What Is Transit Signal Priority?


“Red bus lane for Battery Street” by Seattle Department of Transportation is licensed under CC BY-NC 2.0

By: Alex Bundics, Journalist

Bus transportation is not an intriguing alternative to driving a car if the bus is always stuck in traffic and cannot get any advantage over cars. There are various solutions to this problem, such as dedicated lanes, revised routes, bus bulbs, queue-jump lanes, and the topic of this article, transit signal priority (TSP). TSP is often paired with Bus Rapid Transit to provide the most effective and high quality bus service possible. Simply put, TSP is a code that will adjust the cycles of traffic signals through various strategies to reduce the amount of dwell time along a given bus route. This is not to be confused with signal override codes granted to emergency vehicles that override the traffic signal’s code, since TSP can only modify the signal’s cycle within a preset amount. Studies have shown this can reduce run times along a standard route by 20%. TSP is also often used alongside queue-jump lanes (short stretches of bus only lanes before an intersection to allow buses to get in front of automobile traffic) to make the bus get ahead of traffic as seamlessly as possible. TSP is without a doubt beneficial to transit users, but how big of an impact does it really have, and is it really worthwhile instead of a more serious project such as bus-only lanes?

First, let’s look more into detail on exactly how TSP works. There are two main ways that TSP systems are operated: centralized TSP and distributed TSP. Centralized TSP has a central control center that receives requests from individual buses and responds to each one individually. Since this is managed from a central location, on time performance and crowding of vehicles can be included in the code used to respond to requests. Distributed TSP has no central management and each intersection operates on its own, so this requires less management, but as a result decisions are often less valid. In each of these systems, a bus approaching a traffic signal passes a sensor and determines at what part of the signal’s cycle the bus will arrive. If needed, it responds by sending a request to employ one of various strategies to reduce or eliminate the need to wait at the signal. The two most common strategies used in TSP are green extension and red truncation, which as their name suggests will extend a green phase or shorten a conflicting red phase to help speed up the bus. Red truncation usually impacts more buses than green extension, but provides a much smaller benefit to each bus. The two can be paired together to create the most simple TSP code. A similar strategy to these is early red, which in theory tracks a bus that is far enough away that it would arrive at the intersection during a red phase, so the conflicting phases in the cycle are served early to allow for the signal to turn back to green earlier than usual. This is similar to red truncation but does not actually reduce the length of the conflicting phases. Early red is not commonly used as it requires much more complex sensors and has been shown to not provide any significant benefit over red truncation. Phase shuffling, phase elimination, and phase insertion are strategies that affect the order of the phases in a traffic signal’s cycle. Phase shuffling adjusts the order of the phases to allow the bus to be served as soon as possible. For example, a protected left turn phase may be moved from before adjacent through traffic to afterward. Phase elimination is not commonly used but the idea is that a phase is removed completely, which would usually be a protected left turn phase. Phase insertion adds an extra phase anywhere in the cycle to allow a bus to pass through an intersection without waiting its turn. This is usually paired with queue-jump lanes to allow for a more effective queue-jump than with the lanes alone. The last common strategy is called an actuated transit phase, which means a phase is inserted only when a bus is present. This is used situationally in cases such as entrances to transit hubs, bus-only lanes, and other cases where only buses are permitted, such as bus-only left turns.

So, what effect does this actually have on the quality of bus service? There have been many studies where TSP was implemented, and they all show some considerable improvements. One notable example is after a 10 million dollar project in Los Angeles, California many arterial routes had TSP implemented which led to an overall 25% reduction in travel times, with some lines having as much as a 29% reduction in travel time. Other cities have seem similarly big improvements- as much as 30% reductions in travel time in some cases. In general, though, studies show that on the average arterial route, implementing TSP will reduce travel times by about 15%. That number varies greatly depending on the exact TSP code and the circumstances. Leading companies in the TSP industry claim that their TSP technology can increase ridership by up to 10%, decrease fuel consumption by 19%, prevent bus stop crowding by 43%, and reduce time spent not in motion by 40%. Aside from the obvious benefit of reducing run times, shorter run times make fewer vehicles and drivers capable of providing the same service, so operational and maintenance costs can go down as a result of TSP implementation.

TSP clearly greatly benefits bus service, particularly along arterial routes and Bus Rapid Transit corridors. This is proven by the fact that in today’s world, almost every major bus transit project includes some sort of TSP code. Any city can make a serous positive impact on their bus system by simply using TSP- no major infrastructure required. Studies have found that TSP generally costs about $13,500 per intersection, so for the price of a TSP network many other improvements could be made instead, for example creating dedicated bus lanes or simply purchasing additional vehicles. For this reason, cities must always consider if TSP is worth it or if a bigger project (such as bus-only lanes) should be taken on instead. TSP is often used as a part of bus-only lanes to help speed up the bus even more, and in situations like that or where queue-jump lanes are present are where TSP provides the biggest benefit possible. S0, TSP will benefit all bus systems, but in many cases bus-only lanes or other uses of the money would provide a bigger impact to the bus system overall.

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