Active Traffic Management: Signal Priority & Preemption

WHAT IS SIGNAL PRIORITY AND PREEMPTION?

Active traffic management (ATM) is the ability to dynamically manage recurrent and non-recurrent congestion based on current and predicted traffic conditions [1]. Traffic signal priority, one of the key components of ATM, alters a traffic signal’s normal timing to reduce delay for special types of vehicles, such as buses. Another approach is signal preemption, which enables vehicles, often emergency vehicles, to send messages to a traffic signal to grant immediate right-of-way [2].

ITS FOR DISASTER RESPONSE: EXAMPLE USE CASES AND BENEFITS

Click on each use case below from recent ITS for ATM: Signal Priority and Preemption deployments, based on ITS project evaluations contained in the ITS Deployment Evaluation Databases

Use Case		Example Benefit
	Emergency Vehicle Preemption	In southwestern Pennsylvania, 37 sound-activated emergency vehicle preemption (EVP) systems installed as part of a regional traffic signal program helped reduce emergency response times by 14% to 23%.
	Connected School Buses	Two school buses in Alpharetta, Georgia were equipped with a connected vehicle (CV) traffic signal priority system that reduced the number of stops at red signals by 40.4% and travel time by 13.3%.
	Transit Signal Priority	Transit signal priority (TSP) deployed at three Boston, Massachusetts intersections reduced bus waiting time at red light signals by 21%. TSP resulted in 8% faster travel times.
	TSP Data for Signal Retiming	Data collected from a CV TSP application on an 11-mile corridor in Salt Lake County, Utah was used to retime traffic signals, helping to improve bus run-time reliability by up to 2.7%.
	V2X Emergency Vehicle Preemption	Vehicle-to-Everything (V2X) EVP deployed in Seminole County, Florida led to average response time reductions of 90 seconds. and annual maintenance costs are expected to decrease by $200,000.

Emergency Vehicle Preemption

PROBLEM

To reach the scene of an incident, first responders may need to drive through congested intersections and proceed through red traffic signals. They rely on sirens and flashing lights as alerts, which can confuse nearby drivers and result in response delays or even collisions.

TECHNOLOGY

When emergency vehicle preemption systems detect light, audio, or radio signals from emergency vehicles, they override normal traffic signal operations. This gives approaching emergency vehicles a priority green signal, allowing them to more quickly respond to incidents.

BENEFITS

Across southwestern Pennsylvania, 37 emergency preemption systems triggered by sound were installed, which helps reduce emergency response time by 14 to 23% (2023-B01807).

Connected Vehicle School Buses

PROBLEM

School buses often travel long and complex routes with numerous stops to transport students to and from school. Stops, whether for loading and unloading students or at traffic signals, impact mobility, safety, and fuel consumption.

TECHNOLOGY

The Fulton County School System (FCSS) in Alpharetta, Georgia deployed traffic signal priority at 62 intersections. When a CV-equipped bus approaches a CV-equipped intersection, the signal phase shifts to green, allowing the school bus to pass through the intersection without stopping.

BENEFITS

The combined results for the two FCSS school buses showed a 13.3% decrease in travel time, an 18% increase in speed, and a 40.4% decrease in the number of stops (2023-B01804).

Transit Signal Priority

PROBLEM

Up to 50% of transit delay consists of waiting at red lights in signalized
intersections [4]. These signal-related delays reduce bus reliability and
make travel times longer and less predictable for riders.

TECHNOLOGY

The Massachusetts Bay Transportation Authority (MBTA) and City of
Boston worked with a traffic signal vendor that uses machine-learning
TSP technology to track and predict transit bus locations, enabling the
system to prioritize green signals for the bus as it approaches a TSP
equipped intersection.

BENEFITS

After a year of deployment across the three intersections, buses spent an average of 21% less time waiting at red lights, indicating 8% faster travel time for buses (2025-B01960).

TSP Data for Signal Retiming

PROBLEM

As growing travel demand outpaces available road space, increasing congestion is creating significant hurdles for public transit. Buses are frequently delayed at traffic signals and struggle to stay on schedule, which ultimately makes the service less reliable for commuters.

TECHNOLOGY

A project in Salt Lake City developed a TSP algorithm based on CV technology that enables real-time communication between traffic signals and buses, providing access to real-time signal status and bus travel information. This meant signals could be retimed to better align green signals with bus movements.

BENEFITS

After signal retiming, bus travel times had a 11.7% reduction in northbound (NB) travel times and a 2.3% reduction in southbound (SB) travel times. Bus time reliability also improved after signal retiming by 2.7% (NB) and 1.2% (SB) (2023-B01721).

V2X Emergency Vehicle Preemption

PROBLEM

Emergency vehicles may face delays at congested intersections because line-of-sight or audible emergency vehicle preemption systems prevent vehicles from requesting signal priority until they are very close to the intersection. Ultimately this causes delays and slower response times.

TECHNOLOGY

Florida DOT (FDOT) deployed a dedicated short-range communication (DSRC)-based V2X solution to enhance the effective radius from which an emergency vehicle could place a request for preemption.

BENEFITS

The FDOT pilot in Seminole County found that by implementing this V2X system, average response times improved by approximately 90 seconds, and annual maintenance costs are expected to decrease by $200,000 (2025-B01994).

SAMPLE COSTS: TRAFFIC SIGNAL PRIORITY AND PREEMPTION

LESSONS LEARNED: TRAFFIC SIGNAL PRIORITY AND PREEMPTION

	In Tennessee, 1,655 traffic signals were ranked based on the results of a performance evaluation. Machine learning can fine-tune the weighting of different ranked choice factors in the calculation of signal priority and refine the algorithm (2023-L01170).
	The North Carolina Department of Transportation (NCDOT) and North Carolina Emergency Management partnered to develop the real-time flood warning system FIMAN-T (Flood Inundation Mapping and Alert Network for Transportation), which leverages real time 3D inundation mapping with Light Detection and Ranging (LiDAR)-derived roadway elevation layers to compute flooding depths over roadways. Key lessons learned from using the tool include (2023-L01195): Consider developing an interactive dashboard allowing users to navigate between current conditions, modeled scenarios, and forecasted conditions. Leverage existing asset databases, such as the NCDOT’s bridge database, to provide real time bridge hydraulic performance dashboards during flooding events.
	The Weather-Responsive Management Strategies (WRMS) initiative of FHWA’s Every Day Counts program promotes use of road weather data from mobile and connected vehicle technologies to support traffic and maintenance management strategies during inclement weather. WRMS can mitigate impacts of flooding events through preparation and prediction efforts that build and leverage data, tools, and relationships. Key lessons learned from flood events include (2021-L01024): Data and models can be useful to predict and prepare for flood events. Useful data and tools include: river gauge data, hydrologic modeling, LiDAR mapping, data repositories, and sandbag and barrier systems. Establishing collaborative intra- and inter-agency partnerships is important during a major flood event.

 REFERENCES

1. Federal Highway Administration. “Active Traffic Management: Approaches: Active Transportation and Demand Management - FHWA Operations” Aug. 2023. [Online]. Available: ops.fhwa.dot.gov/atdm/approaches/atm.htm .

2. Federal Highway Administration. “Traffic Signal Timing Manual: Chapter 9 — Advanced Signal Timing Topics.” Apr. 2021 [Online].
   Available: https://ops.fhwa.dot.gov/publications/fhwahop08024/chapter9.htm

3. Applied Information & Kimley Horn. “School Bus Priority: Connected Vehicle Student Safety Pilot Program.” Oct. 2022. [Online].
   Available: https://appinfoinc.com/wp-content/uploads/2022/10/school-bus-priority-pilot-overview.pdf.

4. Massachusetts Bay Transportation Authority. “MBTA and City of Boston Announce Partnership to Improve Bus Reliability by Expanding Transit Signal Priority City-wide After Successful Test Along Brighton Avenue.” Jan. 2025. [Online]. Available:
   https://www.mbta.com/news/2025-01-27/mbta-and-city-boston-announce-partnership-improve-bus-reliability-expanding-transit.

5. Wang, Q., Yang, X. T., and Yuan, Y. “Transit Signal Progression Algorithm for Supporting Redwood Road Transit Signal Priority (TSP).” Oct. 2020. Report No. UT-20.16. [Online]. Available: https://drive.google.com/file/d/10-AlujqlbspZZObah_mfdbPacZUzDlO/view.

6. National Operations Center of Excellence (NOCOE). “FDOT NTCIP Based Emergency Vehicle Preemption.” Aug. 2025. [Online]. Available: https://www.transportationops.org/case-studies/fdot-ntcip-based emergency-vehicle-preemption.