The physical TSP components:
- Vehicle detection system
- Traffic signal control system
- Communications system
Transit Signal Priority (TSP) refers to the use of sensors and/or traffic signal timing to detect approaching transit vehicles and grant them priority passage at an intersection. TSP is a tool that can be used to help make transit service more reliable, increase ridership, reduce travel time, and cut transit agencies’ operating costs. The most common uses of TSP are for fixed-route buses and light-rail systems. Allowing transit vehicles unhindered passage through an intersection improves on-time performance and reliability.
TSP can be managed at the intersection level or at the system level. At the intersection level, transit vehicles autonomously request priority at a specific intersection’s signal equipment. Priority is granted to the transit vehicle according to rules determined by a transit agency in cooperation with a traffic signal owner (typically a state DOT, city or town). Transit vehicles can be granted unlimited priority during all times, during specific times of day, or when a transit vehicle is delayed. On the system level, a more centralized approach is used by timing signals to allow for the smooth flow of transit vehicles according to their route schedules.
TSP is used to:
- Grant transit vehicle priority at intersections
- Grant emergency vehicle priority at intersections
- Improve on-time performance and schedule reliability
- Decrease bus or light rail vehicle headways
- Decrease travel time on a route and provide travel time savings to transit users
A TSP system is composed of three major components: the vehicle detection system that detects transit vehicles and generates priority requests; the traffic signal control system that receives and processes the request for priority at the intersections; and the communications system that links the vehicle detection system with the traffic signal control system.
There are many different TSP strategies that can be implemented to achieve improved transit performance, including:
- Extending the green light phase to allow transit vehicles to travel freely through intersections
- Providing an early green light phase to allow transit vehicles to spend less time idling at an intersection
- Dedicating bypass lanes, also known as high occupancy vehicle (HOV) lanes, or queue jump lanes for buses
Logical structure of a TSP System (NTCIP 1211)
The use of TSP with an automatic vehicle location (AVL) system is the most common combination. Agencies that have AVL technology on board buses with TSP are better able to quantify the benefits of TSP by monitoring the performance of buses. Also, AVL-based TSP systems improve the efficiency of decisions to grant priority by supplying constant information about delays before a bus even reaches an intersection. TSP is also often a critical component of bus rapid transit (BRT) systems. A BRT bus is given priority at intersections before and after it enters a grade-separated lane or is given priority along at-grade lanes.
TSP systems most commonly use inductive loops, sound-based detection or GPS-based technology. The detection and radio transponders extend the green phase or shorten the red phase of traffic signals, therefore reducing bus delays at intersections. TSP systems can also provide real-time arrival times to passengers at stations or through smartphone applications.
Systems Planning and Fleet Management
The deployment of a TSP system is dependent on the existing signal controller hardware and software systems. The availability of centrally controlled traffic signal systems allows for more TSP possibilities. It is cost effective for transit agencies with a large number of signalized intersections to use centralized traffic signal control software at their transit management center.
As traffic controllers and software acquire higher functionality, and communications systems expand, transportation systems can track and provide priority to public transit vehicles, and therefore move more passengers through the transportation system.
Factors to Consider
A TSP project should be consistent with the regional goals of the implementing transit agency and the signal owners.
It is recommended that TSP projects follow the ITS traffic standards that are part of the National Transportation Communications for ITS Protocol (NTCIP). As part of the planning process, the NTCIP 1211: Object Definitions for Signal Control and Prioritization (SCP)standard can be used by transportation agencies, including transit and traffic engineers involved with the design, specification, selection, procurement and installation, operation, and maintenance of TSP systems. ITS product hardware and software designers, as well as computer program and smartphone application developers, should find this standard relevant to their efforts.
Transit signal priority using optical detection
It is important to use a systematic approach to the planning and implementation process.
Following the system engineering process, the proposed steps are:
- Operations and maintenance
- Evaluation, verification, validation, and
- Building on TSP
It may be necessary to analyze the integration of TSP with emergency management system (EMS) pre-emption. Signal priority is not preemption, which is reserved for emergency vehicles. Signal priority modifies the normal signal operation process to better accommodate transit vehicles, while preemption interrupts the normal process for special events (e.g., an emergency vehicle responding to an emergency call). When a traffic signal is preempted there is no consideration given to maintaining the existing signal timing plan.
After installation of the TSP software, it is recommended that the traffic control system be optimized and new signal-timing plans be implemented. This results in smoother traffic flow and a further reduction in delays on the overall traffic system. TSP provides priority service within the coordinated operation of the traffic signal. A TSP system facilitates the provision of enhanced rider information by enabling real-time detection information to be used for other purposes.
Benefits and Costs
Expected benefits typically include: improved transit schedule reliability, reduced transit travel times, less stops (which lead to reduced wear and tear on equipment), increased rider comfort, and decreased emissions. Ultimately, transit becomes an attractive alternative to the single-occupancy automobile. Experience from prior deployments indicates that bus travel time savings on the order of 15 percent have been attained with very minor impacts on overall intersection operations.
Costs can vary substantially based on the desired functionality of the system. Costs are dependent on the configuration of the system, with somewhat higher costs associated with signal upgrades, equipment/software for the intersection, vehicles, or the central transit management center system. However, TSP systems can be implemented without costly upgrades.
Dedicated queue jump lane
Factors that can affect costs include:
- Design and desired functionality of the TSP system
- Type of roadside and onboard equipment
- Developing new equipment vs. use of off-the-shelf equipment
- Upgrading signal controller firmware to provide TSP
- Operations and maintenance of equipment
- Training personnel in how to program/use TSP equipment
- Trenching required to access power and to place in-road detection equipment (a wireless solution may provide cost savings)
- Ease of installing onboard equipment
- Pilot studies and before/after studies
- Time needed to establish interagency relationships and form agreements
The need to upgrade or replace traffic software and controllers is a critical issue to consider, since it represents the most significant cost items in a TSP project. If existing software and controller equipment can be used, costs can be under $10,000 per intersection, but rise to $25,000 to $35,000 per intersection if traffic control equipment and/or systems need to be replaced.
Equipment and Implementation
All equipment should be pre-tested and proven before installation. A TSP will require installation of equipment in the buses and in the roadside traffic signal equipment. Tests on the communications system are also needed.
Field equipment installation may involve any or all of the following tasks:
- Installation of detector/priority request generator (PRG) units on arm masts or utility poles
- Connection to communications system
- Installation of priority request server (PRS) unit within the controller cabinet
- Connection of PRS to power and communications lines and controller or wireless router
- Cutting of new embedded loops in pavement, unless wireless communications detection technology is chosen
- Replacement/relocation of controller cabinet and possible construction of new concrete pad and connections
- Remote or on-site downloading of software and/or new signal settings to controller
With any installation, documentation of the equipment installed should include:
- Wiring diagrams
- Installation drawings
- Detailed description of timing plan adjustments made by priority type
- Documentation for any special equipment, cards, and/or relays
- Catalog cuts and shop drawings of equipment stating specifications for each piece of equipment
- Manuals for maintenance and operations
- Equipment warranty information
Standardized equipment can save money and time. Installation, operation, and maintenance are all easier on standardized equipment. Agreements between transit agency and signal owners’ efforts to standardize TSP controller equipment are valuable. Stakeholders should discuss standardizing controller and TSP equipment.
Operations and Maintenance (O&M)
Ongoing performance monitoring and management requires data collection such as travel time and signal timing. For the purpose of monitoring, managing, and analyzing the overall system, it is beneficial to collect data.
It is further recommended that procedures be developed to track system operations. These procedures should include collecting information to confirm:
- TSP system is operating properly;
- System is generating priority requests;
- System is processing the requests and communicating them to the controller; and
- Controller is granting the appropriate priority action.
Routine maintenance should be performed on the system according to the equipment manufacturer’s recommendations. Maintenance tasks should be incorporated into standard maintenance activities and operational procedures.
It is recommended that a transit agency require training as part of a TSP installation. The contract documents should require the vendor to:
- Develop training documents for operations and maintenance personnel, including field technicians, bus mechanics, and bus operators who will deal with
any and all TSP equipment
- Perform training for operations staff and bus operators
- Perform training of transit maintenance personnel, including mechanics
- Perform training of field technicians who will respond to any TSP equipment failures in the field and/or connections to controllers