Most basic stoplights have one light for each condition, red = stop, yellow = caution, and green = go.
What’s the straight dope on stoplights with two red lights? Here in Texas, you see them at just about every intersection that has a dedicated left turn lane. Sometimes you’ll see two green lights, but one is a solid green lens and the other is a green arrow, showing when you have a protected left turn. But then you have two solid red lenses that come on at the same time. What is their purpose? What’s the difference between one red light and two? Why do you need two if they come on at the same time?
Does one of the two red lights sit on the same “tree” as the green left-turn arrow? If so, I’d assume it informs left-turning people that no, they can’t make a left turn right now. Why, you may be asking, wouldn’t they figure that out from the fact that the other light, the one on the “tree” pertaining to proceeding forward, is red?
Well, bypassing the obvious Texan jokes, the assumption here is that the red light that prohibits you from going forward may turn green while the red light that prohibits you from making a left hand turn may remain red, i.e., as of now you can go on through the intersection, but if you wanna turn left, just hold on for a bit.
Most intersections have more than one traffic light suspended above it , for the multiple lanes. This serves another purpose, the signals act as a backup in case one of the red lights burn out.
I’d guess, in this case, there is no other light over the intersection, and the second red light is a backup.
Hmmm… I guess I didn’t describe the situation clearly enough…
Say you’ve got three lanes of traffic, each with their own cluster (or “tree”) of lights. R = Red, Y = Yellow, G = Green, A = Arrow. The leftmost lane is a dedicated “left turn only” lane.
Looking out your windshield at the lights suspended on the line across the street, you would see:
RRYGA RYG RYG
At any given time, the leftmost cluster of lights will either show:
green arrow (Go ahead and turn. If they hit you, it’s their fault.)
solid green light (Go ahead and turn, but if they hit you, it’s your fault.)
yellow (light’s about to change, gun it and hope you don’t hit anything! )
Two solid red lights. (Stop. And since there are TWO red lights, I really mean it!)
If one of the red lights were on during either of the greens, that might serve to warn people to stop before turning left, but the red lights come on by themselves and always together, and only by themselves. As far as I can tell, it doesn’t serve any separate purpose to have to red lights that only come on together, unless it’s just an attention getter. But I’d love to know the real reason behind it, other than to give me something to ponder while waiting for the light to change!
That does seem to be odd. In Albuquerque, where there are a lot of partially and fully protected lefts, the system is as follows:
For a partially protected left, there is a green and yellow arrow placed under the green light on the same tree as the basic red, yellow, and green. The green arrow is generally on at the same time as the green for traffic heading in that direction. For example, at an intersection, for traffic heading north/south, the green light and arrow would be for the northbound cars and people turning west while southbound and east and west all had red. Eventually, the green arrow will turn yellow and southbound traffic will get the green light. It is still possible to make a left turn either east or west as long as it possible to do safely until the overall yellow light.
For a fully protected left, there is a seperate tree consisting of red, yellow, and green arrows. At an intersection, for traffic heading north/south, the green arrow will come on for both cars turning west and east and eventually go back to a red yellow. At this point, the no-left-turn on the red arrow rule comes into effect and you cannot turn until the cycle comes around again.
My first thought, on reading the OP, was that it was similar to the fully protected left described above, but with a more confusing use of two red lights instead of a red light and a red arrow. However, if both red lights are on or off simultaneously all the time, I have no idea. Perhaps there’s something in the municipal or state code that will explain it.
I asked a traffic engineer (here in PA), he’s never heard of that feature.
I could make a guess that it’s for redundancy if one of the red lights burned out. The light trees for straight-thru lanes usually have at least two light trees, but the left turn lane has only one.
Additionally, I find having redundant but separate traffic light trees helpful for visibility when you’re following a tractor trailor or other tall opaque vehicle, if one light tree is obscured you can sometimes see the other one. I sure wish they’d improve on that though; I’m not sure this feature is even intentional.
In Quebec, each light has two rwed lights too. I think it’s simply to make them more visible - whoever made the rules decided that two was safer than one.
Those lights tend to be horizontal, like this:
(A)RYGR(A)
With approropriate arrow on each end depending on if the light block is on the left or right, or possibly without arrows at all.
One reason may be so that you can spot the light if you’re driving behind a big truck. A little more angle on a light can make a lot of difference in brake time. If you’re too close to a semi there’s almost no chance you’ll see the light in your own lane, with the middle 30 degrees of your visibility blocked. Solution? Put more lights above an intersection to give people ample warning to stop.
Are you in Dallas, by any chance, or another city in Texas with a light rail system? I’ve seen special signals like that in cities with light rail, where one of the lights indicates the signal status of the train.
Close to Dallas, but the lights I’ve seen like I’ve described aren’t near rail systems.
Although I have noticed now that it can be different depending on the intersection and city. For instance, this morning I noticed the lights with five lenses were arranged Red, Yellow, Yellow Arrow, Green Arrow, Green. This makes perfect sense since you’d need to let the people turning left know when the protected turn period is about to end, so you have a green and yellow arrow in addition to a regular green and yellow lens.
It could be that the stoplights with two red lenses only allow for protected left turns, but if that were the case, why didn’t the city just hang a standard 3-lens stoplight?
Aren’t there also traffic-control lights with a larger bulb for red or green?
I bet it’s to make it obvious to the color blind or others who can’t see so good that it’s red or green. In other words, 2 lights = stop, one light/one + arrow = go.
Asking a friend who is a highway engineer got me this answer (simple, and rather obvious in hindsight):
When they added the left-turn arrow, the highway department crew probably had a 5-light fixture laying around in the front of their storage room, and so they used that, rather go searching for a 4-light one, or buying one.
He said that’s the way highway crews tend to operate, and it makes sense to me.
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