Left Turns, The Flashing Yellow Arrow, and the Yellow Trap

September 3, 2016 at 1:48 am | Posted in Traffic Signals | Leave a comment

It didn’t take long after the introduction of traffic signal until left turning traffic became an issue. These were the days before dedicated turn lanes became common, so stuck traffic unable to turn would back up through traffic behind that would otherwise be able to go straight. To solve this, a left turn indication was added that would light for a few seconds.

Note how there’s no warning, the “clearance interval” in traffic signal speak, that the turn phase is going to end, Later came phases designed to let a significant amount of traffic through, not just a few stuck cars. Also there were  longer delays after the left turn phase ended before releasing oncoming traffic, but there was initially no indication for this. This “blind clearance” has been banned for many years but is still present in some older installations in other parts of the country. (This absolutely freaked me out the first time I encountered it, in Keokuk, IA).

Early Indications

With the introduction of protected left turns, it took a while for this indication to be standardized. Some very early indications were white balls, rather than arrows. The indications were originally 8”, but soon grew to the 12” standard. Before the standard pattern came about, there was an alternative style (called the “Chinese Arrow” in collector / enthusiast slang) that had notable curves to the strokes, and another style with the arrow heads filled in. There was an arrow that was neon and even a white arrow has turned up, no doubt related to early white ball indications.

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Early left turn indication with a white ball in the middle section.


Also of note was the “Arroway”, which was a panel that replaced the green indication. There was a 8″ green ball in the center and  red bars and green arrows extending outward would light up indicating prohibited and protected turns.

Later Indications

Later, with left turn lanes becoming more common and roads getting wider, left turns indications started being handled by a dedicated head. The yellow and red were standard 8” balls. Uniquely, California used visors on the red and yellow sections so through lanes would not see them.  Red and yellow arrows came much later, in the 1970s, and red balls in left turn displays were still permitted until the most recent standards update. At one time red arrows were under consideration for being banned due to drivers not understanding them, but this is no longer a problem, drivers have since gotten used to them.

Left Turns 02

California Classic Left Turn Arrangement

Bimodal Arrows:

There exist arrows that can light up either yellow or green in the same section. They’ve never been used in Minnesota until recently, but were much more common out east. One use was to eliminate blind clearance without adding another section, but some agencies used them for new installations, either for signals hung on overhead span wires due to clearance issues, or just to save the cost of another signal section. Uniquely, Bennington, VT has a two section signal with a red arrow and a bimodal that operates as protected only, they didn’t have clearance for a standard three section signals. The older models were fiber optic, but they now come in LED models.

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Unusual two section bimodal in Bennington, VT. The red sign says “No Right Turn on Red Light”

3M also had a programmable visibility signal that was bimodal, it worked by rotating an internal color filter

Protected vs Permitted Phasing

Permitted operation is simply the default, where you have a single green light for all traffic, and left turning traffic has to yield to oncoming traffic. Protected is where you have the three left turn arrows and traffic is not allowed to turn when oncoming traffic has the green. The tradeoffs are efficiency vs safety. With protected only you remove 4 conflicts from the intersection, but dedicated arrows take up time in the cycle, and turning traffic has to wait even if there’s obviously no-one coming. A hybrid approach is protected/permitted, where both modes operate during the same cycle. This is a split the difference approach between efficiency and safety. Left turning drivers are less likely to take chances if they know they’ll get a protected movement eventually, and the permissive phase allows gaps in the oncoming traffic to be utilized.

Left Turns 03

Typical protected / pemissive signal as used in Minnesota

Lead vs Lag

There’s also various trade-offs as to whether arrows should come on at the beginning or end of the cycle. (Lead vs Lag Lefts) Lead lefts have the advantage if there’s no or inadequate left turn lanes and the traffic tends to back up, blocking the through lanes. A lead left allows them to clear before they’re in the way of through traffic. Second, if there’s an unusual volume of left turning traffic, extending a lead left impacts traffic operations less because you can still release the through traffic the same direction during the left turn phase. (And they actually did a survey and found lead lefts are what drivers want.)

Lag lefts have the advantage in that drivers have less of a tendency to continue going through as the arrow turns yellow and red. I’m sure everyone has witnessed it where left turning cars keep going through until oncoming traffic notices they have a green and start edging forward. Secondly, the overall left turn phase can be shorter, as more turning traffic has an importunity to find gaps before the phase begins. Finally, pedestrians have the tendency to step into the street as soon as they see a red on a side street, which would put them in conflict with left turning cars. (I see this all the time at Calhoun parkway and William Berry Parkway; this would be a good location to modify to a lag left just because of pedestrian behavior).

Split phase

Sometimes traffic one direction will have both a through and protected left turn simultaneously, then it’s the other sides turn. The left tun traffic signal heads will be four lights, as in lag lefts the  arrow always ends at the same time as the ball so a yellow arrow isn’t needed. This usually isn’t particularly efficient but is used where there’s a high volume of turning traffic relative to through (like a pair of shopping mall exits at a wide suburban road), or there simply isn’t room for adequate left turn lanes and it was determined a protected phase was necessary. In this setup it’s really easy for lanes to be used for both through and turning traffic as cars will never wait to turn blocking through traffic.

Split phase display, can also be used for lag protected / permissive as neither require yellow arrows.

The Problems with Permissive

Over time permissive turns became an increasing safety issue, as traffic volumes in general grew, and drivers becoming used to protected turns didn’t realize they had to yield on green ball. (Yes, this is a real issue, a permissive phase had to be yanked from a traffic signal in Stewartville, MN because too many left turning drivers were not yielding on green) Another issue is that and lead/lag protective/permissive phasing (the most efficient way to operate coordinated intersections) creates an extremely dangerous situation called the “Yellow Trap”. Various local solutions were tried, including “Dallas Phasing”, (in Texas), flashing red arrows (in Delaware), flashing red balls- (in Michigan- where they were treated like a yield in practice, and Maryland), and flashing yellow balls (Washington state). So it was obvious a national solution was needed. This led to the development of our new friend, the flashing yellow arrow.

The Flashing Yellow Arrow

The flashing yellow arrow is really the first new indication since the hand/man pedestrian signal was developed in the 1970s. Besides the desire to increase efficiency, it was driven by two problems. The first is that in practice some drivers really were too stupid to realize that a green ball means they had to yield to oncoming traffic. Mn/DOT actually had to convert the light on US 63 in Stewartville to split phase because of this issue.

The second reason is it eliminates the “yellow trap” in lead lag protected/permissive operations The most efficient way to operate a protected/permissive operation with coordinated signals is lead/lag. This was figured out not too long ago with computer modeling which is why lag lefts have not appeared en masse until recently. But this introduces a huge problem, the “yellow trap” is created, an extremely dangerous situation where two drivers each expect the other one to stop. This is a lot easier to explain with pictures; the red car is about to get “yellow trapped”.


The Red Car pulls into intersection waiting for a break in traffic to make a left turn.


The red car driver doesn’t get a chance because of the steady stream of grey cars. Then the lights change to yellow and he or she starts to panic.


What the red car driver thinks is going to happen, “Cross traffic will get a green and I’ll be blocking traffic, or worse yet, get run into by it. That blue car is getting a red too, so now’s my chance to move since it’s going to stop…”


The “yellow trap” is so dangerous that in an almost unprecedented move in 2003 federal standards required existing equipment to be modified so it’s either eliminated or warned of with a sign (typically  reading “Oncoming Traffic has Extended Green”) within 5 years. More typically non-compliant installations may remain for the remainder of their service life.

The issue is created by the fact that conveying a permissive for left turn and a stop for through traffic was problematic. You can’t just display a green ball to left turners, because through traffic would see it while they had the red, and get confused. For a while Texas would add louvers to the left turn signal (Called “Dallas phasing”), but this only saw limited use, was problematic in that louvers (and programmable visibility heads) tend to make signals more difficult to see, even in the desired direction, and didn’t solve the emerging problem of drivers not yielding on green balls. Traffic engineers are a pragmatic bunch; if they see what they perceive as a real problem they will try to see if there’s a way to fix it rather than retort that drivers should just go back to traffic school, and the flashing yellow arrow was born when multiple agencies realized a national fix was needed.

Dallas Phasing signal

Issues with Flashing Yellow Arrows

There are a few issues that have come up. One of them is the “disco effect”. In an urban environment with traffic signals every block (or even more), all the arrow flashing at different rates can definitely add to the visual clutter. Since traffic is typically slow moving in this situation, some agencies put lower wattage lamps in the flashing yellows, but with the conversion to LEDs all of them are the same brightness and they cannot be dimmed.

The second is a thing called the  “perceived yellow trap”, if the driver was looking down texting on his cell phone, and then looks up at the flashing yellow arrow when it’s illuminated at the same time the other lights are turning yellow, the driver may think may think he or she’s about to get “yellow trapped”. But this is much less of a problem than the real yellow trap, because it will usually be obvious he or she is mistaken before getting a chance to start out.

Finally, some drivers don’t yet understand the meaning. The “Left Turn Yield on Flashing Yellow Arrow” signs are meant as an interim measure to help drivers get used to them; discussion is under way whether the signs should remain for the remainder of their service life, or be removed after a couple of years. But more signs aren’t always better, besides the expense signs that warn of ordinary conditions add to clutter and cause drivers to tune out signs. So policies vary as to when to erect signs. In Cloquet they installed some flashing yellow arrows without the usual sign in an area without any previous installations, and drivers thought it meant they had the right-of-way, causing several accidents. Mn/DOT disabled it until they could get a sign erected.

After this article was first published, in Dakota County, which does not use signs, there was a fatal crash which was alleged to be caused by a flashing yellow arrow. A member of the TV media found my home phone number and called me up wanting to help do a story. I never got back to him, primary because generally the only people that leave messages on my land line want to help me lower my interest rates or take advantage of the financial stimulus, but more-so I didn’t want to be a talking head on TV, and had nothing to offer beyond the content of this article. Just by allowing permissive terms, flashing yellow arrows are less safe than protected turns, but with how much more efficient they are while being safer than green balls, it many situations it’s an allowable risk. The same head can operate in protected only mode during peak periods if safety requires it.

Flashing Yellow Arrows for Right Turns

There’s also been limited use of flashing yellow *right* turn arrows, in heavy pedestrian areas to reinforce the need for right turning traffic to yield to pedestrians. A creative but unapproved proposal has a white pedestrian “man” flash alternatively with the yellow arrow in the same indication to remind drivers that pedestrians may be present. In some countries in Europe right turns on red are usually not allowed; the presence of a flashing yellow arrow indicates and exception, and drivers must yield to pedestrians but do not need to come to a stop before turning.

Bimodal Arrows:

There exist arrows that can light up either yellow or green in the same section. They’ve never been used in Minnesota until recently, but were much more common out east. One use was to eliminate blind clearance without adding another section, but some agencies used them for new installations, either for signals hung on overhead span wires due to clearance issues, or just to save the cost of another signal section. Uniquely, Bennington, VT has a two section signal with a red arrow and a bimodal that operates as protected only, they didn’t have clearance for a standard three section signals. The older models were fiber optic, but they now come in LED models.

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Unusual two section bimodal in Bennington, VT. The red sign says “No Right Turn on Red Light”

Flashing Yellow Arrow Configurations

Four light heads: This is of course the standard configuration. It was thought that having the yellow arrow change position as well as light steady would focus attention on the fact that the phase was about to end. An additional benefit over the old permissive /protected standards is that lead lefts have a short red phase. This allows oncoming traffic a chance to take over at the very start of their turn, rather than wait until someone turning left finally stops.

Three light heads: Due to the obvious advantages of being able to use existing wiring and hardware, various configurations using thee section heads have been proposed. The first is simply replacing the green ball with a flashing yellow arrow in the lower section for permissive only turns. But utilizing only three sections for protected/permissive operations has also been proposed in two different configurations. One of them uses a bimodal arrow in the lower section.

A later proposal was to maintain the standard configuration but to either flash or light steady the yellow arrow. They did a study and found that the former increased accidents but the latter did not, so the latter is now allowed as an option. Presumably it will primarily be used in retrofit applications, and Mn/DOT has stated it is not allowed under any circumstances in Minnesota. The three light permissive only indication was originally allowed by Minnesota, but pulled before any installations actually went up.

Left Turns 07

Three Section Head

Five Light Heads: Minnesota has quite a few four-lane undivided “death roads”, some of which have protected/permissive phasing. Since there is no left turn lane, standard four section head with arrows wasn’t an option. Instead Minnesota, with consultation with the Federal Highway Administration,  developed a 5 light configuration. The lower left arrow can light either steady green or flashing yellow. This is the first use of a bimodal arrow (and the first use of the so-called “doghouse” layout for permanent mountings) in Minnesota. This is also used for split phasings, where it operates part time as split phase and part time as permissive.

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“Doghouse” Flashing Yellow Arrow, started by Minnesota, used by other states


The rate of adoption varies; retrofitting an existing installation requires a substantial investment, besides new signal head,  wiring, and a firmware upgrade sometimes a complete new cabinet is required. The city of Woodbury is in the middle of a five year project to retrofit all the signals it owns that are eligible and is also working with other agencies that own signals in the city. Meanwhile Minneapolis continues to violate state and national standards by erecting new “yield on greens” over turn lanes. Most agencies, besides using them for all new installations, are retrofitting existing signals here and there.

Overall, flashing yellow arrows are safer than green balls for protected/permissive, but more dangerous than protected only. However traffic engineering is always a balance between efficiency and safety, and they are safe enough that in almost all cases it tips the balance to allowing permissive turns at least part of the time. The next article in this series will look at pedestrian signalling.

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