Transit Signal Priority

Transit Signal Priority (TSP) is considered to be one of the popular transit management strategies that can be used to help transit service become more reliable, faster, and more cost-effective. TSP has little impact on general traffic and is a relatively inexpensive way to make transit more competitive with the automobile. Transit signal priority modifies the normal signal operation process to accommodate transit vehicles better. RTD acknowledges TSP as one of the advanced tools to improve bus speed and reliability. Consequently, RTD developed a new TSP operating concept, and the designed TSP concept has been deployed at 18 intersections in the district to improve transit service and performance.

Typically, TSP operation allows traffic signal controllers to skip conflicting signal phases, to shorten conflicting phases, to lengthen compatible phases, or to modify phase sequence to serve the transit vehicle. Two primary TSP strategies are 1) Green Extension and 2) Early Green (Red Truncation). More specifically:

• A green extension strategy extends the green time for the TSP movement when a TSP-equipped transit vehicle is approaching. The green extension is one of the most effective forms of TSP since a green extension does not require additional clearance intervals.
• An early green strategy shortens the green time of the preceding phases to expedite the return to green (i.e., red truncation) for the movement where a TSP-equipped vehicle has been detected. This strategy only applies when the signal is red, and a TSP-equipped vehicle is approaching. Both green extension and early green strategies are available together within TSP enhanced control environment but are not applied to the same signal cycle.  

After assessing the state of the practice, recent experience and modern communications technology available for the bus TSP design, RTD developed a unique TSP approach by utilizing RTD’s cellular communications technology to implement TSP operation. The most significant advantage over other methods studied is that this approach fully utilizes the bus on-board equipment and system software that RTD has been applying to CAD/AVL and Automatic Passenger Counting (APC) systems. All that was required was some modification of the on-board programming module to enable TSP triggering and purchase of the relay device to bridge TSP communications between bus and traffic signal controller. This additional equipment is inexpensive and allows for simple maintenance of the system. Figure 1 shows the eight (8) components and steps to generate and process a TSP request. 

The Computer-Aided Dispatch/Automatic Vehicle Location (CAD/AVL) system (1) that is currently equipped on each bus will be utilized to trigger TSP-related functions. In addition to the route, schedule, and stop information that the system stores, the system will also maintain the geographic locations to trigger and cancel TSP requests. The TSP check-in requests generated by the on-board equipment will be forwarded to the existing wireless gateway (2) and the cellular modem (3) on the bus. The cellular modem will use the cellular network (4) to send the TSP requests to a cellular modem located in the traffic signal control cabinet (5). Once received, the modem will forward the TSP requests to the relay (6). Then, the TSP requests will be forwarded to the traffic signal controller (7), via a relay circuit, for TSP executions. Similarly, the check-out requests can be used to cancel TSP at the traffic signal controller and can be configured for a different port in the contact closure.   The traffic signal controller contains direct inputs to activate and deactivate TSP requests. When the relay forwards the requests to a traffic signal controller, the controller will respond with TSP specific actions to either extend the TSP phase or terminate non-TSP phases early. The TSP changes will remain in effect until the predefined maximum green extension/early-green times are met, the TSP request is canceled by a bus traveling through the check-out zone, or the TSP request times out based on parameters set with the agency. 

As of Summer 2020, the TSP system has been installed at 14 intersections along E. Colfax Ave. (see East Colfax Improvement Project) and four (4) intersections along the US-36 corridor in Westminster. Review a summary of where and when TSP has been activated below. 

• The TSP system has been successfully promoting transit services by improving transit reliability and speeds. In addition to the current TSP locations, RTD collaborates with local agencies to explore new TSP locations along Havana St (Aurora section). and Federal Blvd. (Denver section) to advance regional mobility and connectivity.
• A Before-After TSP study covering the measures of bus dwell time between and at stops, speed between stops, running time between stops and punctuality at stops will be published in early 2021. Based on the TSP performance, RTD and the participating stakeholders will make modifications to their operation and control systems. For example, RTD can adjust the transit schedules to optimize transit operation. As for the local stakeholders, they may adjust signal timings and splits to not only prioritize transit operation but also optimize the vehicular operation.
• RTD was recently awarded two DRCOG Technology set-aside funding totally around 1 million dollars to enhance RTD's integrated TSP/Transit real-time data sharing system focusing on GTFS-RT feed and real-time bus occupancy notification system. Meanwhile, the funding will be using on migrating the existing TSP system to the Conditional TSP system, which is inherently more robust in utilizing TSP operations and eliminating the impact of TSP on traffic signal systems.
• Collaborate with Denver to launch a cloud-based Enterprise Data Management (EDM) system to store our transit TSP data. After the implementation of the EDM system, retrieving and analyzing TSP data will be earlier for both agencies. The EDM system is expected to be completed in early 2022.

An "RTD Transit Signal Priority (TSP) Application Guidelines" has been prepared and published. The application guidelines explain what TSP is, why it is important, what the benefits are, and the important issues surrounding the topic. This report contains the steps one should follow to implement a TSP project in the district successfully. To request a copy of the report or have any other Transit Priority questions (e.g., TSP, Queue Jump, Bus Bypass ), please email [email protected].