Giving buses the green light

Posted in: Bridges, Construction, Transportation


Bus line transit in Alexandria
credit: Flickr user Beyond DC

Transit Signal Priority (TSP) is a traffic management strategy that uses software and hardware to modify traffic signal operations—often in real-time—to prioritize the movement of a transit bus through an intersection. TSP is typically implemented conditionally. This differentiates it from signal pre-emption, which provides unconditional priority to vehicles such as fire engines. TSP offers a city numerous benefits as described here.

Earlier versions of TSP provided basic algorithms for interrupting normal traffic signal operations to provide extensions of green intervals, early start of green intervals or unique bus-only phases to expedite the passage of bus transit vehicles through an intersection.  These basic algorithms often employ strict conditions about how long and how often the additional signal time or phase could be provided. Other factors considered include how full the bus was, in which direction it was traveling, or how far it was behind schedule. The reasoning behind such rigid TSP protocols was that adjusting carefully balanced signal timing patterns that served all movements and modes could be highly disruptive to traffic along congested arterial streets, ultimately resulting in slower travel speeds for all vehicles.Now, TSP is shifting. Emerging technology and artificial intelligence can combine ‘real-time’ traffic data—like approaching traffic volumes, signal operations, vehicle detection and bus positioning—along a corridor to leverage TSP system capabilities more effectively. This ‘next generation’ TSP aims to increase bus travel time speeds and reliability, and maintain or increase transit mode share. This is accomplished by implanting real-time signal timing optimization of cycle lengths, coordination, and intervals to compensate any borrowed time for the bus priority back to other modes within the intersection and corridor. This allows for the provision of maximum green priority for buses with minimal impact to overall vehicle delay and travel time.

Working closely with software developers, Mead & Hunt used an advanced traffic simulation software program to simulate this enhanced TSP along the US 1 transitway in Alexandria, Virginia, location of the new Amazon Headquarters #2. This corridor is anticipated to run buses on 5-minute or less headways and carry up to 4,000 passengers per day. Watch this short video to see the difference this enhanced TSP makes. The results of simulating this short 4- signal segment of US 1 indicated a 30% reduction in bus travel time.


Paul Silberman

About the Author

Paul Silberman is a Department Manager of Transportation Planning. He’s passionate about providing successful, workable transportation solutions to clients that better communities. When not focused on transportation, Paul is an avid sports fan, and enjoys traveling and volunteering for engineering outreach initiatives.

Read more posts by Paul Silberman

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