The Flashing Yellow Arrow and the Yellow Trap

Here is part 3 in my ongoing series of articles on traffic signals. Part 1 covered vehicle signals in general, and part 2  covered left turn indications specifically. This is really a continuation of part 2, so I won’t re-explain some terminology covered there. But flashing yellow arrows are important enough to deserve their own article.

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”.

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

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

Red car sees the lights change to yellow and starts to panic.

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 thinks is going to happen: cross traffic will get a green and s/he will block traffic, or worse yet, get run into by it.

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…”

Better move before cross traffic gets the green. The light's turning red so that blue car is going to stop.

But this happens. Red car driver starts to make the turn figuring that the blue car has a red and is going to stop. But the blue car driver actually has and sees the green and thus isn’t paying close attention since sh/e has the right of way. Pretty soon red and blue are exchanging insurance information with the white car as a witness.

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

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. 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. And I’ve personally witnessed drivers sitting at an arrow patiently waiting for a green, even with no one obviously coming. 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.

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.

Fiber Optic Bimodal Arrow- Green Mode

Fiber Optic Bimodal Arrow- Green Mode

FIber Optic Bimodal Arrow- Yellow Mode

FIber Optic Bimodal Arrow- Yellow Mode

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

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.

Three Section Flashing Yellow Arrow- Not for use in MInnesota.

Three Section Flashing Yellow Arrow- Not for use in MInnesota.

Five Light Heads: Minnesota has quite a few four-lane undivided 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.

Flashing Yellow Arrow indication for "Four Lane Death Roads"

Flashing Yellow Arrow indication for “Four Lane Death Roads” and split phase / permissive.


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.

About Monte Castleman

Monte is a long time "roadgeek" who lives in Bloomington. He's interested in all aspects of roads and design, but particularly traffic signals, major bridges, and lighting. He works as an insurance adjuster, and likes to collect maps and traffic signals, travel, recreational bicycling, and visiting amusement parks.

13 thoughts on “The Flashing Yellow Arrow and the Yellow Trap

  1. Sean Hayford OlearySean Hayford Oleary

    I appreciate your explanation of the yellow trap issue. I genuinely had assumed the main benefit of the flashing yellow arrow over Left Turn Yield on Green was “some drivers really were too stupid to realize that a green ball means they had to yield to oncoming traffic” — and the flexibility to use a red arrow. That seemed ridiculous.

    But the yellow trap issue is a real one, even when both directions get a red light at the same time.

    However, I don’t think the annoying factor of the flashing yellow should be overlooked. The only thing equivalent is the flashing upraised hand on signals that have pedestrian signals by default — and that is not as bright, as large, or as high off the ground. Is there a reason dimmer LED modules couldn’t be used like the dimmer bulbs, for more urban environments?

    Also: any thoughts on applicability to Hiawatha Avenue? I believe it previously had permissive lefts (like Olson Memorial Highway) but went to fully protected lefts with the installation of light rail. Is it conceivable they’d go to flashing yellow arrow (when no trains are in the vicinity) since they still have the option of enabling a red arrow when trains are around?

    1. Monte Castleman Post author

      There’s specs for how much light a traffic signal needs to put out, and they design LED modules to meet them over the life of the module, therefore LEDs are even brighter than incandescents when new to offset lumen depreciation so they’re still meeting specs several years down the road. Companies don’t offer dimmer modules that don’t meet specs, and they feel no need to bother making them capable of being dimmed. An incandescent signal that’s dimmed or has a lower wattage bulb probably isn’t meeting specs either, but it’s not like anyone is going around checking; that option just doesn’t exist any more with LEDs.

      Conceptually I see no reason why flashing yellow arrows wouldn’t work on Hiawatha; but practically the city of Minneapolis seems loathe to use them to the point of erecting new “Yield on Green” signals in violation of state and national standards, and the Hiawatha signals interacting with light rail and each other are so complicated that altering the phasings would be extremely tedious and costly.

  2. Matt SteeleMatt Steele

    I enjoy and appreciate these articles. Thank you for writing them.

    I echo Sean’s take on the disco effect. Although I would actually reverse it, and say that flashing do-not-walk hands I’ve seen are likely more intrusive, since they are on the sides of streets and at a level that enters ground floor windows. I doubt louvers are needed for flashing left yellow to counteract disco effect except in unique placement circumstances. Though I’d love to see many do-not-walk hands dimmable so they can be bright during the day and less bright at night.

    I also like Sean’s idea for permissive lefts on Hiawatha when no train is present.

    Finally, I can imagine a variety of places, especially in the suburbs, where five light doghouse configurations would allow for split-phase operation and permissive lefts during peak periods, but would remove a necessary phase during off-peak periods (Buck Hill Road at Southcross Drive in Burnsville as an example). Though I’d rather just see a 4-3 road diet restripe on many of those roads.

    1. Monte Castleman Post author

      A big flashing hand is a probably a lot more annoying than the old 9″ Don’t Walk’s, also LEDs are deliberately made brighter than incandescents so they still meet specs after a number of years of lumen depreciation. It would be trivial to make dimmable LEDs, but there’s been no interest in doing so.The old 3M and McCain programmable visibility heads actually had automatic dimmers that need to be disabled when they’re retrofitted for LEDs, and most modern traffic controllers could dim the outputs, however it would damage any LEDs.

      There was a company called Optisoft that made LED signals with automatic dimmers, as sense and compensate for lumen depreciation so they didn’t need to make them overly bright to start, but they were run out of business when they’re LED conversion kits for neon pedestrian signals had a design flaw and burnt out prematurely, leading to a lawsuit by the city of LA.

      At Buck Hill, that could easily be a four-light flashing yellow arrow protected / permissive, but I don’t see that as a split phase. Split phases are very inefficient unless a large percentage of the traffic is turning and I don’t see that as the case there. The first 5 light “doghouse” in Minnesota was Eden Prairie at Valley View Road and Prairie Center road. One side of the intersection is an office building and the other side is a major street, so very little traffic goes straight and the signal operates at split phase during peak periods, and permissive (with flashing yellow arrow) at other times.,-93.4185816,3a,75y,249.1h,86.31t/data=!3m4!1e1!3m2!1sJm2vsLn8yl6HqcnHNiDnqA!2e0

      1. Matt SteeleMatt Steele

        I’m fairly confident Buck Hill Rd operates as a split phase 100% of the time. At off peak hours, traffic is usually so light that you just wait for the loop detectors to kick in and give you a green. But I’ve never seen an instance where the green arrow is separate from the green ball (and that would necessitate a yellow arrow, correct?), and I’ve never seen an instance where I get a green at the same time oncoming traffic gets a green.

  3. Joe

    Great article! Very good information!

    I had a question about protected left turns in St. Paul. Prior to the Green Line running down University, I believe there was very few protected left turn signals in the city. The one protected left turn signal that always left me scratching my head was the signal at 7th St. and Payne Ave. Does anyone know the reasoning behind having a protected left turn signal there to go north on Payne from East 7th?

    Travel anywhere down 7th and from my memory I don’t think there’s another protected left turn signal at even busier intersections. There is many protected/permissive signals along 7th, but none like on this particular intersection.

    Would love to hear some insight if you guys know what signal I’m talking about. Thanks!

    1. Students_TT

      Usually if there’s sight distance issues, they won’t put in flashing yellow arrow. Is this spot on a grade or something? Or maybe there’s quite a bit of thru traffic westbound so the engineers thought that it would be too hard to find a gap to cross? Raw speculation though.

    2. Sean Hayford OlearySean Hayford Oleary

      There don’t appear to be any sight line issues, or anything unusual about this intersection (like five legs or something). My only observation here is that it appears to be a very old signal that also has a dedicated left-turn lane. It is possible that at one time, St. Paul’s preference was to do fully protected lefts when a turn lane was present. (Almost all of the older permissive lefts are for left turns made from the main travel lane, and the adjacent ones corresponding to dedicated left-turn lanes are newer equipment.)

      1. Sean Hayford OlearySean Hayford Oleary

        A similar example is Cedar and Riverside in Minneapolis — also sort of a three-way (technically a four-way, but the fourth leg is very low-traffic). It may be the only fully protected left on Cedar, but there it is.

  4. Mikey

    Personally, I’d love to see St Paul upgrade the left turn on Shepard Rd to Washington (upper landing) to a flashing yellow – it would likely shave two minutes off my morning commute.

    On the topic of conversions, Dakota Co recently switched the protected/permitted signals at Oakdale and Mendota Rd to yellow flashers. Unfortunately, the yellow only flashes after 7PM, basically it’s now a protected phase only all day long – even on weekends. It’s frustrating having to wait now because some traffic engineer somewhere decided I only was smart enough in the evening

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