In Los Angeles, there are a number of intersections with traffic signals that sometimes offer a left arrow, and sometimes (within the same light cycle) do not. My question is, what is the benefit of this configuration over others where the left turns are always regulated by an arrow?
The actual signals look like this:
R e d : O
Yellow: ← O
Green: ← O
(← is a left arrow, in case it doesn’t display)
The first part of the cycle, you get both ← and O green. Fine; you may go straight on or turn left without interference. But soon the arrow disappears, and only the regular green light is on. Then you have oncoming, two-way traffic. But without a red arrow, it is still legal for cars to inch their way into the intersection, and make a left turn if it is safe, which in practice means once the light is red in all directions a couple of cars will zoom diagonally across the intersection.
I understand the benefit of this configuration over a normal light, but how is it better than:
r e d : ← O
yellow: ← O
green: ← O
…which is how they do it in San Diego? Bear in mind that these streets are old, poorly maintained, and overcrowded often to the brink of gridlock, but they do at least have separate left-turn-only lanes.
I’m sure there is a factual answer from the traffic engineer types.
Because when the green left arrow is on, opposing traffic has a red light and you have the right of way. When the main green light is on, you must yield to opposing traffic.
Was it here or another forum where we had a big hairy debate about whether the left turning car is allowed to inch into the intersection and then turn at the instant both directions turn red?
Yeah it was definitely here. One of the big points I remember cause it jibes with my experience is that there are some streets/intersections where if you don’t inch out and then turn on the red, simply no one will be able to turn. Legality aside, it’s more of a practical issue. One left turn per light cycle is better than none.
In the city where I live, the left turn arrows don’t come on when the light turns green. It’s after the going-forward green light goes to yellow then red that you get the green left arrow. You can turn left out of the seperate turn lane at any time while the green light is on if you have the opportunity, or you can wait, knowing you’ll get an arrow momentarily.
It’s quite nice actually, I wish more lights had this timing. Anyone else seen this?
Having a green phase with no red arrow adds a few extra vehicles per cycle. After the arrows are off, but before the light is red, you can still turn left if the traffic will allow it. Optimizing the vehicles per signal cycle is one traffic engineering goal. (+ throughput = no red arrow)
Another goal is safety. Having a few left turning cars leaking through at the end of a signal phase, or allowing left turners the opportunity to dart through mid-phase, may increase the accident rate at a given intersection. So the red arrow may be added for safety. (+ safety = red arrow)
Another goal is flexibility. A signal with a red arrow could be programmed to allow a green phase with or without the red arrow. There are more options available. (+ flexibility = red arrow)
Another goal is cost savings. It’s going to cost more in equipment, maintenance, and electricity to have that extra red arrow light there. That won’t mean much for one intersection, but multiply it out, and the cost is significant. (+ cost savings = no red arrow)
You can see that there’s no clear imperative to either have or not have the red arrows as a general thing, although certain intersections may require the red arrow. So you get into the area of fads/historical artifacts and personal preferences. If the red arrows were the latest and greatest when a bunch of signals were installed in a given city, well, it’s easier on the maintenance crew if there’s just a few standard signal configurations. So the fad becomes locked in. Also if the Director of Traffic Engineering has a preference, well, that’s the way it’s going to be.
Where there’s an extra arrow on a regular signal here in Stockton, it’s just a green arrow. The arrow turns on toward the end of the through-phase and both greens shift to the regular yellow at the end. So that’s a third way of doing it. It’s got less phasing flexibility, but less cost and less visual clutter in the intersection.
Of course, most dedicated left-turn lanes have their own separate signals, now.
For what it is worth, I was told during my driver training (on road / student driver) that a green left arrow was a “Protected left turn” meaning that while you had the green arrow, you had the right of way, etc.
But then, this is GA traffic lights.
You know, GA, the state with Atlanta… one of the worst crop of drivers.
I run red turn arrows when I feel it is safe for me to do so. I do not need a traffic engineer to make decisions for me.
I have encountered some intersections where the red turn arrow has a purpose: limited visibility of oncoming traffic. In this particular case, a driver can not determine if he has time to make the turn. But in many cases of red turn arrows, visibility is not a problem, and a safe driver can determine if he has time to turn, or not.