Understanding Transit Signal Priority (TSP)

In early December 2025, the Finch West Light Rail line (Line 6) opened. The line was almost immediately met with negative attention due to it running much slower on opening day than the existing bus, partly but not entirely fueled by left-turning cars typically being given the green light before the line's vehicles. Improved Transit Signal Priority (TSP) has been presented as a solution. I appreciate the increased attention to TSP and hope it leads to speed improvements for all transit in the near future, but we must ensure we are doing it right. We should also understand what TSP can and can't do, including the true differences between having TSP and not having it. TSP is much more complicated than the public seems to realize. Due to my participation in UTTAN's 2025 Student Competition, I (along with my peers, see #Acknowledgements) have looked into the complexities of TSP as part of a potential solution to congestion in Toronto. This page summarizes my understanding of TSP.

I have published a video about TSP and how it can and can not improve Line 6's speeds. Much of the info in that video comes from this page.

Passive and Active TSP

TSP encompasses any modifications to traffic signals that advantage transit over other modes. Thus, the most simple things can be counted as TSP. Passive TSP is anything that does not require vehicle detectors. This typically means timing the signals to give vehicles a green wave, though there are other strategies such as shortening other signal phases. Due to the nature of intersections a green wave may be difficult to achieve in both directions with existing infrastructure, and without vehicle detectors influencing the signals a slow zone or other issue could prevent any time savings. The slower signal timing may also slow car traffic even when a bus or train is nowhere near the intersection. As a result, active TSP is generally preferred.

Active TSP is any signal which prioritizes transit using vehicle detection. The signals may be able to shorten, lengthen, or skip a signal phase when a transit vehicle wants to go through the intersection. It may also reorder phases, insert a signal phase exclusively for transit vehicles, or a combination of these five to help transit vehicles get through the intersection faster. More advanced active TSP systems may also be able to decide when to give a transit vehicle priority. All implementations of TSP discussed on this page are some form of active TSP.

"Aggressive" TSP

This is not actually a technical term I have seen in literature. When aggressiveness is referred to by public figures, it means the transit priority activates more often with the primary goal of making transit vehicles run faster. Aggressive TSP is not necessarily unconditional, but conditions would not be very restrictive. Other means of increasing aggressivess include granting the signals the ability to reorder signal phases, increasing limits on green extensions and shortening red lights, and allowing phase insertion at more points during the signal cycle.

Unconditional TSP

Most TSP in Toronto is unconditional. When a transit vehicle is approaching an intersection, the signal will do whatever priority measures it is allowed to do, assuming it is working properly. Over 400 intersections in the City of Toronto already have TSP, but it may not be noticeable as it usually is in the form of a green extension either before or after the pedeestrian countdown. If you've ever seen a Toronto bus get through an intersection after the pedestrian countdown, you might have witnessed TSP in action. See a 2023 map.

Toronto has been using signal priority since the mid-1990s. At the time, there were technological barriers to implementing anything other than unconditional TSP. It is much easier to implement unconditional TSP as the only information the signal needs to process is whether a vehicle is present. This leads to the signals letting a vehicle through even when it is ahead of schedule. While this makes trips nominally faster, it also decreases reliability and increases bunching potentially increasing the amount of time journeys take through the increase in wait time. Due to this, I do not believe all TSP in Toronto should be unconditional. Instead, conditions need to be added.

Conditional TSP

Conditional TSP is exactly what it sounds like. The vehicle will now have to send the signal additional information which it will then use to determine whether it requires priority. If the vehicle is early, it might deny priority but it will not intentionally give it a red light (unless you program it to do so, perhaps this can be called "anti-priority"). Since any TSP with conditions counts as "Conditional TSP", it is a very broad category. You may even make some actions conditional while others are unconditional, for example extending a green light unconditionally but not allowing phase insertion unless the vehicle is not early.

We can go further. Schedule adherence is not the only condition you can use. For more frequent routes, headway regularity may be a more important priority than schedule adherence. As a result, you may want to avoid giving full priority to a vehicle if another already passed through the intersection two minutes ago. I do not expect Toronto to implement much conditional TSP in the near future apart from Metrolinx projects such as Lines 5 and 6 (they do have TSP, though I do not know whether it has been working properly). This is due to the complications with upgrading the TTC's bus fleet to be able to provide signals with more information. I do not expect a political push for this either, as not many people understand the different types of TSP (hopefully this page will change some of that).

Most TSP systems around the world are conditional, with some unconditional and fewer adaptive systems.

Adaptive TSP

Adaptive TSP is the only type I don't quite fully understand, so I would take this section with some amount of skepticism. I know it is not well-used at the moment as it is a more recent innovation which is getting more feasible over time. Since Finch's signals were designed and constructed years ago, I doubt this would be implementable without major signal upgrades which other lines might need more.

Adaptive traffic signals change their timing based on real-time conditions. Adaptive TSP would therefore adapt their priority based on real-time conditions. This may allow signals to be more aware of not just traffic conditions but conditions on the whole line, perhaps enhancing the signals' ability to deal with bunching and similar problems. Adaptive TSP is getting more feasible over time thanks to advances in AI applications, so we may be able to see it on future projects if any get built (e.g. Eglinton East LRT). Adaptive TSP may also be able to adapt to conditions outside the vehicle such as crowds at later stops or on the train already.

Modifying the Signal Cycle

All active TSP systems modify signal phses in some way. The most simple forms of this involve modifying existing phases. As discussed above, green extensions are a common occurrence in Toronto. The green extensions come with a time limit, if a vehicle does not move through the intersection fast enough, the light will turn red anyway. This is to ensure opposing traffic can get a green light within a reasonable timeframe. The maximum green extension is typically some number of seconds, though I can not remember the exact number (20 seconds is around the order of magnitude for this). The other kind of phase modification is shortening your red light. This is limited by how far the system can detect a vehicle and how much time it has to give pedestrains to cross the street. If given enough distance, one could finish the pedestrian countdown early enough to avoid the red light. However, the further you might want to detect a vehicle the less reliable your predictions of when the vehicle will come will be.

The next most common modification is adding a transit-only phase. This is the most obvious form of TSP you can get. A common example is letting transit through before a left-turn phase. A small number of intersections along York Region's Viva Bus Rapid Transit (BRT) system do this conditionally at intersections with a longer left-turn phase (Keele/Hwy. 7 and Weston/Hwy. 7 come to mind). These can be implemented wherever in the cycle you want, but unless the transit vehicle needs to turn they are most typically done just before the left-turn phase from the street the vehicle is running on.

Apart from those three, there are less common implementations. Reordering phases may allow you to get a green light immediately like with phase insertion, but also would allow drivers to go straight at the same time before a left-turn phase, which would occur after. You could also skip the left-turn phase altogether, though I would not recommend this if left turns are only allowed during the left-turn phase.

Conflicts

As a hypothetical, let us assume that the Jane LRT proposal is built. It will intersect with Line 6 Finch West at-grade at Jane and Finch. Let's say the Jane LRT is at a red light and the Finch LRT is approaching the intersection at a green. Would you stop the Finch LRT and let the Jane LRT through with a phase insertion? Would your answer to that question change if you found out the Jane vehicle was late and the Finch vehicle was early? When two routes which are supposed to be given priority intersect, the question of how to implement TSP becomes much more complex. Originally, the TTC refused to touch these intersections. Nowadays, the priority given is first-come, first-serve. If red light truncation were enabled in this way at Jane and Finch, this may result in the Finch train being hit with a red light.

In this kind of situation, I would attempt to determine which train is worse off (whether due to a delay or headway issue), and prioritize appropriately (perhaps only truncating Line 6's green if that train is early and Jane's is late). Alternatively, I would let the Finch train through then immediately give a red, or simply provide two straight phase insertions for both trains. I personally do not know which option works better, and such a decision may depend on the circumstance. I think you can see now why TSP at major intersections is much more complex. Additionally, while in some cases you would want to prioritize a higher-ridership route, the buses along Finch West and Jane had similar ridership in 2024, so that would not be a factor at Jane and Finch.

Influences on TSP's Effectiveness

TSP is only effective if you can make it work and there are external factors that impact whether it can succeed. Of these, stop location is perhaps the most obvious. When a vehicle stops, you do not know how long it will need at the stop. Even on a rapid transit line, the vehicle may hold due to a headway adjustment or disorderly passenger. Thus, failed priority requests (such as when a green extension runs out of time) are much more common when stops are placed nearside. When stops are placed farside, the stop will occur after the intersection improving the reliability of systems predicting when the vehicle will clear the intersection (including TSP systems).

Frequency is another factor influencing TSP effectiveness. If your line runs too frequently, the signal cycle may be too short to accomodate every TSP request and it may need to activate during every cycle. TSP's reliability begins to decline when frequencies improve beyond every 10 minutes, which would mean a vehicle enters the intersection an average of once every five minutes. If the line needs to carry more people than TSP can handle, perhaps a longer train or an elevated line may be a better solution.

Traffic can also play a role in TSP reliability for mixed-traffic systems (it is much less effective in heavy traffic). This is likely irrelevant to Line 6, but the City may be less willing to implement more aggressive TSP measures as it would most likely be less optimal for drivers in the short-term.

TSP is Not the Only Problem With Line 6

As of Dec. 2025, Line 6 has noticeably low speed limits. Line 6 vehicles are only allowed to travel through intersections at 25 km/h (though some have been measured faster), and some street-running sections have also been slow. My initial ride on opening day seems to confirm this. This is not an issue TSP can fix, but rather one involving other systems attached to Line 6 and by internal TTC policy. I hope the City can fix this, as it may be an even bigger problem than the signals along Line 6.

Shortly after this page's initial publication, I heard from others that Line 6 had been running faster. I took the line again two weeks after it opened and the run-time had improved to approximately 40 minutes. Other passengers who travelled around the same time had travel times ranging from 37 to 48 minutes, but generally between 40 and 42 minutes. Further investigation showed some unreliability, which allowed me to outrun an LRV from Finch West to Humber College. Better TSP will remove some of the unreliability, but it may not solve it completely.

The Takeaway

Transit Signal Priority is not a silver bullet. It can not fix every problem your transit line has. If red lights are a major issue then better then TSP might help, but if the required service frequency is too high it may not help as much or at all. Unconditional TSP sounds fast, but it might make bunching worse particularly if the line is susceptible to bunching. Exactly what priority measures should be unconditional and which should be conditional should be discussed with potential bunching and speed issues in mind. Additionally, if your transit line is slow even with good TSP it will need more than just better signals to get to the speeds riders expect.

Acknowledgements

This page would not have been possible without the 2025 UTTAN Student Competition, my peers who worked on it (Lucas Gaspari and Thanusan Ragulan) alongside me, and our mentors (Joshua Wang of York Region and Tony Zhuang of Burlington Transit). Much of this page is taken from Wenxun Hu's Literature Review on the subject done under the supervision of Prof. Amer Shalaby, who also helped us as an expert on TSP during the 2025 Student Competition. Much of the information not in the Literature Review was taken from August 2025 meetings with Prof. Shalaby and Marc Tan, Transportation Engineering Manager at the TTC.

Last updated: 01/01/2026