Very frequently there will be a multilane road with a left hand exit lane with a stoplight. There will be a sign hanging next to the left lane’s light which says “turn on green arrow only” or something like that.
But the thing is - the only states that leftmost light has is red, yellow, and a green arrow. They never ever just have a plain green light. There is never a situation in which you would alter your behavior because of that sign.
So why are they there?
I assume they are there to ensure that people don’t turn left on a red light.
That is, they warn drivers that, just because the lanes to their right have a green light to go straight ahead, the cars in the turning lanes are not allowed to turn left, even if there is no oncoming traffic, until the green arrow comes on.
I guess that’s a logical interpretation, but it seems unnecesary. If the light for your lane is red, you don’t go anywhere, that’s obvious. You might as well put a sign on top of every normal stoplight that says “do not go through intersection on red” just in case people forget.
I agree with you but, having been on the roads for more than 20 years, i’ve learned not to overestimate the intelligence of the average driver.
Nor the intelligence of the civil engineer. Especially the civil engineers in Las Vegas. 5 god damn minutes to make a left hand turn!
Some intersections have a light that is either a green arrow (meaning oncoming traffic is blocked, and left-turners have the right-of-way) or else that light shows nothing at all, even while the main traffic signal for that side of the street is green (meaning that if the general signal is green, left-turners can still turn if it is safe to do so, but must yield to oncoming traffic). The “turn on green arrow only” signs are to emphasize that this is NOT one of those intersections; those wishing to turn left must wait for an explicit green arrow.
deleted
In Montreal there actually are signs at many intersections that say (translated from French) “wait for the green light”.
In my experience signs like that are placed in response to drivers not doing the right thing. Sometimes it might be at a particular intersection or if it’s a common problem the sign will be at all suitable intersections.
There are lots of traffic lights, at least around here, that don’t have totally separate lights for each lane. Instead there’ll be the usual red, amber and green, with a separate green arrow bolted onto the side, which normally isn’t lit at all. In these cases, a “turn only on green arrow” sign would be useful. There never seem to be any, though.
This is the kind of thing I mean. Imagine just the bottom green light is on (but not the arrow). Some people might think that gave them the go-ahead to turn left as well as proceed straight ahead, as there is no “red arrow”.
In some states a green light in the main traffic lane(s) indicates that you may also turn left or right, even if a red arrow is showing for a left or right turn. The sign is there to modify that general rule, allowing turns only when there is a green arrow. Sometimes a light has been re-timed so that there is never a red arrow and green light together, and the sign was just left there, perhaps in anticipation of a future change.
On Kedzie and Fullerton in Chicago, there is a “Turn left on green arrow only” sign.
It’s used because then you’d have people sitting in the lane on the green light waiting to turn left, and they would clog the road. This way the green light means you can go straight or turn right. Then the light turns red and the green left arrow comes on, and those cars get a turn, then it goes to right and the other street goes
You think that’s bad…how about “yield to pedestrians in crosswalks” signs? Drivers are supposed to yield to pedestrians everywhere, all the time. This sign doesn’t so much remind me to yield to them here, too, as it does cause me to speculate if it would be okay with the authorities if I run over anyone who strays outside the crosswalk lines.
Don’t any of y’all have “left when clear” rules?
If there’s a “left on arrow only” sign, then you must wait for the green arrow to turn left. The green arrow means that you have clear passage and right-of-way.
If there’s no “arrow only” sign, you can still turn left when it’s clear to do so. However, it’s your job to determine that it’s safe to turn and you must yield to all other traffic.
At least that’s how I learned it, back when dinosaurs ruled.
That’s not the same type of light the OP is describing (at least as I understand the OP). The Fullerton and Kedzie light (if I’m thinking of the right one) governs both the turn and main lanes has states:
RED, YELLOW, GREEN, YELLOW ARROW, GREEN ARROW
Normally, a green at an intersection (as you say) would permit advancing in either the main traffic lanes or the turn lane. For that intersection, though, they only want left turns explicitly on the green arrow.
The OP’s light, located in the exit lane, has three states: RED, YELLOW, and GREEN ARROW. There should be no confusion, as there is no plain green to be confused by.
My only guess is it’s to make clear that, hey, there’s another set of lights here you need to follow, so don’t pay attention to the lights for the through traffic lanes. I’m pretty sure I’ve seen lights set up like this around the Loop area, perhaps on Michigan Ave., and the extra sign has kept me from turning when I otherwise might have.
Another vote for the “It’s a reminder not to follow the traffic going straight in whether or not you proceed”, noting that it’s often possible that other vehicles may restrict your range of vision, e.g., a large truck ahead of you whose box kieeps you from seeing one light or the other, or positioning of the lights such that the drivers of the first cars in line can only see the lights by craning their heads forward and upward, meaning there is a tendency to rely on traffic in other lanes for guidance.
Although the thought did strike me that “Turn on Green Arrow only” signs may be a good way to keep you from getting Speedy. 
Not much help unless you stop at the intersection and wait for the light to cycle through its states.
I’m not following you.
Anyhow, I just passed an intersection in Chicago which I think has the OP’s configuration, except with states as red arrow, yellow arrow, green arrow.
Unfortunately, the Google vehicle is all the way in the right lane, so let me describe what you have:
You have the main traffic signals ahead of you, with the green arrows pointing up. Where you have that island separating north and southbound lanes is another set of traffic signals governing the turn. That set of lights has three states: red arrow, yellow arrow, and green arrow. There is a sign there now (although it doesn’t seem to be in the Google capture) that says “Left turn on green arrow only” in addition to the other sign prohibiting left turns during certain hours.
Around here, the DOT apparently feels it necessary to remind people making U-turns that they must yield to right-turning traffic on the cross-street. I mean, really? You have to remind people at the absolute bottom of the right-of-way chain that they’re supposed to yield to other drivers?
Oh, and don’t even get me started on the ridiculous blinking turn arrows we have now.
No, your intersection is considerably weirder than what I’m describing. I didn’t think to use google street view to demonstrate my point though, good idea.
This is the sort of thing I mean. Totally typical intersection, 3 lanes on the right going straight and one lane on the left that’s a turn only lane. You can’t see it from the angle on the picture, but that light on the left only has red/yellow/green arrow states, and you can see (but not read) the little sign next to it indicating to only go on green arrow.
This isn’t a weird intersection where people might be confused - it’s a totally run of the mill turning lane.
