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Flashing Yellow Arrows

This page last updated June 3, 2023

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Over the past few years, the implementation of the flashing yellow arrow (FYA) traffic signal has become widespread. While the meaning of the signal is generally well-understood ("yield to turn left"), the reason for its replacement of previous "permissive left" signals is not widely understood, with some cynics asserting it to be an intentional waste of money. This page is intended to explain the reason why the signal was developed, and why it is superior to previous permissive left signals.

(A "permissive left" is when a signal permits you to turn left but the turn is not protected.)



The "Yellow Trap"

To fully understand the reason for the FYA, you have to first understand a traffic situation known as the "Yellow Trap." This occurs when a driver enters an intersection on a green circular signal to wait for a gap in oncoming traffic before turning left (which, by the way, is perfectly legal and a common practice in many places.) While waiting, their signal turns yellow. Wanting to get out of the intersection before the signal changes to red, and believing that oncoming traffic also has a yellow and therefore expecting oncoming traffic will slow down and stop, the driver turns left only to be T-boned by an oncoming vehicle because the oncoming traffic actually still had a green signal. 


Sometimes in this situation, a driver will instead smartly opt to wait for oncoming traffic to actually stop, or may somehow know that oncoming traffic does still have a green, and so they will then continue to wait in the intersection while their light turns red. However, sitting in an intersection with a red light often leads the driver to quickly become anxious, making them prone to take more risky turns, or they will try to back out of the intersection, both of which are undesirable outcomes.

Previous solutions
Because of these dangers, traffic engineers for years had been trying a variety of solutions for the Yellow Trap. The most basic was signage like those below indicating that oncoming traffic had an extended green. While a cheap and easy fix, many drivers did not understand what the signs meant (or just didn't read the sign to begin with) and still ended-up in the Yellow Trap situation.

Traffic engineers in Dallas came up with what is regarded as the first signalized solution. With conventional permissive left turns, the left turn signal shows the same circular signal as the adjacent through signals (see first example below.) In other words, when northbound through traffic has a green or red, the northbound left turn also shows the same green or red circular signal. The Dallas solution was to de-couple the left turn permissive signal from the adjacent through signal and instead sync it with the opposing through signal. That way, when the northbound left turn signal was turning red, the southbound through signal was also turning red at the same time, thus meeting the northbound left turning motorist's expectation that opposing traffic will stop and thus allowing them complete their turn safely. This became known as "Dallas Phasing".

(Confused? Watch the animated comparison of left turn signals in the first site under "Other sites of interest" at the bottom of this page.)

A side benefit of Dallas phasing was that there was increased time for left turns, which helped move more vehicles through the intersection each cycle, thus helping to ease congestion.

To accommodate this change using existing signal displays, engineers had to install louvers in the green circular signal for the left turn so that the adjacent through traffic would not see it and perhaps get confused. These louvers worked reasonably well for that purpose, but also reduced the visibility of the green signal markedly, especially if the signal got out of alignment or was on a span wire and being blown by the wind. So it was less than ideal.

Another problem, though, was that some drivers became confused when seeing red signals for the adjacent through traffic but a green signal for the left turn lane. Because drivers are accustomed to usually seeing this when they have a protected left, many drivers-- either due to instinct and/or not paying close attention-- started turning before recognizing that the green signal was not actually an arrow and that they in fact were supposed to yield. The results of this were often disastrous.

Typical signal display for conventional permissive left turn.
Drivers generally understand to yield when turning left in this situation.
However, the signal phasing could result in a "Yellow Trap".

"Dallas phasing" signal display for permissive left turn when oncoming traffic has a green signal for through traffic and a protected left. Note the louvers in the left turn lane through green signal. While this solved the "Yellow Trap", drivers sometimes misinterpreted this to indicate a protected left turn.

FYA to the rescue
In addition to Dallas phasing, several other jurisdictions across the country were experimenting with other types of displays for permissive left turns. So to establish a national standard, federal traffic safety officials began a study in the mid '90s to evaluate and select the best permissive left signal. Various combinations of different colored flashing and solid arrows and circular signals were evaluated. The FYA, which had been widely used in Europe for some time, was included in the evaluation.  

In driver comprehension studies, the FYA was found to have a high inherent understanding. Perhaps more importantly, drivers who did not understand its meaning tended to interpret it to mean "wait", (i.e. a "safe failure") whereas with the green circular signal, drivers who misunderstood it usually interpreted it as a protected turn (i.e. a "critical failure".)

The FYA also provided a signal indication that was exclusive for left turns, i.e. it obviously had no meaning for through drivers, so therefore seeing it would not possibly cause confusion for through drivers.

And the FYA proved to be more versatile and flexible in terms of possible signal displays and operation than the other displays studied.

Based on the results of the studies, the FYA was field tested beginning in 2006 in localities in Maryland, Oregon, Florida, and Arizona. Those tests confirmed that the FYA solved the Yellow Trap and did so with a generally-understood signal that, when misinterpreted, typically resulted in a safer default outcome. As a result, the signal was approved for general use in the 2009 Manual on Uniform Traffic Control Devices, and Dallas displays were prohibited for new installations.

Why the FYA is easily understood
The FYA is instinctively understood by most drivers because of its conformity with other flashing yellow indicators. With traffic signals, a flashing yellow light means "proceed with caution", so a flashing yellow arrow inherently conveys the meaning "turn with caution".

Safety studies
Several studies of intersections where FYAs replaced legacy permissive left turns have consistently shown significant reductions in left turn crashes. One study of 307 intersections in four states showed a 15-50% reduction in such crashes, and another study of 222 intersections in North Carolina showed a 16-59% reduction in those crashes. (Links to those studies is below.)

Summary
While the problem of the Yellow Trap was mostly solved with Dallas Phasing, using the traditional green circular signal with Dallas Phasing to indicate a permissive left had several problems that resulted in drivers turning when it wasn't safe, as well as problems at intersections with span wires, far side signal requirements, or other geometry that made the use of the required louvered signals difficult or impossible. Additionally, other signal options could cause driver confusion in certain circumstances and also had other operational issues. The FYA was determined to be the best solution for all of those problems.

In short, here are the benefits of the FYA:

  • Solves the Yellow Trap
  • Uses a signal that is generally well-understood
  • Uses a signal that, if not understood, usually results in a safe outcome
  • Uses a signal that is flexible and can be used universally
  • Uses a signal that is exclusive for left turns
  • Increases left turn capacity, thus helping to reduce congestion and driver frustration
  • Reduces crashes 

Other sites of interest

Animated comparison of the various protected-permitted left turn displays
https://www.youtube.com/watch?v=KPKjcPI5Sko
Evaluation of Traffic Signal Displays for Protected/Permissive Left-Turn Control
https://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_493.pdf
Wikipedia article on traffic signalling
https://en.wikipedia.org/wiki/Traffic-light_signalling_and_operation#Indication_of_permissive_turn
Flashing yellow arrow for safer left turns
https://onlinepubs.trb.org/onlinepubs/nchrp/impacts/NCHRPImpacts_493.pdf
streets.mn - The Flashing Yellow Arrow and the Yellow Trap
https://streets.mn/2015/01/07/the-flashing-yellow-arrow-and-the-yellow-trap/
Safety Evaluation of Flashing Yellow Arrow at Signalized Intersections
https://www.fhwa.dot.gov/publications/research/safety/19036/19036.pdf
Evaluating the Safety Impacts of Flashing Yellow Permissive Left-Turn Indications
https://www.mass.gov/doc/evaluating-the-safety-impacts-of-flashing-yellow-
permissive-left-turn-indications-in-massachusetts-approach-level-analysis-final-report/download
Safety Effectiveness of Flashing Yellow Arrow
https://www.safemobilityfl.com/pdfs/Safety%20Effectiveness%20of%20
Flashing%20Yellow%20Arrow_research.pdf


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