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Why aren't left turn signals "yields" rather than "stops"?
January 5, 2014 11:59 AM   Subscribe

I have seen many controlled intersections that have one (or more) left turn lanes with one (or more) straight-through lanes with separate signals for the left turn lane(s) versus the straight lane(s). In general, those intersections are designed to have straight-through traffic going from two opposing directions and then alternate with left-turning traffic going from two opposing directions. When the straight-through traffic is passing, why do the left-turning lanes get a red circular light rather than a green circular light or a yellow circular light?

My questions relate to scenarios where the intersection is passing straight-through traffic but there is no straight-through traffic on the road. In that case, those in left-turning lanes have to wait until the intersection changes. During a left turn, drivers facing a green or yellow circular light must yield to oncoming traffic anyway, so it seems more efficient to give the drivers in the left-turning lanes at least the chance to proceed through the intersection when there's no oncoming traffic rather than force them to stop.
posted by saeculorum to Travel & Transportation (21 answers total) 1 user marked this as a favorite
 
Jerks who partially turn into oncoming traffic in order to slowly force their way through, thus bringing the flow to a grind, probably have something to do with it.

Beyond that, the same question can be asked of red lights in general - forget about left turn lights. Why are there red lights when stop signs are perfectly adequate? And why can't I go through a red if I see there's no traffic coming on the cross street? The answer is of course that stop signs aren't perfectly adequate; at some point they'd be dangerous, so red lights are put in place. And I can't go through a red light that looks safe to me because it's more important for everyone to be consistent and safe than to allow potential idiots to take it upon themselves to decide that they want to save thirty seconds.
posted by Flunkie at 12:10 PM on January 5 [3 favorites]


I don't know. If I had to guess it's because highway designers want to add in extra safety measures to keep idiots from causing wrecks.

But here in Springfield MO we've gotten these new fangled things called "Diverging Diamond Interchanges" They have sprung up all over town, and my understanding is that it's a trial basis....guinea pig thing to see how they work before they go national. In these interchanges you drive on the left, making all left turns open and right turns stops. It also allows for a driver to exit a North or East bound freeway and cross the overpass to get back on the South or West bound freeway without stopping (or in some cases slowing down!). I really like them cause they speed up heavy traffic flow areas, and it's always funny cause you can always tell who is the out of towners...they get to the interchange and stop-confused-not knowing what the hell to do.
posted by QueerAngel28 at 12:12 PM on January 5 [1 favorite]


You are making a case for "unprotected left-turns" (ie all traffic proceeds on the same green light) vs. "protected left-turns" (ie a red or green left-turn arrow controls turns). I do not have a cite handy, but unprotected left turns are considered one of the most dangerous driving manuveours because it introduces additional room for error in human judgment ("I can make this turn between these two cars!") and obstructed views (when two cars going opposite directions are both turning left, they obstruct each others' view of oncoming traffic). Many professesional delivery companies forbid their drivers from making left turns for this reason.

So the answer is: protected left turns exist to make your driving experience safer, and ultimately faster, by reducing the number of accidents on the road.
posted by samthemander at 12:13 PM on January 5 [7 favorites]


I think it has to do with the flow of traffic. In areas where there isn't as much traffic throughout the day, you can have a left hand turn on a green circle (aka a yield turn) because the likelihood of people turning left being able to make that left turn is higher given that they will have to wait for fewer cars to go straight than they would elsewhere. In extremely high traffic areas, or in areas where there are multiple stop lights in row over a relatively short distance, dedicated left-hand turn lanes where a driver can only turn left on a green arrow are more the norm because the flow of traffic must be stopped completely to avoid accidents since too many cars would be going straight whenever a left-turning car would be trying to make its turn. There are many lights in my neighborhood that ought to be converted to dedicated left hand turns now that the city's population has increased in the way that it has. I regularly have to do some death defying things to make certain left turns, and that is not the way lights should operate.
posted by These Birds of a Feather at 12:20 PM on January 5


We have both types here. The problem with unprotected left turns mostly seems to come when the light is yellow. The turning car expects oncoming traffic to stop, so they attempt a turn, while the oncoming car may decide to "race the light". I imagine it's a leading cause of injury inducing accidents, and most of the busier intersections have changed to protected left turns.
posted by backwards guitar at 12:29 PM on January 5


("I can make this turn between these two cars!")

A person turning left across 2 or more lanes has an interesting visibility problem.

If the street has lanes that I've numbered 1|2|L|3|4, and a person in L is going to turn left across lane 2 and then lane 1, an oncoming car in lane 2 can hide another oncoming car in lane 1 from the left-turn driver in L. Likewise, the car in lane 1 (which is trailing the car in lane 2 at just the right distance) also can't see the car in L.

The hazard of unprotected lefts is much lower when there's just 2 traffic lanes plus a left-turn lane.
posted by Sunburnt at 12:31 PM on January 5


Just to be clear, the case you are describing does exist. In that case there is no red light and left turns are protected when the green turn light is lit and unprotected when the green for straight is lit and no turn lights are lit.

Now that traffic signal controllers are more advanced, it's possible for the lights to detect when there is no opposing straight traffic and turn on the green for turning out of order, which can at least partially solve the traffic flow issues.
posted by ckape at 12:33 PM on January 5


Sunburnt, I don't think that configuration is what the OP is talking about. They're wondering why the left hand turn lane is often a dedicated turn that can only be completed when all straight going traffic is stopped, rather than a turn that requires a left turn car to yield to oncoming traffic before making their turn.
posted by These Birds of a Feather at 12:35 PM on January 5


With a left turn signal, whether an arrow or a flashing green, there's always the risk of people just blindly following the traffic in front of them. They see the cars in front of them freely turning left, don't notice the light change from an arrow or flash to a solid green, maybe look and see green, green means go, so they go. Into oncoming traffic. Usually the worst that happens is someone will honk a horn at them, but still.

That said, I've only seen separate left-turn red on crazy intersections where it's totally necessary because Problems. Around here arrows go green arrow - yellow arrow - solid green, and flashing green goes straight to green. But that'll vary depending on how they do things in your jurisdiction.
posted by Sys Rq at 12:36 PM on January 5


What you're describing is common in places I've been. In many places the left turn lanes will be protected during certain hours and allow unprotected left turns during other times. As traffic systems are updated that can also have protected left turns turn green when there's no traffic from the other direction and not have to wait so long.
posted by birdherder at 12:41 PM on January 5


Many of the lights for left turn lanes like this in my city have been or will be upgraded to a new system that gives a flashing yellow arrow for the duration of the green for straight two-way traffic, indicating that you can make a left turn whenever it's clear. They also have the protected green arrow mode for a briefer period during each light interval.
posted by planetesimal at 12:49 PM on January 5


Where I drive, a common commute or trip to a specific store often spans multiple townships. Some jurisdictions only have protected left turns, some have none, and (more commonly) there is mixed use, dependent on the average volume of traffic, number of prior accidents at a given intersection and how many lanes one needs to cross in order to complete a turn.

It is primarily annoying late at night or early in the AM, as you can practically see open road for a half mile yet not have the "permission" to turn.

I mostly chalk the existence of protected turns up to the theory of Lowest Common Denominator.

It's the municipalities' responsibility to keep everyone safe. As such, for those jurisdictions concerned with the safety of left turns, they take into account the unfamiliar driver, poor driver, the new driver, and the impaired driver into their decisions, which ultimately can add needless minutes to a "good" driver's commute, but ensure physical protection from the former.
posted by Debaser626 at 1:07 PM on January 5


Going back to the original question about phasing order, it depends on the sophistication of the phasing of the traffic signal. Many older traffic signals are on a fixed phased system. Look at Figure 4-4 of this sample phase diagram, for instance. Some may be able to mix things up, where if there's no traffic on Phase 2 (the southward side) but traffic on Phase 6, it will display Phase 1 in the interim, until cross traffic is given a green. Dumber signals will insist on progressing through the cycles, or waiting until what would be a cycle for the minor street, before giving a left.

The protected left turn is safest but protected-permissive ("doghouse" signals) enable more traffic flow, although at the risk of the pedestrians who have two turning movements potentially coming at them at all times (as opposed to the right turning movement). Different jurisdictions seem to like different kinds of signals. For instance, the City of Los Angeles has historically just had permissive signals (green balls, no turn arrows) even on major intersections, and only recently added protected-permissive phasing. Meanwhile, suburban cities in Orange County, California have protected left turns everywhere. You also want to use protected left turns if there are sight distance issues.
posted by calwatch at 1:28 PM on January 5


A left-turning driver may be watching for a gap in oncoming traffic and fail to notice a pedestrian.
posted by domnit at 2:13 PM on January 5 [1 favorite]


Just a few data points to aid your search for answers:
posted by WasabiFlux at 2:19 PM on January 5


When the straight-through traffic is passing, why do the left-turning lanes get a red circular light rather than a green circular light or a yellow circular light?

Depends on the intersection. Some are like you describe. Others have a left turn signal indicating that the cars turning left have the right of way. But plenty of intersections have a "Left Turn Yield on Green" sign, indicating that if the left turn signal is not illuminated, cars may still turn left but must yield to oncoming traffic.

Which one you get depends on the characteristics--both physical and traffic--of the particular intersection. Sometimes it makes sense to let cars turn left at will. Other times it makes sense to force them to wait for a protected turn.
posted by valkyryn at 2:56 PM on January 5


We have a flashing yellow arrow on protected left turns here (after the solid green arrow protected phase of the light, it stays flashing yellow until the left turn and straight lights all go red), but not on the busier intersections. There's a particularly annoying one that gives you about 2 seconds of protected green left, so that only one car can get through protected before everyone else waits for an opening during the flashing yellow for the duration of the cycle.
posted by jason_steakums at 4:30 PM on January 5


There are lots of intersections that lack a distinct left-turn signal, and instead just have a sign that says "Left Turn Yield On Green" or something similar. Generally they are lower-volume interchanges.

They are kind of a mess, and I think they are rightly abhorred by modern civil engineers.

One of the reasons is probably that high-speed side impacts — the so-called "T-bone" collision — are really dangerous and tend to cause a high degree of injuries / fatalities. Compared to a head-on or rear-end collision, the car can't protect you nearly as well against another vehicle striking it from the side. So it follows that turning, or really any maneuver when you expose the side of your vehicle to oncoming traffic, should be done with extreme care.
posted by Kadin2048 at 4:42 PM on January 5


What you talking about it now being implemented in many places with the flashing yellow arrow. This is called a permissive left turn meaning you must yield to oncoming traffic.

This is better than just having a circular green light with permissive left turn because it prevents what is called the yellow trap. The yellow trap is when the circular green goes to yellow, people automatically assume the other direction sees the same thing, thinking you are free to turn left as the circular green goes to yellow. However this can be a fatal trap if the lights are sequenced so that the oncoming traffic is still green while you attempt a left turn on yellow. The yellow arrow prevents this by explicitly indicating that you still must yield to oncoming traffic.
posted by JackFlash at 6:36 PM on January 5


I've noticed that the left turn is frequently red when the crosswalk they would be crossing through is green. At least at smaller intersections. I imagine that is because while people may look for cars they rarely look for pedestrians.
posted by magnetsphere at 6:59 PM on January 5


As magnetsphere pointed out, sometimes the red is there to protect heavy pedestrian and/or bike traffic that might otherwise be missed.
posted by lab.beetle at 9:22 PM on January 5


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