King Street Pilot (Streetcar Transit Priority) | Page 242

Discussion in 'Transportation and Infrastructure' started by AlvinofDiaspar, Mar 22, 2007.

  1. TransitBart

    TransitBart Senior Member

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    Time for a new thread. If I recall correctly, the capital was allocated for a new traffic signal control computer system. The current one is something like two decades old. The upgrade is in progress.
     
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  2. reaperexpress

    reaperexpress Senior Member

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    When did the CEO say that the firmware issues are hampering the TSP rollout? Any chance it's in writing so I can follow up on it?
     
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  3. reaperexpress

    reaperexpress Senior Member

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    We have a thread for this, it's here: Roads: Traffic Signals.

    I may provide some background once I do some research to see what information is publicly available, but long story short the City already replaced that decades-old MTSS central system which runs almost all of the city's 2000+ signals. What is being replaced now is the SCOOT adaptive control system which runs the signals on a handful of highly-variable traffic routes such as Lakeshore Blvd, The Queensway and Don Mills.

    The City makes use of virtually every available strategy to prioritize transit vehicles, though a given intersection doesn't necessarily use every tool available - that's a matter of policy and debate regarding the level to which transit vehicles should be prioritized at that intersection. The system is by no means limited to just extending the green, it can do pretty much anything you can think of.

    But in the case of King Street, with simple two-phase intersections running fixed-time, green extensions (and sometimes red truncations) are all that really makes sense anyway.

    At minor intersections, it is not possible to shorten the red light when a streetcar is approaching, because the north-south green is already equal to the minimum pedestrian crossing time.

    For example, look at King & John. Given its nature as a minor cross street, it's almost certain the north-south green will be as short as possible, which according to the City Traffic Signal Operations Policy is:

    7s Walk
    13s Flashing Don't Walk (15m @ 1.2 m/s)
    3s Amber
    3s All-Red
    Total: 26 sec

    But it's actually not a big deal that truncation is not possible here, because you can get very good results just with 'rudimentary' extended greens.

    Assuming a 70-sec cycle length (standard for downtown), here is the chart of streetcar delay throughout the cycle with no TSP:

    The worst case scenario is if the streetcar arrives just at the end of the green, in which case it sits through all the clearance and red time*. Each second later that the streetcar arrives, it would have to wait one second less until the next green, until eventually the streetcar arriving just at the start of green has a delay of zero.
    Screen Shot 2018-05-05 at 12.48.02.
    The average delay over the 70-second cycle is the total delay (area under the graph, [Max Delay]² /2) divided by the cycle length, which here works out to 7.3 seconds.

    With TSP
    The City allows up to 30 seconds of green extension at minor intersections. So before the light changes to yellow, it checks whether there is a streetcar estimated to be less than 30 seconds away and if there is, it will hold the light green until it passes through. The problem is that if the streetcar actually ends up taking longer than 30 seconds, the light would extend to the max and then change to yellow before the streetcar made it through. In that case, TSP has actually increased the delay to the streetcar by 30 seconds because if the green hadn't been extended the light would have been about to turn back to green by then, and instead the streetcar needs to sit through the entire red (this is called a "failed extension").

    So to accommodate variability, you need to leave a margin of error. With a near-side stop, there is a huge amount of variability from one streetcar to another, so you may for example leave a 14-second margin of error to minimize failed extensions. So instead of extending for vehicles that are estimated to be less than 30 seconds from the intersection, you would only extend for vehicles estimated to be less than 16 seconds away (in case they actually take longer):
    Screen Shot 2018-05-05 at 12.47.31.

    Moving the stops to far-side as has been done with the King Pilot can bring a big improvement by removing the variability of the stop, allowing you to use a much smaller margin of error, and therefore a longer horizon for approaching vehicles. Within the same 30-second maximum, one might now use a 6-second margin of error, allowing the signal to extend for vehicles estimated to be within 24 seconds of the intersection.

    Screen Shot 2018-05-05 at 12.44.25.

    THE BOTTOM LINE:

    Just using the City's current standard of 30-second maximum green extension along with the new stop layouts, TSP could reduce signal delays by a whopping 93% at John St to an average delay of only 0.5 seconds. I seriously challenge anyone to come up with a different way of prioritizing streetcars that can achieve those kinds of savings without massively screwing over everyone else at the intersection.

    __

    *Actually streetcars can proceed during the yellow, but there is also a couple seconds lost at the beginning of the green before vehicles actually start moving which mostly cancels that out. Convention is to assume that the effective green is 1 sec greater than the displayed green, but I haven't accounted for that here to keep the illustration more intuitive.
     
    Last edited: May 5, 2018
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  4. mdrejhon

    mdrejhon Senior Member

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    If you have added intelligence for multiple cycles, one could theoretically extend greens over 2 cycles instead of 1.

    That requires very reliable streetcar arrival prediction times, but if you know a streetcar is already accidentally "stuck" behind a long red light (45 seconds) at an important artery like University further down the road, then you have time to do a 2-cycle minor green extension (instead of a 1-cycle major green extension) to realign the timing of green to a predicted arrival of streetcar. This can help the streetcar catch up from its accidentally getting stuck behind an earlier red light.

    Basically, you try to never have long red lights for a streetcar, but if a streetcar does get stuck (light goes red the moment the streetcar approaches one) for any reason, then the downstream lights can theoretically reliably predictively adjust their cycles over a 2-cycle period if there's no conflict with a streetcar in the opposite direction.
     
  5. reaperexpress

    reaperexpress Senior Member

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    Yes, this is called predictive priority, and can work really well where there are no stops between signals and priority requests are not frequent (typically at least 10 minute headways for service).

    But in the King Pilot area there are stops at almost every signal, which makes travel times unpredictable from more than a block away. And there is on average a streetcar every 55 seconds during rush hour (every 110 seconds in each direction), which makes it impossible to look more than a cycle ahead given that each cycle is 70-110 seconds long. Even if only the streetcars with above-average headways get priority, that's still a TSP request every 110 seconds on average.
     
    Last edited: May 5, 2018
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  6. steveintoronto

    steveintoronto Senior Member

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    Excellent posts! To which I wonder: "Why could the King Pilot Project Team supply this sort of answer?" And that makes a massive point in itself. The left hand doesn't know what the right one is doing when it comes to integrating and *intuitive prediction* of Toronto traffic flows.

    Reaper has supplied an excellent post, in fact UT should offer to have him write a feature on this.

    Without poring over the minutiae yet, this claim has to be stepped up a good notch:
    But can it/does it?

    What layers of parameter are the TTC supplying real-time from their vehicles to the control functions of the intersection lights? Either directly, or through a central control system that integrates in real-time two way vehicle position, speed, acceleration, deceleration and the approaching light cycle parameters?

    In other words, a fully integrated CBTC system also encompassing the intersection lights. This would by definition assume PTO of the streetcars. This is a decision the City is wholly unprepared to take, and thus the King route never progressing past where it is now.

    This might be what @dowlingm meant by "firmware" albeit the term infers a proprietary system is already up and running, or at least in place, but not actuated. Without having even closely examined Reaper's excellent post in detail, something that has been noted in other systems as per "green light priority" is that if it doesn't take account the oncoming tram/streetcar traffic, it spites Peter to pay Paul, even if intersecting vehicular traffic be damned (edit: Which is exactly what is done in a number of US and other nation's cities)....thus the emphasis on the need for PTO/PTC to co-ordinate those 'priority' lights to maximize benefit to what is prioritized, while minimizing the imposition on all other traffic affected, which must include pedestrians!
    Which all indicates the need for an overall reactive dispatch system integrated with the signal lights, not only controlling the light cycles and parameters, but also the arrival and departure times of streetcars including velocity control to optimize the performance of that loop.

    Which brings me back to the "King Balooza Karnival". The City had better make up its mind on this. Either the tracks are going to have to be controlled access from pedestrians too, or King just becomes an amusement park, not a Transit Mall. And the third most heavily used transit route in Toronto has now reached its peak, and can only slide backwards.
     
    Last edited: May 5, 2018
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  7. BMO

    BMO Senior Member

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    While this may work in theory, typically TSP uses subsequent cycles to go back to the original coordinated signal plan. When you have a high frequency line like King it leaves the signal in a perpetual state of playing catch up, especially at major intersections which is where the highest delays are. Many would argue the signals work great at John already, the issue is the major intersections, compounded by their large walk time requirements since intersections like Spadina and University don't have a median for pedestriana to complete only part of the crossing.
     
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  8. reaperexpress

    reaperexpress Senior Member

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    Well the for the majority of streetcars TSP doesn't do anything because they were going to get a green light anyway. And while the signal is in offset recovery, priority keeps working so that's not really an argument against it. Furthermore, with conditional priority the number of TSP requests is typically cut in half, which allows the signal to stay in sync more, not that that really matters on King Street where there's no traffic to coordinate for anyway.
     
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  9. dowlingm

    dowlingm Senior Member

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    Here's mention in the March 2018 CEO report
    Interestingly, this text appears in other past reports - September 2017:
    I note however this refers to buses, but there definitely was an intent to have TSP on King in February.
     
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  10. steveintoronto

    steveintoronto Senior Member

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    https://www.ttc.ca/About_the_TTC/Commission_reports_and_information/Commission_meetings/2018/March_20/Reports/1_Chief_Executive_Officer's_Report_March_2018_Update.pdf


    Many thanks for that. There's a series of questions to be asked of the TTC and City, because what's been and is still being stated, and what's extant, functioning or not, doesn't add up.
     
  11. reaperexpress

    reaperexpress Senior Member

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    Ah okay, that checks out. Evidently this is a general item not related to the King Pilot. It's talking about new intersections to be equipped with TSP, not the re-activation of existing TSP in the pilot area.
     
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  12. Leo_Chan

    Leo_Chan Senior Member

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  13. EastYorkTTCFan

    EastYorkTTCFan Senior Member

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  14. steveintoronto

    steveintoronto Senior Member

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  15. TheKingEast

    TheKingEast Senior Member

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