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Flashing Yellow Arrows

This page last updated July 29, 2016

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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 asserting it to be a 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.  While waiting, his signal turns yellow.  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. 


Why did this happen? 
Because the waiting driver did not realize that oncoming traffic still had a green light and that his signal was turning red to allow for a protected left for oncoming traffic.

Sometimes a driver will realize that oncoming traffic does still have a green and so they will then continue to wait in the intersection while their light is red and their blood pressure increases, 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.  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 allowing a northbound motorist waiting to turn left to complete his 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 each cycle for left turns, which helped move more vehicles through the intersection.

To accommodate this change using existing signal displays, engineers had to install louvers in the green circular signal for the left turn so that through traffic could not see it and perhaps get confused.  These louvers worked, but reduced the visibility of the green signal markedly (especially if the signal got out of alignment), so it was less than ideal.

A major 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 an arrow and that they in fact were supposed to yield.  The results of this were often disasterous.

Typical signal display for conventional permissive left turn.
Drivers generally understand to yield when turning left in this situation.

Typical signal display for permissive left turn in "Dallas phasing" during
oncoming protected left phase. 
Drivers sometimes misinterpreted this
to indicate a protected left turn.


FYA to the rescue
To solve the problems of the Dallas display, federal traffic safety officials began a study in the mid '90s to evaluate and select a better signal solution.  Various options were considered including various combinations of different colored flashing arrows and circular signals (e.g. flashing red arrow, flashing red circle, flashing green arrow, etc.)  The FYA, widely used in Europe for some time, was included in the evaluation.  

In driver comprehension studies, the FYA was found to have the best inherent understanding of the solutions studied.  Furthermore, 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 proved to be more versatile 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 confomity 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 left with caution". 


Other sites of interest

Animated comparison of the various protected-permitted left turn displays
https://www.youtube.com/watch?v=KPKjcPI5Sko
Wikipedia article on traffic signalling
https://en.wikipedia.org/wiki/Traffic-light_signalling_and_operation#Indication_of_permissive_turn
FYA research project website
http://projects.kittelson.com/pplt/index.html




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