After Travel Time runs not lining up with Time-Space Diagram

Questions and discussions about operating Tru-Traffic

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After Travel Time runs not lining up with Time-Space Diagram

Postby ChrisBurke » Wed Aug 15, 2012 1:34 pm

Attached is my Tru-Traffic (v9) file showing six southbound AFTER runs. The file was created using a Synchro Combined CSV export to include correct signal timings, offsets, etc.

Field runs 2-5 seem to match up well with the time-space diagram, but runs 1 and 6 don't seem to match up well (i.e., travel time run moves through red phase, and stops on green). Is there any general reason for this, and is it fixable? Does it have to do with what part of the cycle the travel time run began during?

Thanks for the help,

Chris Burke
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Re: After Travel Time runs not lining up with Time-Space Dia

Postby bullock » Wed Aug 15, 2012 6:07 pm

(A) One possibility is that the side-street green gapped out early on those runs, giving an early green to the artery, or the side-street phases were skipped altogether. This seems somewhat unlikely, as the runs pass through the red pretty early. Can you rule out this possibility? That is, do you know there was sufficient traffic from the side-street to ensure that the phases were served and got their full split?

(B) Another possibility is that when runs #1 and #6 were recorded, a different timing plan was in effect.

(C) If those possibilities are ruled out, then the most likely cause is that the GPS clock and the signal controller clocks, as far as Tru-Traffic is concerned, were not synchronized. That is, the Signal Cycle Start Time was not set accurately prior to recording the trip logs. Not to worry, though, as we can correct this after the fact. It will be a two-step process.

(C.1.a) Do you know the Master Reference Time for the signals? In Tru-Traffic's vernacular, this is the time of day at which all signals, in timing plans (regardless of cycle length) have a common t=0 for their cycles. (Typically it's either midnight or 3:00AM, but other values are possible). If you know the Master Reference Time for the signals, then you can synchronize the clocks now using the time of day box at the top of the Clocks tab of the GPS View, in the Option (a) section.

(C.1.b) If you don't know the Master Reference Time, then you can help Tru-Traffic identify a Signal Cycle Start Time (which need not be the Master Reference Time) using the controls in the Option (b) section the bottom part of the Clocks tab of the GPS View. Best to connect a GPS receiver to Tru-Traffic running on a laptop, carry it out to the field, and watch the phase transitions for one of the intersections. If possible, choose a critical intersection, or any intersection with sufficient demand that all phases are getting served with the full split. In any case, try to point the controls to a phase and transition whose occurrence is not sensitive to whether a movement "gaps out." Typically, this is the yield point of the synchronized phase. Then click the Synchronize Now button when you see the phase transition occur in the signal head. I like to use start of Red for that phase, as the Yellow serves as a warning to get ready to click the button.

(C.2) Once the clocks are synchronized, go to the Trip Logs tab of the GPS View. Use the Shift key to highlight all the trip logs, then right-click and choose Signal Cycle Start Time: 110s (which is the name of the only timing plan in your file). This sets all trip logs to assume the Signal Cycle Start Time that you specified in step (C.1).

If the runs still pass through red times, and you're confident that it's not due to possibilities (A) or (B), then either the timings in Tru-Traffic do not accurately reflect what's in the controllers or the clocks are not synchronized correctly. Both of these possibilities are more common than many people think, and they're both pretty easy to diagnose with a single test out in the field. The clocks can get incorrectly synchronized if you use step (C.1.a) (or Option (a) on the GPS View's Clocks page) but your signal controller clocks are unstable or are a few seconds off from standard time. The timings in Tru-Traffic can inaccurately reflect what's in the signal controllers for a variety of reasons: they may have been entered incorrectly in the controllers, they may have been adjusted in the field such that your Synchro file is now out of date, and other possibilities as well.

In any case, if the runs still appear in the wrong time within the cycle, the step (D) should get to the bottom of it. Step (D) can also be useful for uncovering subtle problems, even if the runs all appear OK within the cycle.

(D) Connect a GPS receiver to Tru-Traffic running on a laptop and carry it out to the field. On the GPS View: Options: GPS Tracking page, check the box labelled "Mark all positions at current time with a horizontal line" (in v9 and prior) or "Mark "now" at all positions with a horizontal line" (in v10 and above). Alternatively, click the Cycle Timer option under the View menu. As the moving "now" line crosses different phase transitions in the time-space diagram, it shows you when Tru-Traffic thinks those phase transitions occur. Compare that to what you see in the field. Any observed discrepancies that aren't obviously due to gap-outs indicate errors -- errors in either the file in Tru-Traffic or possibly in the timings entered in the controllers. These errors could been
  • in the timings (most likely the splits or offsets, but possibly also the phasings or even cycle lengths), or
  • in the clocks, either the controller clocks or in Tru-Traffic's Signal Cycle Start Time.
There may be more than one error. Watching the "now" line on the time-space diagram and comparing it to what you observe in the field, you should be able to identify and fix these errors until the "now" line agrees completely with your observations (allowing for any gap-outs).

Please let us know what you learn, especially if this doesn't seem to be sufficient -- if there seem to be problems deeper than these.

Regards.
Greg
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Re: After Travel Time runs not lining up with Time-Space Dia

Postby Chris » Tue Sep 04, 2012 5:44 pm

Two more possibilities of running through red:

1) Pedestrian Clearance times are not as long as split time for phase. In this situation the signal runs its coordinated plan until a pedestrian pushes the button causing the side street to stay green longer than its coordinated plan. Signal goes free for a few cycles as it catches back up

2) Signal Preemption. In this situation an emergency vehicle is traveling across the intersection causing the signal to go free for a few cycles.

But I agree with Greg since the situation appears to be system wide its more a clock issue. See how this lines up. Right click on your first trip log and go to prroperties and add 30 secondes to the "time shift on diagram" Add 45 seconds to the last trip log.

Chris
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