FREQUENTLY ASKED QUESTIONS
ABOUT
FLASHING YELLOW ARROWS

Many people don't understand the purpose of flashing yellow arrows (FYA) on traffic signals. Here are some questions and answers to help you understand them:

• What does it mean when a traffic signal shows a flashing yellow arrow?

  It means that traffic turning in the direction the arrow points may turn, but must first yield the right-of-way to conflicting vehicles and pedestrians before turning.

• Isn't this the same as a circular green?

  No.

  • For left turning drivers, flashing yellow arrow and circular green have the same meaning.
  • But their meanings are very different for drivers who are NOT turning left.
    • Drivers not turning left must look at other signal faces, instead of looking at the flashing yellow arrow.

  This lets the signal completely separate control of the permissive left turn from control of the straight-ahead traffic.

• Isn't this just dumbing down traffic signals? Do drivers need special instruction to know when to yield?

  No, it is not. That is not why the flashing yellow arrow is used.

  Flashing yellow arrow provides an indication that can not otherwise be displayed. This indication lets drivers make permissive turns while the straight-ahead traffic is stopped with a circular red. The circular green can't possibly do this.

  This new indication provides the following advantages:

  1. It prevents a hazard called yellow trap (caused by permitting left turns on the circular green).
  2. It gives traffic engineers more choices of light sequences the signal can use.
  3. These sequences include a lead-lag sequence with permissive turns.
  4. The lead-lag signal allows traffic engineers to progress green lights on more two-way streets.

  More permissive turns and more progression mean saving gasoline.

• So why did a local politician say it was meant to increase driver understanding?

  He did what politicians always do - opened his mouth.

  There are several possible reasons:

  1. As a layman in the field of traffic control, he probably does not understand either yellow trap or signal progression.
  2. He may have read the report on driver understanding of the flashing yellow arrow. Since a new indication was needed, it had to be tested for driver understanding, to prevent unexpected hazards. But this was not the intended purpose of the flashing yellow arrow.
  3. He might have been asked a question he didn't know the answer to. So, instead of saying, "I don't know," he guessed.
  4. He might have asked someone in the traffic department who also didn't understand it.

What's wrong with just using the circular green? We know what that means. Nobody can easily obtain the meaning shown at right using a circular green indication to indicate a permissive left turn. The display shown indicates: Straight and right turn indications are red, so those drivers must stop. The left turn is a permissive turn through gaps in oncoming traffic. The following capabilities are revealed: The flashing yellow arrow can allow the permissive turn to be made while the straight ahead signals are red. The circular green can't do that, because it also tells other movements to go.

Special Movement With Flashing Yellow Arrows

Dallas Phasing: Dallas Phasing (also called the Dallas Display) was an attempt to use the circular green for a permissive left turn without causing yellow trap. It was installed in some states between 1992 and 2009. Dallas Phasing required vision-restricting louvers or optical programming on the circular green and circular yellow in the left turn signal, to keep drivers going straight or right from seeing them. This was intended to keep from confusing straight-ahead drivers. Drivers turning left had trouble understanding this display. Many didn't understand the meaning of the one circular green light and two circular red lights they saw. Also, Dallas Phasing can't be used on span wires without the wind twisting the signals, showing the hidden signals to the wrong drivers. The 2003 Manual on Uniform Traffic Control Devices (MUTCD) approved Dallas Phasing, but the 2009 MUTCD prohibits it in favor of flashing yellow arrows.

This display is the Dallas Phasing equivalent to the flashing yellow arrow display shown above, as seen by a left-turning driver.

• Why do traffic engineers need to do that?

  For the following reasons:

  1. To prevent the hazard of yellow trap.
  2. To have a longer permissive turning period in the signal cycle.
  3. To have more signal sequences available without the hazard of yellow trap.
  4. To be able to put signal progression on more two-way streets.
  5. To make traffic use less gasoline.

  The benefits of these changes far outweigh any problems caused by removing permissive turns on circular green from the signal.

• What is "yellow trap"?

  Yellow trap occurs when all of these come together at the same signal:

  • Left turns are allowed on the circular green on the affected street.
  • The signals for both directions of the same street are showing circular green.
  • The signals in one direction turn yellow, while the signals in the other direction remain green.

  • 1 Circular Greens	2 Signal Changes	3 Yellow Trap

  • Most drivers expect the circular greens facing in both directions to end at the same time. Then they come to a signal that doesn't do this.
  • The driver getting the yellow light thinks the signals in both directions changed to yellow.
  • He thinks that the traffic coming from the other way will stop.
  • He might turn across live traffic that still has a green light. A crash might result.
  • If he doesn't turn, he might be trapped in the intersection with a red light.

  Yellow trap is mainly caused by allowing permissive left turns on a circular green.

  Yellow-Trap Sequence

  
• What is "second yellow trap"?

  Second yellow trap occurs when all of these come together at the same signal:

  • Left turns are allowed on the circular green or flashing yellow arrow on the affected street.
  • Oncoming right turns receive a green arrow before or during the yellow clearance for that left turn.
  • The right turning traffic blocks the path of the left turns as they try to clear the intersection.

  • 1 FYA Left Turns	2 2nd Yellow Trap	3 Next Phase

  • The driver getting the yellow light thinks oncoming right turns will stop. He turns across live traffic that has a right green arrow. If he doesn't turn, he might be trapped in the intersection with a red light.
  • The right turn green arrow must not be displayed until left turns have had time to clear.
• What is "third yellow trap"?

  Third yellow trap occurs when all of these come together at the same signal:

  • Left turns are allowed on flashing yellow arrow on the affected street.
  • The flashing yellow arrow is inhibited or delayed during the pedestrian WALK interval
  • The left turn signal is showing flashing yellow arrow. Oncoming traffic has a circular green.
  • There is no traffic waiting on the cross street.
  • A pedestrian wanting to cross the crosswalk to the left of the left turning driver pushes the pedestrian pushbutton.
  • The left turn is illegally given a steady yellow arrow when the oncoming circular green is reserviced to answer the pedestrian pushbutton.

  • 1 FYA Left Turns	2 3rd Yellow Trap	3 Illegal Next Phase

  • The driver getting the yellow light thinks oncoming traffic is being stopped, and turns across live traffic that still has a green light. If he doesn't turn, he might be trapped in the intersection with a red light.
  • The flashing yellow arrow was illegally ended so a pedestrian phase has no left turns crossing it.
  • The next phase is illegal only because it follows the flashing yellow arrow phase with no intervening all-red period for the straight-ahead movements.
• What is Little Yellow Trap?

  Little yellow trap is when the circular greens for both directions end at the same time, but the circular yellows do not. Little yellow trap happens whenever the yellow clearance intervals of the two circular green phases are set to different values.

  When the shorter yellow turns red, the driver thinks the other direction is red too, and completes his turn. Although it is unlikely to happen, an oncoming straight-ahead driver could continue on through during this period, causing a crash.

• What is Green Trap?

  Green trap occurs where there are no left turn phases, but the circular greens facing opposite directions can start at different times. This can fool a left turning driver when an opposing driver suddenly gets a circular green and speeds up.

  In green trap, a left turning driver sees an oncoming car slowing down for a red signal, so he thinks he has the right-of-way. As the oncoming driver enters the detector, his signal suddenly turns green, and he speeds up to go straight or right. He enters the intersection in the path of the left turning vehicle. This can cause a crash.

  Green trap happens when opposite circular greens on the same road have their own phase units, but no other phase units are in the concurrency group. The problem happens when one phase stays red until a car appears on the approach.

• Isn't this "yellow trap" just a case of drivers breaking the law?

  Yes and no. The problem is that each driver thinks a different law applies at the moment:

  • The driver receiving the yellow light thinks both directions have a yellow light. The law he is concerned with is getting out of the intersection before the red light. And he expects the driver coming from the other direction to stop for a yellow light.
  • The driver with the green light does not know the signal has changed. He still sees a green light. The law he is concerned with is that turning drivers must yield to oncoming straight ahead drivers.

  The problem is that each driver expects the other driver to stop. When the other driver does not stop as expected, the accident occurs.

• How does the flashing yellow arrow prevent yellow trap?

  The following is a case of yellow trap, followed by a case where flashing yellow arrows prevents yellow trap.

  • Yellow trap occurs when all signals in one direction turn yellow, while the signals in the other direction remain green.

  • 1 Circular Greens	2 Signal Changes	3 Yellow Trap

  • The driver thinks signals both ways turned yellow. To leave the intersection, he might turn across live traffic with a green light.
  • Yellow trap is prevented when the flashing yellow arrow permits the turn to continue until the oncoming signal changes to yellow:

  • 1 Circular Greens w/ FYA	2 Change FYA Still On	3 Safe Turns w/ FYA

  • Circular signals change to yellow and red. The FYA keeps flashing. The driver knows the oncoming traffic still has a green.
  • Note that the flashing yellow arrow must be flashing at ANY time the oncoming circular green is lighted.
• Why are flashing yellow signals needed in both directions on the same road?

  Each flashing yellow arrow prevents the yellow trap caused by the circular green and green arrow facing the other way on the street.

  Even if only one direction of flow has a green arrow, the flashing yellow arrow is needed in both directions.

• How do I know if the flashing yellow arrows have protected me from yellow trap?

  When you see that the circular signals are yellow or red and the flashing yellow arrow is flashing, the sequence has protected you from yellow trap.

• Can the flashing yellow arrow signal face tell you what color the oncoming circular signals are?

  Yes. The flashing yellow arrow left turn signal face tells you the color of the oncoming circular indications whenever it permits a movement:

  LEFT TURN SIGNAL	LEFT TURN MEANING	SIGNALS FOR ONCOMING TRAFFIC		ONCOMING TRAFFIC CAN NEVER HAVE THESE SIGNALS
  		EXPECTED SIGNAL	ADDITIONAL POSSIBLE ONCOMING SIGNALS
  	Stop and stay	ANY		None
  	Stop, turn when safe		Double Clear:	* *
  	Prepare to stop	or		* *
  	Yield to conflict		Double Clear:	* *
  	Go - protected turn
  Night Flash	Stop, turn when safe	or		*
  Night Flash	Yield to conflict			*

  * Right turn indication may be allowed if the right turn enters an exclusive lane.

• What is progression?

  Signal progression is the timing of traffic signals relative to each other, so the lights on the progressed street turn green as the driver comes to them. Progression is beneficial, because drivers don't have to stop as often for red lights. This saves gasoline.

• Isn't progression as simple as finding the travel time to the next signal, and making it start the green then?

  That works only when the street is an isolated one-way street.

  The problem with doing that on a two-way street is that any timings set that way are wrong for traffic going the other way on the street. A complicated procedure is needed to cause progression to occur in both directions on a two-way street. Even with a one-way street, progression on the cross streets must also be considered.

  One tool that can make progression work on more two-way streets is the lead-lag signal sequence. But without the flashing yellow arrow, the lead-lag sequence must have left turns stopped at red arrows, instead of allowing permissive turns through gaps in oncoming traffic.

• What is a lead-lag signal?

  The following are facts about the lead-lag signal:

  • A leading left turn arrow occurs before the oncoming traffic gets its circular green.
  • A lagging left turn arrow occurs after the oncoming traffic gets its circular green.
  • The lead-lag sequence (using flashing yellow arrows) operates in the following order:

  • 1 Leading	2 Thru	3 Lagging	4 Cross street

  • The orange dashed arrows show left turns made using flashing yellow arrows.
  • If flashing yellow arrows are not used, the orange arrows would have to be deleted from diagrams 1, 2, and 3 to prevent yellow trap. Those turns would be stopped with red arrows, instead of being permissive. (In either case, the orange arrows in diagram 4 show left turns made from the cross street on a circular green.)
  • The lead-lag sequence allows the traffic engineer to adjust the signal to meet the arrival times of the platoons of cars coming from each direction. In this case, the timing plan has cars arriving from the left coming earlier than the cars that arrive from the right.

  FYA Lead-Lag Sequence

  
• Why not make all signals use the same sequence of lights in all directions? It's simpler and more uniform.

  There are several different reasons that different sequences are needed:

  • The alignment of the intersection might not allow simultaneous left turns in both directions.
  • If one or more approaches don't have exclusive left turn lanes, then the left turn signals facing them can't be green when the circular signals are red.
  • Widely differing traffic demands on different approaches might demand another sequence.
  • Different sequences are needed at different intersections in a progression system to optimize progression. This saves gasoline.
• Why does the flashing yellow arrow face require two yellow arrows? Why not just flash the one used for clearance?

  The Manual on Uniform Traffic Control Devices (MUTCD) requires that the yellow arrow used for clearance must be above or to the left of the yellow arrow used for the flashing yellow arrow permissive turn. This is so the arrow changes sections in the signal face when the signal changes, in the same way that the lighted section moves when the circular green changes to circular yellow.

  Look at the model at right. Notice how the arrow moves up when the signal changes. It is much more visible than just stopping the flashing of the arrow.

  It also makes the signal face more distinctive, so the driver knows it is a flashing yellow arrow face instead of an exclusively protected face. The vertical 4-section signal face is not used for much else except in signals using split phasing.

• The flashing yellow arrow is flashing while the circular lights are red. Isn't this wrong? It seems wrong.

  It is operating correctly. This is the part of the sequence that prevents yellow trap and provides all of the other advantages.

  The flashing yellow arrow flashes at all times when the oncoming circular green is lit. This is the only way to remove yellow trap from permissive turns.

• Why not just replace the circular green in the left turn signal with a flashing yellow arrow, but use the same sequence the circular green used?

  This is the wrong way to install a flashing yellow arrow. It causes the same yellow trap the circular green causes.

  In order for the flashing yellow arrow to prevent yellow trap, the flashing yellow arrow must be tied to the oncoming circular green, not the circular green facing the same way the flashing yellow arrow faces. The circular green facing the same way has the wrong timing, causing yellow trap.

• If I don't understand the flashing yellow arrow, can't I just look at the circular indications and obey those?

  NO!!! That is dangerous and illegal.

  When a flashing yellow arrow face is installed, it is the only face controlling left turns. The circular indications often show indications that are wrong for the left turn driver to obey.

  The entire purpose of the flashing yellow arrow is separating the permissive turn timing from that of the circular green.

  If you can't learn this simple addition to traffic signals, or if you can't drive anywhere signal sequences are different, you do not belong on the road.

• Why are they wasting money fixing something that isn't broke?

  It is broke. It has been broken for at least 80 years.

  Yellow trap has been around as long as left turn signals have existed. And up until now, there has been no standardized fix that worked:

  • The problem was not recognized until the mid-1950s. Officials and police assumed the accidents caused by yellow trap were caused by driver inattention, failure to yield, or someone in too much of a hurry.
  • The original "fix" was to avoid lagging left turns. This is why many city signal systems are inefficient.
  • Another "fix" was to prohibit the left turn from the direction opposite to the lagging left turn - inconvenient for other drivers.
  • Indiana used a steady yellow arrow for a permissive turn in the 1950s. The arrow went out for a clearance period.
  • Special circuitry had to be installed in most signal controllers to keep them from skipping phases into a yellow trap.
  • The first real development was the flashing circular red used for permissive turns in Michigan. But it required each turning vehicle to stop. Maryland and Virginia later used the same signal.
  • Delaware, Maryland, and California used a flashing red arrow instead. It still required each turning vehicle to stop.
  • Washington (state) adopted a flashing circular yellow in the position normally occupied by the circular green. It was very effective.
  • In the late 1980s, Dallas Texas started using what is called "Dallas Phasing" to eliminate yellow trap. It showed the opposing circular green and yellow to left turning drivers, instead of the adjacent ones. Visibility-limited signals were used. But it confused drivers who saw their own left turn signal with a circular green, while the other signals had circular red. This, plus the fact that straight ahead drivers thought the circular green was burned out, caused many false maintenance calls.
  • Nevada first experimented with the current flashing yellow arrow signal face in the 1990s.
  • After a 10 year study period, the flashing yellow arrow signal was adopted into the US Department of Transportation's Manual on Uniform Traffic Control Devices (MUTCD) on December 12, 2009.
• Have there been any problems with the flashing yellow arrow?

  Yes. There have been a few problems:

  • Some states used to define the flashing yellow arrow as the way to display a privileged movement when the signals are on night flash. People from those states got into accidents when they saw flashing yellow arrows while driving in other states.
  • One flashing yellow arrow installation in Oregon was converted to exclusively-protected left turns, due to unusual intersection geometry causing accidents.
  • A study showed that most drivers looking for gaps in heavy oncoming traffic don't look for pedestrians in the left hand crosswalk before turning through a small gap. Their recommended solution of inhibiting the flashing yellow arrow while a pedestrian phase is timing will cause third yellow trap if the phase is reserviced to serve a pedestrian pushbutton call with no calls on the cross street.
  • A very few cases exist where the flashing yellow arrow was installed wrong, causing a yellow trap accident. Most of these have been removed.
  • One accident was caused by a driver from another state who remembered and applied an old definition of a flashing yellow arrow that gave a privileged right-of-way, causing a fatal accident.
  • Some liberal reporters do not understand the concept of the flashing yellow arrow, and write scary stories about it.
• Are there intersections where flashing yellow arrows can not be used?

  Yes. Flashing yellow arrows can not be used where any of the following conditions exist:

  • A single approach lane
  • Any approach where a lane is shared between the controlled turn and other traffic movements
  • Where more than one permissive left turn would turn into the same intersection leg at the same time
  • Where more than one permissive right turn would turn into the same intersection leg at the same time
  • Where the flashing yellow arrow movement would cross more than one other movement
  • Where conflicts develop in multilane left turns
• How many states are now using the flashing yellow arrow? Just a few?

  At least 48 states and the District of Columbia now have at least one flashing yellow arrows signal within their borders.

  Look here to find out: The Box Scores

• How much more do these flashing yellow arrow intersections cost, as compared to the 5-light signals in use now?

  There are several factors:

  • The 4-light flashing yellow arrow face costs less than the 5-light traditional shared left turn face.
  • In most cases, the signal faces that are taken down can be reconfigured and used again. The lamp sections come apart for this purpose.
  • The flashing yellow arrow face can not be used as one of the two required faces for straight ahead traffic.
  • In a signal being converted from a 5-light shared signal, two more signal circuits per approach are needed. This might mean replacing cables.
  • An older signal controller and/or conflict monitor may have to be replaced to get the ability to operate flashing yellow arrows.
  • This is a job for a trained engineer, not a local politician, police chief, or electrical contractor.

  A new installation where no signal existed before should cost less than the equivalent signal with a shared 5-light left turn display.

  The main cost of converting an old signal is the labor. It would be equivalent to the cost of adding or removing a left turn phase.

  Labor is also the main cost of a sign. A new stop sign ready to be installed costs under $50. After installation, the cost is over $200.

• Does the flashing yellow arrow always prevent yellow trap?

  Only if it is carefully installed, with particular attention paid to the following:

  • It must be installed correctly. If it is installed wrong, a flashing yellow arrow can cause yellow trap as badly as a circular green.
  • Attention must be given to second yellow trap. This is where a right turn overlap can cause yellow trap for a permissive turn.
  • Attention must also be given to third yellow trap. This is where the oncoming phase is reserviced for pedestrian reasons and either delays the start of flashing yellow arrow or prevents its display for that cycle.
  • Pre-emptions for fire, police, ambulance, railroad, drawbridge, queue discharge, or other special circumstances must be specially designed to prevent yellow trap. A common mistake is to value government workers more than the lives of others.
  • Unusual intersection geometry, including offset legs, skewed approaches, and more than 4 legs, must be specially designed.
• What is the correct way to install flashing yellow arrows?

  See the box at right.

• How can a flashing yellow arrow be installed wrong?

  If flashing yellow arrows are installed wrong, they can be more dangerous than turns on circular green:

  • The flashing yellow arrow is installed with the flashing yellow arrow operation tied to the adjacent circular green, instead of the oncoming traffic circular green on the same road.*

    A flashing yellow arrow must be flashing at ANY time the oncoming circular green is lighted.

  • Only one flashing yellow arrow signal face is installed on any road where both left turns can be made from that road.*

    Flashing yellow arrows faces must be installed in pairs for left turns from both directions on the same road.

    Note that if a green arrow left turn phase is not needed on one approach on the road, a flashing yellow arrow face must still be provided for that approach to prevent yellow trap. But it does not need to have the green arrow or its controller phase unit. Even if it has no green arrow, the face must still be tied to the oncoming circular green phase.

    Note that if any approach on the street has no left turn lane, a flashing yellow arrow cannot be used on that street. Other methods of preventing yellow trap must be used.

  • Using a green right turn arrow as an overlap phase on a road with flashing yellow arrow left turns can cause second yellow trap.
  • Allowing a phase with a delayed-start flashing yellow arrow to be reserviced for pedestrian purposes causes third yellow trap.
  • Allowing a pre-emption to end the flashing yellow arrows display without ending the oncoming circular green causes yellow trap.
  • Making the wrong change interval display to appear:

    A flashing yellow arrow must be followed by a steady yellow arrow before a red arrow or a flashing red arrow appears.

    The flashing yellow arrow must continue to flash until the change period ends before a green arrow appears. A steady yellow arrow must not appear.

  The key: Any installation where the flashing yellow arrow can end before the oncoming circular green ends is installed wrong. It causes yellow trap.

  * Yellow trap can be prevented through other methods used with permissive turns, but the main advantage of having the flashing yellow arrows is lost.

  FLASHING YELLOW ARROWS - WHAT CAN GO WRONG?

• What signals are now allowed for left turn signals?
  1. The 4-arrow flashing yellow arrow face.
  2. A 3-arrow version with a dual color indication section that displays both a steady green arrow and a flashing yellow arrow.
  3. A 3-arrow version with no green arrow. This is used where yellow trap must be prevented, but the approach has no green arrow left turn phase.
  4. A 3-arrow face without a flashing yellow arrow can be used for an exclusively protected turn or a flashing red arrow face.
  5. A 4-arrow face with a flashing red arrow can be used for a flashing red arrow face.
  6. A 5-arrow face with a flashing yellow arrow and a flashing red arrow can be used for a flashing yellow arrow/flashing red arrow face.
  7. The 5-light 2-arrow signal may be used where it doesn't cause yellow trap. It must be used where a shared left and straight lane exists.
  8. A 4-light 1-arrow face can be used for split phasing (each leg has its own green).
  9. Where no left turn phase exists and no yellow trap is possible, a 3-light no-arrow signal may be used for permissive left turns.
  10. A six-indication five-section face is allowed where a shared-use lane changes left turn mode at different times of day. The flashing yellow arrow and green arrow share the same lens.
• I don't like the flashing yellow arrow. What can be done to stop it?

  The federal government has already approved it in the 2009 MUTCD, as the preferred left turn signal where it can be applied.

  If you don't like it, you don't really understand the benefits of having it. It appears to be the left turn signal of the future. The safety record proves that.

  You are in the minority, and will not be able to stop it.

Links:

• All Yellow Trap Links are here:
  YELLOW TRAP INDEX PAGE
• FYA - What can go wrong?
• Do you understand yellow trap?
• Do they think we're that stupid?
• A Visual Example of Flashing Yellow Arrows
• Unusual Traffic Problems, Hazards, and Solutions
• Two New Approaches to the Problem
• Yellow Trap in Bloomington Indiana
• HOME