may be implemented at individual intersections or across corridors or entire street systems. As the Federal Transit Administration’s TSP Planning and Implementation Handbook points out, the distinction between Transit Signal Priority and signal pre-emption is an important one because: “signal priority modifies the normal signal operation process to better accommodate transit vehicles, while pre-emption interrupts the normal process for special events such as an approaching train or responding fire engine...”

TSP systems require four components: a detection system aboard transit vehicles; a priority request generator which can be aboard the vehicle or at a centralized management location; a strategy for prioritizing requests; and an overall TSP management system. There are a variety of software and hardware systems that may be used for TSP management. Both the California Department of Transportation (Caltrans) and the City of Los Angeles have developed software to suit their specific needs.

TSP System Architecture

Centralized TSP

In this type of TSP system, a central system organizes and manages requests for priority from many vehicles. This type of system is best for jurisdictions that have control of the traffic signal system and the transit system, and the two systems can communicate with one another. The ‘priority request system’ may be located on the transit vehicle if that vehicle is equipped with a Global Positioning System (GPS) and can communicate directly with the traffic management center. The priority request system may also be located at the transit management center, where the decision to request priority can be determined based on the requests coming from all the vehicles in the field. A third scenario is possible in this type of system wherein the priority request system is based at the traffic management center and happens in real-time as vehicles approach intersections. In this type of system, the amount of communication between vehicles, signals, and the two types of management centers can become somewhat burdensome, especially if a pre-determined ranking of priority is not established.

Distributed TSP

The key difference between the two system types is that in the Distributed TSP system, all priority decisions are made at the intersection level, rather than at a central location. Distributed systems take less management, but the decision to grant priority can be less nuanced than if a centralized location were managing it.

In this type of system, the transit vehicle itself delivers a request for priority each time it approaches an intersection. This method requires less communication between traffic and transit management centers than Centralized TSP, but one problem that arises in this system is the possibility of granting priority to vehicles that are on-time or ahead of schedule.

Benefits

According to the federal Department of Transportation, the benefits from using TSP improvements include “reduced transit travel times, improved schedule adherence, improved transit efficiency, and increased road network efficiency as measured by person mobility.” The Federal Transit Administration also considers TSP to be important to the successful implementation of Bus rapid transit systems.

In a demonstration project on busy Los Angeles County Metro bus lines, the Transit Signal Priority improved travel time savings by 25 percent and, on one line, increased overall travel speeds by 29 percent.

Costs

TSP system maintenance is a relatively insignificant cost that can be incorporated into regular maintenance of transit vehicles and systems. Additionally, costs to other users of roadways tend to be minimal, or even imperceptibly small because the adjustments to traffic signals are made in terms of seconds at a time and signal timing readjusts quickly. The costs associated with implementing TSP will also include the costs of any studies required to analyze needs for the system. The cost of implementing a TSP demonstration project in 2000 in Los Angeles averaged $13,500 per intersection, $75 per transponder per bus, and $10 million overall.

Evaluation Approaches

Because there are so many types of TSP technologies, it is important for transit operators to be able to compare them fairly. The report, “Transit Signal Priority Research Tools,” offers a set of Methods of Evaluation (or MoEs) for doing this. The three key components to measure are whether the technology is functioning properly, whether transit system performance is improving in terms of time savings and reliability, and whether the TSP technologies have affecting the other users of the roadway.

Additional Reading

United States Department of Transportation. “Transit Signal Priority: A Planning and Implementation Handbook.” 2005.

This handbook offers technical information about implementing TSP and includes several case studies of transit systems that gained major cost savings after implementing a TSP system. This handbook thoroughly addresses the state of the practice as it stood in 2005.

California Department of Transportation and University of California PATH Program. “Transit Signal Priority Research Tools.” 2008.

Sponsored by the United States Department of Transportation, this report is more technical than the handbook listed above and offers a detailed examination of the different types of TSP technologies.

Fehr & Peers. "University of California, Davis Transit Signal Prioritization Implementation Study." 2011.

This study offers an account of how the University of California, Davis and the City of Davis may choose to prioritize certain intersections for improvements that would improve the reliability and overall service of the university's transit system, Unitrans. These improvements include transportation signal coordination and Transit Signal Prioritization. This study is a strong example of the type of analysis that could be used to determine where and how to implement TSP.

US Department of Transportation. Research and Innovative Technology Administration, Intelligent Transportation Systems Professional Capacity Building Program. “Lessons Learned: Improving Reliability with Transit Signal Priority Systems - King County Metro Transit and Los Angeles County MTA.” 2008.

This webinar features a federal perspective and lessons learned from King County in Washington, as well as a presentation from the demonstration projects and expansion efforts from Los Angeles County. Individual powerpoint files are available along with the transcript from the Question & Answer session following the webinar.