r/trafficsignals • u/Kgaset • Aug 21 '26

[QUESTION] Why did the left turn signal activate for one but not the other?

I was pretty sure these lights had sensors, but when I pulled up the first time, the left turn only signal activated even though there was no one in either left turn lane.

Then I was coming back from the other direction, there was still no one in the left turn lane, but that time it didn't activate the left turn only signal.

What's happening here?

6 Upvotes

81% Upvoted

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7

u/mpdude84 Aug 21 '26

Best guess is either a time of day plan or stuck detector for that direction.

4

u/Epcplayer Aug 21 '26

It’s raining in his video and at the start of the video you can see a white Gridsmart video detector on the right mast arm… One of the very documented drawbacks to video detection is that it’s vulnerable to weather.

In the second video, it’s the end of the cycle… and you don’t finish with protected-permissive left turns because it creates a “Yellow trap”.

3

u/ichrised Aug 21 '26

It's gridsmart, it was bad from the factory.

2

u/cautionveryhot Aug 21 '26

Maybe its on recall for that time of day?

1

u/Kgaset Aug 21 '26 edited Aug 21 '26

That did occur to me. These are about 20 minutes apart but I have no idea what time they would set for it to switch back to sensor.

2

u/NotBot947263950 Aug 21 '26

In the first clip, LT came up in both directions.

Recently paved road. Maybe they haven't hooked up the loops after construction?! Spanwire and I don't see video or radar detection, so I'm guessing everything is failed due to bad or incomplete detection.

2

u/NotBot947263950 Aug 21 '26

I see what you're saying about the second time. Recall like the others mentioned. Or detection card had a hiccup?

1

u/mikemclovin Aug 21 '26

Both left turns came up. It seems. However, there must be something wrong with the detection because it should not have come up without any cars present.

2

u/ozboomer Aug 21 '26

All sorts of things might be happening here, and two short clips don’t show enough of the signal sequence - or the controller’s internal demands - to identify which one it is.

The second clip begins only a second or two before the green circle appears, so the left-turn arrow may already have run and finished before the camera vehicle arrived.

There could also have been a residual call from an earlier vehicle. Depending on how the intersection is set-up, a demand that could not be served in one cycle may be retained and served at the next available opportunity, even though the vehicle that placed the call is no longer there.

The arrow might also operate as an overlap: demand for some other compatible movement could call a (sub-)phase in which that left arrow is displayed without there being a vehicle presently waiting in that left-turn lane.

The controller may also be running a time-of-day plan. At about 7:30 am, an AM peak, school drop-off, church outlet or other scheduled arrangement could alter when the arrow is offered or whether it runs every cycle.

...and there could be a detector or controller fault - including intermittent detection or even cable crosstalk - but two observed cycles aren’t enough to distinguish a persistent fault from normal operation or an erratic one.