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:
Why do traffic engineers need to do that?
For the following reasons:
- To prevent the hazard of yellow trap.
- To have a longer permissive turning period in the signal cycle.
- To have more signal sequences available without the hazard of yellow trap.
- To be able to put signal progression on more two-way streets.
- 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
- 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.
What is "second yellow trap"?
Second yellow trap occurs when all of these come together at the
- Left turns are allowed on the circular green or flashing yellow arrow on the affected
- Oncoming right turns receive a green arrow before or during the yellow clearance for that
- The right turning traffic blocks the path of the left turns as they try to clear the
- 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
- 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
- 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
- The left turn is illegally given a steady yellow arrow when the oncoming circular green is
reserviced to answer the pedestrian pushbutton.
- 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
- 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
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 starts to move.
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
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 occurs when all signals in one direction turn yellow, while the signals in the other
direction remain green.
- 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:
- 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
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
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
Yes. The flashing yellow arrow left turn signal face tells you the color of the oncoming circular
indications whenever it permits a movement:
|SIGNALS FOR ONCOMING TRAFFIC
CAN NEVER HAVE
||Stop and stay
||Stop, turn when safe
||Prepare to stop
||Yield to conflict
||Go - protected turn
||Stop, turn when safe
||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
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:
- 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
- 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
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 later used the same
- 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
- 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
- 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
- Where more than one permissive left turn would turn into the same intersection leg at the
- Where more than one permissive right turn would turn into the same intersection leg at the
- 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
- 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
- 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
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
- 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
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?
- The 4-arrow flashing yellow arrow face.
- A 3-arrow version with a dual color indication section that displays both a steady green
arrow and a flashing yellow arrow.
- 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.
- A 3-arrow face without a flashing yellow arrow can be used for an exclusively protected
turn or a flashing red arrow face.
- 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.
- A 4-light 1-arrow face can be used for split phasing (each leg has its own green).
- 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.
- 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.