ABOUT FLASHING YELLOW ARROWS
The latest solution to the yellow trap, lag trap, or left-turn trap problem is the Flashing Yellow Arrow. It redefines the flashing yellow arrow to have a new meaning of "Yield to opposing (oncoming) traffic." But it is not a panacea. If not properly implemented, it causes hazards as serious as the yellow trap it is meant to eliminate. So the engineer who implements it must be careful to implement it correctly, and not make the mistakes listed later.
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Flashing Yellow Arrows was NOT created solely for driver understanding of signals. It's needed for these reasons:
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Note: This page is written for locations that drive on the right. For locations that drive on the left, replace the words "left" and "right" with each other throughout the page. |
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−−−−−−− SPECIAL REPORTS: 1. WHY FLASHING YELLOW ARROWS WORK 2. EXTRA TURN TIME WITH FLASHING YELLOW ARROWS −−−−−−− TOPICS: DO THEY THINK WE'RE THAT STUPID? −−−−−−− |
MEANINGS OF SIGNAL INDICATIONS
| ♦ Steady Red Arrow | − Stop and stay |
| ♦ Flashing Red Arrow | − Stop; proceed after yield |
| ♦ Steady Yellow Arrow | − Stop if you can |
| ♦ Flashing Yellow Arrow | − Yield to conflict |
| ♦ Steady Green Arrow | − Go, Protected turn |
| ♦ Steady Circular Red | − Stop |
| ♦ Flashing Circular Red | − Stop; proceed after yield |
| ♦ Steady Circular Yellow | − Stop if you can |
| ♦ Flashing Circular Yellow | − Caution; Turns yield to conflict |
| ♦ Steady Circular Green | − Go; Turns yield to conflict |
HOW FLASHING YELLOW ARROWS WORK |
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HOW YELLOW TRAP OCCURS
If nothing special is done to prevent it from happening, a left turning driver coming from the direction that received the yellow and red signal will usually think that both directions of traffic get a yellow light at the same time. So, thinking that opposing cars will stop, he might turn across traffic that still has a green signal. If so, an accident occurs. This happens because each driver expects the other driver to stop and wait. The thru driver with the green light expects left turns to yield to opposing traffic. Unfortunately, the turning driver with the yellow light expects the thru driver to stop for a light that never changed. Since each driver expects the other to yield, an accident is likely. |
FLASHING YELLOW ARROWS
In this case, the driver making a left turn while the thru signal is yellow or red still has the flashing yellow arrow. The thru signal color is unimportant to him now (unless he misinterprets the meaning, or doesn't see the left turn signal). So he is still permitted to turn left through gaps in opposing traffic until the traffic moving in the other direction is also stopped by the signal. Given an indication that he can continue to try to turn through gaps in opposing traffic, the driver will not tend to turn across vehicles that still have a green light. The yellow trap has been avoided. |
The yellow trap (lag trap) hazard must begin when both straight-ahead (thru) phases on one street are
green at the same time.
At this time, one of the thru phases changes to yellow and red. This happens, either so a left-turn
phase can turn green in the other direction, or because a preemption occurs that requires one
direction to have a green while the other direction must be red. When a circular green is used to
permit turns through gaps, it can't continue to permit turns when the thru movement must be
stopped.
At this time, one of the thru phases changes to yellow and red, just as in the earlier example.
But at this intersection, the left turn has a flashing yellow arrow. It continues flashing even
while the corresponding thru signal is yellow or red. This continues, until the signal facing the
other way also turns yellow and red (when the left turn signal also turns yellow and red).IMPLEMENTATION
Flashing Yellow Arrows requires a different kind of operation than the standard left turn signal requires. So there are several differences in the way the signal controller is connected to the signal display, and several different ways to implement the display:
THE FOUR-ARROW FACE
The four-arrow signal face for Flashing Yellow Arrows requires special logic to be implemented. It needs an overlap phase that is "green" (active) when either the left turn phase for this display is green, or the opposing thru phase is green. It also needs some special logic for some of the lamps:
- The red arrow lights when the overlap phase is red.
- The upper yellow arrow lights when EITHER the overlap phase is yellow, OR the left turn phase is yellow.
- The lower yellow arrow flashes when the overlap phase is green AND the left turn phase is red.
- The green arrow lights when the left turn phase is green.
- The red arrow or the upper yellow arrow flashes when the signal is placed in flashing mode.
An animated visual example of this signal face.
Signal manufacturers are already providing phase display devices and conflict monitors for it.
The four-arrow face does not provide a positive indication that the permissive period is ending, because the flashing yellow arrow is replaced by a steady yellow arrow. A driver glancing at the signal while looking for a gap in opposing traffic might not notice that the arrow is in a different lens, and is not flashing. This effect is more pronounced at night, when the rest of the signal face might be invisible.
See this perception problem yourself.
A reflective yellow border on a signal backplane can make the clearance problem go away.
One nice feature of Flashing Yellow Arrows is that the left-turn mode can be changed by a time-of-day clock, by a computer control system, or by actual traffic conditions. This makes the intersection able to control traffic in a more flexible manner.
| This face can be operated in any of the following left-turn modes: | |
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THE FOUR-SECTION TWO-ARROW FACE #
The four-section two-arrow signal face for Flashing Yellow Arrows has exactly the same logic the 4-arrow face has. This kind also needs an overlap phase that is active when either the left turn phase for this display is green, or the opposing thru phase is green. Some special logic is also needed for some of the lamps:
- The circular red lights when the overlap phase is red.
- The circular yellow lights when EITHER the overlap phase is yellow, OR the left turn phase is yellow.
- The yellow arrow flashes when the overlap phase is green AND the left turn phase is red.
- The green arrow lights when the left turn phase is green.
- The circular red or circular yellow flashes when the signal is placed in flashing mode.
The four-section two-arrow face is not approved by the MUTCD. It gives a positive indication of the end of the permissive period that the four-arrow face does not give: the yellow arrow changes to a circular yellow. This means that the driver does not have to look at the signal face for over a second to determine whether the yellow arrow is the flashing permissive turn arrow, or the steady clearance arrow.
| This face can be operated in the following left-turn modes: | |
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THE FIVE-SECTION THREE-ARROW FACE #
The five-section three-arrow signal face for Flashing Yellow Arrows has a much simpler logic scheme. It needs an overlap phase that is active when either the left turn phase for this display or the opposing thru phase is green. Special flashing logic is also needed for the flashing yellow arrow lens:
- The circular red lights when the overlap phase is red and the left turn phase is red.
- The circular yellow lights when the overlap phase is yellow and the left turn phase is red.
- The upper yellow arrow lights when the left turn phase is yellow.
- The lower yellow arrow flashes when the overlap phase is green and the left turn phase is red.
- The green arrow lights when the left turn phase is green.
- The circular red or circular yellow flashes when the signal is placed in flashing mode.
An animated visual example of this signal face.
The five-section three-arrow signal face is not approved by the MUTCD. It also gives a positive indication of the end of the permissive period. Another feature of this face is that an existing 5-lens signal face can be converted to this face by replacing the circular green lens with a yellow arrow. But the signal heads may need to be moved, because the new display can not be used as one of the two required faces for thru traffic, and it must now be over the left turn lane. An all-arrow version of this would be allowed by the MUTCD, but does not provide the positive indication of clearance.
| This face can be operated in the following left-turn modes: | |
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THE THREE-SECTION THREE-ARROW FYA FACE
The three-section three-arrow signal face for Flashing Yellow Arrows (FRA) is used where no left
turn phase exists for the drivers seeing it, but a left turn phase exists for opposing traffic. This
also prevents yellow trap with preemptions. It has a very simple logic scheme. It uses the opposing
thru phase, plus special flashing logic for the yellow arrow:
- The red arrow lights when the opposing phase is red.
- The upper yellow arrow lights when the opposing phase is yellow.
- The lower yellow arrow flashes when the opposing phase is green.
- The red arrow or upper yellow arrow flashes when the signal is placed in flashing mode.
This face is used when an approach with a permissive left turn, but without a left turn phase, shares the same street with an approach with a left turn phase, or when a preemption cuts off one direction. It is necessary to prevent yellow trap on approaches without left turn phases.
WARNING: If an approach has a lane sharing left turns and other movements, and the opposing left turn has any kind of left-turn signal, the following are true:
- This signal face can not be used on the approach with the shared lane.
- Other means of preventing yellow trap MUST be used.
| This face can be operated in the following left-turn modes: | |
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THE THREE-SECTION ONE-ARROW FACE #
The three-section one-arrow signal face for Flashing Yellow Arrows does the same job the above signal
face does, but would be used where red arrows are not desired. It has the same logic scheme, using the
opposing thru phase, plus special flashing logic for the yellow arrow:
- The circular red lights when the opposing phase is red.
- The circular yellow lights when the opposing phase is yellow.
- The yellow arrow flashes when the opposing phase is green.
- The circular red or circular yellow flashes when the signal is placed in flashing mode.
This face is not approved by the MUTCD. It would be an alternate for use when an approach with a permissive left turn, but without a left turn phase, shares the same street with an approach with a left turn phase or a preemption. It also gives the positive indication of the end of the permissive period that the four-arrow face does not give (see above).
WARNING: If an approach has a lane sharing left turns and other movements, and the opposing left turn has any kind of left-turn signal, the following are true:
- This signal face can not be used on the approach with the shared lane.
- Other means of preventing yellow trap MUST be used.
| This face can be operated in the following left-turn modes: | |
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THE THREE-SECTION THREE-ARROW FRA FACE
(Not for Flashing Yellow Arrows)
The three-section three-arrow signal face for Flashing Red Arrows (FRA) has almost the same logic the
4-arrow face has. This kind also needs an overlap phase that is active when either the left turn phase
for this display is green, or the opposing thru phase is green. Some special logic is also needed for
some of the lamps:
- The red arrow lights when the overlap phase is red.
- The red arrow flashes when the overlap phase is green AND the left turn phase is red.
- The yellow arrow lights when EITHER the overlap phase is yellow, OR the left turn phase is yellow.
- The green arrow lights when the left turn phase is green.
- The red arrow or the upper yellow arrow flashes when the signal is placed in flashing mode.
This face is used when the permissive left turn must make a complete stop before making a turn. Otherwise, it behaves the same as the Flashing Yellow Arrow faces. This face is also used for an exclusively protected left turn phase.
| This face can be operated in the following left-turn modes: | |
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THE FIVE-SECTION TWO-ARROW "DOGHOUSE" FACE**
(Not for Flashing Yellow Arrows)
The five-section two-arrow signal face is NOT for Flashing Yellow Arrows. It causes yellow trap. But
it can safely be used in the following situations:
- Intersections with split phases on one street. Each leg gets a separate phase.
- Legs where other means prevent yellow trap:
- The leg is on a side street, the left turn is leading, and the main street green can not be skipped.
- Special logic (call redirection, false call, or phase inhibit) prevents a yellow trap sequence from occurring.
- Flashing Yellow Arrows can NOT be used where left and thru movements share a lane.
Special logic must be used:
- The green arrow can not be illuminated unless the circular green is also illuminated.
- Split phases can be used on that street. Each leg gets a separate phase.
- Special logic (call redirection, false call, or phase inhibit) prevents a yellow trap sequence from occurring.
- Where only one left turn is possible from the street (e.g. T-intersection, or intersecting a one-way street).
- Where a left turn on a one-way street has a protected phase.
The circular red or circular yellow must flash when the signal is placed in flashing mode.
| This face can be operated in the following left-turn modes: | |
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THE FIVE-SECTION TWO-ARROW SIX-INDICATION "DOGHOUSE" FACE**
(Flashing Yellow Arrows for shared-use lane)
This five-section signal face is for Flashing Yellow Arrows with a shared-use lane. It prevents yellow
trap. It can be used in the following cases:
- Split phases. Each leg gets a separate phase.
- Single-split lead-lag phasing.
- Permissive phasing.
- Single lead phasing.
- Alternating lead phasing (left turn phases take turns on alternate cycles)
Special logic must be used:
- The green arrow can not be illuminated unless the circular green is also illuminated.
- Split phases can be used on that street. Each leg gets a separate phase.
The circular red or circular yellow must flash when the signal is placed in flashing mode.
The green arrow and the flashing yellow arrow share the same lens with a dual color arrow.
| This face can be operated in the following left-turn modes: | |
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Many other variations have been created to deal with yellow trap. Many of these are shown on the page
linked to below. Others attempt to warn the driver of the yellow trap without eliminating it. The
various attempts to deal with yellow trap include:
- Special phase sequence prevents yellow trap from occurring.
- Preventing phase skip prevents yellow trap from occurring.
- Special logic (call redirection, false call, or phase inhibit) prevents yellow trap from occurring.
- A special yellow lens in the shape of a "yield triangle" #
- A special red lens with a black left arrow on it #
- A MUTCD-required sign for yellow trap locations: "ONCOMING TRAFFIC MAY HAVE EXTENDED GREEN"
- A MUTCD-required sign for repeating yellow trap: "ONCOMING TRAFFIC HAS EXTENDED GREEN"
- A custom sign for yellow trap: "NO LEFT TURN ON (circular yellow image)" #
- A custom sign for yellow trap: "LEFT TURN ON YELLOW WITH CAUTION" #
- Prohibiting the opposing left turn
- Using exclusively protected* phasing
- Using a flashing red arrow in the left turn signal.
- Using a flashing circular red in the left turn signal. #
- Using a flashing circular yellow in the left turn signal. #
Here is a page showing most of the left turn displays, and more:
WARNING
Even though the MUTCD now requires all yellow trap conditions to be treated (the 5-year grace period expired in 2008), many locations still have untreated yellow trap. The main reasons for this are:
- Authorities do not understand the hazard.
- Authorities do not know of the MUTCD ban of yellow trap.
- Authorities are unaware that their signals are causing yellow trap.
- Authorities don't think it is a problem.
- Authorities do not want to spend money to fix the problem.
- Politicians voted down the changes.
- Construction contractors in charge of a site don't understand yellow trap.
Note: None of the Flashing Yellow Arrows signal faces shown above can be used as one of the two required signal faces for thru traffic. They must be mounted in line with or over the left turn lane. This may require new mast arms or other signal mounts. An additional signal face may also be needed to provide the two required faces for thru traffic.
Note: Flashing Yellow Arrows are not allowed to be used where turns controlled by the turn arrow share lanes with other movements. These cases must be avoided, because drivers waiting to make a movement permitted by the signal may illegally cross the double yellow line or enter the shoulder to pass other vehicles waiting to make a movement stopped by the display. So either these movements must be given left turn lanes, or some other method must be used to avoid yellow trap.
This means that all intersections with the following characteristics need special treatment:
- Single lane approaches
- Approaches with lanes sharing left turn and thru movements
These intersections where Flashing Yellow Arrows can not be used must prevent yellow trap with other methods:
- Use a single lead left turn phase without cross-street phase skip. A dual lead or dual split lead is not allowed.
- Use call redirection and phase inhibit on a single lead left turn.
- Alternate the single leading left turn on opposite approaches on different cycles, or at different times of day.
- Use false calls to prevent phase skip when the single leading left turn is called.
- Use a 4-way red clear before the left turn phase.
- Use separate phases for the two approaches (split phase, also called unsplit lead-lag).
- If only one direction has shared left turn lanes, make its direction lagging, and the opposite direction leading. Give the leading phase either an exclusively protected phase or a flashing yellow arrow. Inhibit cross street phase skip.
Update: In 2012, the FHWA issued a ruling allowing a 5-section 6-indication signal to allow a flashing yellow arrow with a shared use lane that changes left turn mode at different times of day.
Approaches with multiple-lane left turns should use exclusively protected* left turn phases.
Other intersections where Flashing Yellow Arrows can not be used must prevent yellow trap with other methods:
- Use a leading left turn phase without cross-street phase skip.
- Use call redirection and phase inhibit on a leading left turn.
- Use false calls to prevent phase skip when the leading left turn is called.
- Use a 4-way red clear before the left turn phase.
- Use separate phases for the two approaches (split phase, also called unsplit lead-lag).
Note: Signal controllers and conflict monitors are now available with the Flashing Yellow Arrows system built into them as a selectable option.
The 2009 MUTCD permitting Flashing Yellow Arrows is now in place.
THE DANGERS
There are some dangers inherent in Flashing Yellow Arrows, but they are caused by improper implementation:
- Not testing the design for all possibilities of phase skip.
- Not providing three-section Flashing Yellow Arrow signal faces for left turns without left-turn phases, but facing the opposite direction to approaches with left turn phases.
- Failing to provide other means of preventing yellow trap where any approach has left turns and other movements sharing the same lane. This includes single lane approaches. Flashing Yellow Arrows can't be used in this case.
- Wrong operation of a Flashing Yellow Arrow signal. The flashing yellow arrow must always be controlled by the opposing circular green, NOT the adjacent circular green.
- Right-turn green arrows or flashing yellow arrows in conflict with Flashing Yellow
Arrow left-turn movements during the permissive phase, or that display simultaneously with and/or
immediately following the yellow clearance of the permissive phase. This happens where the
right turn will continue to have a green while overlapping a left turn green arrow on the cross
street. The MUTCD allows this
display, but it causes yellow trap when used with Flashing Yellow Arrows on left turns. See:
A Second Yellow Trap Hazard - Pedestrian WALK or flashing DON'T WALK indications conflicting with Flashing Yellow Arrow permissive movements.
- Improperly designed intersection controls. These include offset intersections, intersections with more than 4 legs, and odd geometries. Bad designs can cause yellow trap events, even with Flashing Yellow Arrows.
- Improperly designed special preemption sequences for emergency vehicles,
railroad crossings, drawbridges, queue discharge, and other special events. These can cause yellow trap
with Flashing Yellow Arrows if not designed properly. See:
Yellow Trap and Signal Preemptions - Burned-out signal lamps on left-turn faces change the meanings of signal displays, often by causing yellow trap events. This happens even with Flashing Yellow Arrows.
- Construction zones: Not paying attention to Preventing Yellow Trap During Construction. Lane closures particularly cause problems if exclusive left turn lanes become shared lanes. Too often the signal sequence is not altered to be safe, or to comply with the MUTCD.
MAKING FLASHING YELLOW ARROWS VIABLE
CHANGING GOVERNMENT STANDARDS
In order for Flashing Yellow Arrows to be allowed to be used, three changes must be made:
- The US Department of Transportation's 2009
MUTCD incorporates the new
meanings of Flashing Yellow Arrow and Steady Yellow Arrow:
- Traffic facing a flashing yellow arrow must yield to opposing traffic.
- The steady yellow arrow is allowed when opposing traffic has a steady circular yellow or steady yellow arrow.
- The steady yellow arrow is redefined to not always be a protected indication.
- The steady yellow arrow is required as a clearance interval for the following signal changes:
- Green arrow to any other indication.
- Flashing yellow arrow to steady red arrow, flashing red arrow, steady circular red, or flashing circular red.
- Flashing red arrow to steady red arrow or steady circular red.
- Prohibiting Second Yellow Trap has not yet been accomplished.
- The Uniform Vehicle Code was amended to make the same changes.
- Once these are done, then each state that has its own uniform traffic control device manual must update its manual to agree with the new federal one.
- Uses implemented before the final MUTCD ruling, and operating under the interim rule with written permission from the US DOT, are now standard.
EDUCATING LEGISLATORS
Too many laymen do not understand either the yellow trap hazard, or how the Flashing Yellow Arrows system eliminates it. Unfortunately, when traffic control is being discussed, most legislators are laymen. They may make the following errors when passing legislation on Flashing Yellow Arrows:
- Not understanding the yellow trap hazard.
- Thinking that Flashing Yellow Arrows adds unnecessary complication to driving.
- Thinking that Flashing Yellow Arrows adds unnecessary expense to the budget.
- Not understanding the difference between circular green and Flashing Yellow Arrow.
(Flashing Yellow Arrow can be used when the straight-ahead signals are red. Circular green can not.) - Not understanding that yellow trap violates federal standards.
- Not knowing that the yellow trap grace period expired in 2008.
- Restricting Flashing Yellow Arrows to only one kind of display.
(Prevents using Flashing Yellow Arrows on approaches with yellow trap, but no left-turn phase.)
(Prevents using Flashing Yellow Arrows to make preemptions safe.) - Refusing to eliminate other laws that conflict with the use of Flashing Yellow Arrows.
(e.g. antiquated laws prohibiting red arrows) - Not understanding Second Yellow Trap.
- Not understanding how to Prevent Yellow Trap During Construction.
EDUCATING THE MEDIA AND THE PUBLIC
The general public must be taught how to drive at an intersection with Flashing Yellow Arrows.
Newsmen and reporters must be taught how to drive at an intersection with Flashing Yellow Arrows.
Newsmen and reporters must understand how Flashing Yellow Arrows prevent the yellow trap hazard. If they are not properly informed, they may make the following errors:
- Not understanding the yellow trap hazard.
- Not understanding Second Yellow Trap
- Not understanding the difference between circular green and Flashing Yellow Arrow.
(Flashing Yellow Arrow can be used when the straight-ahead signals are red. Circular green can not.) - Thinking that Flashing Yellow Arrows adds unnecessary complication to driving.
- Thinking that Flashing Yellow Arrows adds unnecessary expense to the budget.
- Not understanding that yellow trap violates federal standards.
- Not knowing that the yellow trap grace period expired in 2008.
- Demanding an improper installation of, Flashing Yellow Arrows.
- Demanding the removal of Flashing Yellow Arrows.
- Misinforming drivers on how to drive at a signal with Flashing Yellow Arrows.
- Believing a flashing yellow arrow is a protected movement
(This comes from uses where a protected green arrow on a split phase signal flashes yellow during night flash).
MISTAKES AND HAZARDS TO AVOID
IMPROPER IMPLEMENTATION
If Flashing Yellow Arrows are not properly implemented, they can be more hazardous than the circular greens they replaced:
- Analysis: Each installation must be carefully analyzed by a trained engineer.
- Competence: Layman politicians are not qualified to design Flashing Yellow Arrows installations.
- Testing: All possibilities of phase skip must be tested, to make sure the proper signals are produced without inducing unwanted hazards.
- Opposing left turns: Where left turns exist from both directions on the same street, and EITHER direction has a left turn signal or a preemption, BOTH directions need Flashing Yellow Arrow signals, unless the signal sequence can never produce a yellow trap event.
- Shared lane use: Where left turns exist from both directions on the same street, one direction has a left turn signal or a preemption, and the other direction has a lane shared by left turns and other movements, means other than Flashing Yellow Arrows must be used to prevent yellow trap.
- Opposing thru control: Where Flashing Yellow Arrow signals are used for left
turn control, each one must be controlled by the opposing thru movement. A flashing yellow arrow for
left turn control must NEVER be connected to operate from the adjacent thru movement signal.
Right turn signals with flashing yellow arrows overlapping a pedestrian movement may be connected to the adjacent thru signals. - Notice of clearance: Use of the four-arrow face can cause confusion when the steady yellow arrow clears the flashing yellow arrow. Drivers might not notice the change until the red arrow appears. A reflective-border signal backplane can help here.
- Right turn conflict: Right turn green arrows must not overlap any phase where permissive turns enter the same intersection leg. This causes yellow trap when the flashing yellow arrow ends. A circular green or a flashing yellow right turn arrow should be used during the permissive left turn instead. This turn must be cleared with yellow and red, before the green arrow is displayed for a cross-street left-turn overlap. Right turn arrows must also be tested for all phase-skip possibilities. See:
- Pedestrian conflict: No crosswalk can have a WALK or flashing DON'T WALK indication when a flashing yellow arrow that must turn across it changes to steady yellow arrow. This causes yellow trap when the flashing yellow arrow ends.
Special care is needed when evaluating intersections: Unusual intersections can
cause unusual cases of yellow trap in locations without left turn arrows, in addition to causing
sequences that defeat the safety of the Flashing Yellow Arrows system:
- Offset intersections with the near leg on the right:
If the greens in opposite directions do not terminate simultaneously (e.g. to display a lagging left turn phase), yellow trap occurs at the intersection that does NOT display the green left turn arrow. This combination might also occur due to a phase skip.An example of such a case is a pair of intersections operating as a single intersection, using optically programmed signal faces to control exit from the short section of street between the intersections. Yellow trap events can occur at one intersection, even though the other intersection is the only one that has the left-turn signal face (see diagram).
- Offset intersections with the near leg on the left:
Not only can ending one direction's green earlier cause yellow trap, but the normal practice of
having a double clearance also causes yellow trap. This happens without any left turn signals.
Flashing yellow arrows must be used in both directions, and must continue to be displayed until
the opposite direction signal with the extended clearance time also clears. - Intersections with more than 4 legs:
When two legs on one side of the intersection share the same leg on the other side, yellow trap occurs if the same approach gets a circular green for the green phases of both of the other legs. That phase may be green with the green for one of the two legs on the other side, but not also with the other leg on that side. - Intersections with odd geometries:
Places to look here include:- Intersections where the left turns in opposite directions have to cross each other's paths.
- Left turns where U-turns also occur frequently.
- Intersections with odd approach angles.
- Intersections with multiple-lane left turns.
- Offset intersections with the near leg on the right:
- Preempted signals need special attention: Special considerations must be taken into account when designing signals that are preempted for railroad trains, drawbridges, emergency vehicles, queue discharge, transit priority, and other special events. They can cause special hazards when used with Flashing Yellow Arrows. This hazard now has its own web page:
- Construction: The need to Prevent Yellow Trap During Construction
The only signals that do not need Flashing Yellow Arrows faces are:
- Signals with no left turn phases, no odd geometries, and no possible preemptions on any approaches.
- Signals with exclusively protected* left turn phasing on all approaches.
- Signals with all left turns prohibited or diverted.
The only intersection approaches that do not need Flashing Yellow Arrows are where:
- The street has no left turn phases, no odd geometries, and no possible preemptions.
- The street has exclusively protected* left turn phases in both directions.
- One of the left turns from the street is impossible, prohibited, or diverted.
- Approaches with shared left and thru lanes require yellow trap prevention methods other than Flashing Yellow Arrows.
MISTAKES AND HAZARDS TO AVOID
THE HAZARD OF BURNED-OUT LAMPS
When a signal lamp on a left-turn face burns out, especially at night, it can change the meaning of the signal display, sometimes in a hazardous way:
- If the circular red or red arrow burns out, it causes the intersection to revert to the old system of using the circular green to allow turns through gaps in opposing traffic. This makes the intersection vulnerable to yellow trap.
- If the circular yellow or upper yellow arrow burns out, the left-turning driver does not receive a proper clearance interval display. This can cause the driver to rely on the thru signals. Yellow trap is a possibility.
- If the flashing lower yellow arrow burns out, it causes the intersection to revert to the old system of using the circular green to allow turns through gaps in opposing traffic. This makes the intersection vulnerable to yellow trap.
- If the green arrow burns out, it causes the intersection to revert to the old system of using the circular green to allow turns through gaps in opposing traffic. This makes the intersection vulnerable to yellow trap. It also causes drivers to start moving when the yellow arrow clearing the green arrow comes on. They may mistakenly think it is a flashing yellow arrow. This becomes a hazard if the flashing yellow arrow is not installed, or when it will not be displayed next.
The use of LED signal lamps will greatly reduce, but not eliminate, lamp burnout. LED signal lamps should be designed so that a single component failure does not darken the entire lens or change its shape.
THE FUTURE
Some predictions of future implementations of Flashing Yellow Arrows:
- The 5-light 2-arrow "doghouse"** shared left turn signal will disappear, except where left and thru movements share the same lane.
- The 4-section all-arrow Flashing Yellow Arrows faces will become the dominant left turn signal display.
- The 5-light 2-arrow 6-indication "doghouse"** shared left turn signal will be used where left and thru movements share the same lane and the left turn mode can automatically change.
- The use of lagging left turns and lead-lag left turns will increase.
- Signals will be able to change between leading, lagging, and lead-lag phasing, based on time of day or actual traffic conditions.
- Engineers will no longer have to choose between providing protected/permissive phasing and providing signal progression. With Flashing Yellow Arrows, both can be used simultaneously without causing yellow trap.
- More streets will be able to have green progression from signal to signal in both directions.
- More signals will be able to have protected/permissive phasing. This is not possible for signals with lagging or lead-lag phasing without Flashing Yellow Arrows.
- Many signals will change left turn modes (exclusively permissive*, exclusively protected*, and protected/permissive phasing) as needed, either by time of day, or by actual traffic demand.
- Because a shared flashing-yellow-arrow and thru-movement display is discouraged, shared left turn and thru lanes will be removed in many places. Left-turn lanes will be added where they are needed.
- Some jurisdictions will install Flashing Yellow Arrows faces wherever left turns exist, even though no protected left turns are present.
- Most traffic signals will have reflective border backplanes, because these show the existence of a signal at night if the power or display fails.
- With the increased use of LED signal lamps, battery backup will also find universal use. But electrical storms and power surges will always have the potential to damage traffic signals.
- Yellow trap will disappear in most installations.
- It is possible that some jurisdictions will always use the same display for all left turns from exclusive-turn lanes, whether using exclusively permissive*, exclusively protected*, or protected/permissive phasing. The only difference is the use or non-use of the green arrow and the flashing yellow arrow indications in different applications.
- In some places, power-hungry politicians will improperly install Flashing Yellow Arrows, causing hazards.
- Many construction sites will violate the MUTCD and allow yellow trap hazards, because the people in charge don't understand either.
CONCLUSION
- Proper design will allow the Flashing Yellow Arrows method to prevent accidents, saving lives and property.
- Improper design will make the hazard worse.
- It is imperative that the government agencies responsible for traffic control do the job correctly.
- Trained engineers must do the job.
- Untrained politicians should never have the power to design these installations themselves.
- Construction men must be trained how to prevent yellow trap. Construction sites must be inspected for this.
WHY FLASHING YELLOW ARROWS WORK
The cause of yellow trap, and the cure, are both embodied in the dual timing rings of an 8-phase controller.
Definitions:
- Ring, or timing ring: One of two units that times the various phases of the signal. They are called rings, because each will service a rotating ring of signal faces. The timing rings usually advance through the lists of phases they control, each returning to the beginning of its list after the last item.
- Phase: One set of signal faces controlling one traffic movement or set of movements at the intersection.
- Barrier: A restriction on the movements of the timing rings. Both rings must be on the same side (above or below) of each barrier at the same time. The rings must also cross barriers at the same time.
The barrier is the | or the −−−−−−−−−−−−−−− in the tables.
EXCLUSIVELY PROTECTED*
In the original "quad" left turn cycle, the signals had separate faces for left turns, with "left turn signal" signs over the signals. Left turns were stopped by red lights, and were not permitted on circular green. The signal controllers have two timing rings, arranged like this:
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Since only one phase (movement) in each ring can go at any one time, each timer has only one movement to deal with at any one time. This is why the left turns do not have to end at the same time. Each traffic movement's timing is always controlled by one ring. The timing normally moves down the list in each ring. But if no phases on the other side of the barrier have vehicles waiting, the ring can back up and return to an earlier phase on the same side. But with exclusively protected* turns in all directions, yellow trap can't happen. |
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In the following scenario:
- Phases 1, 2, and 6 receive detector calls (traffic). Phases 1 and 6 are active (display green) first.
- When phase 1 runs out of traffic, phase 2 becomes active. Phases 3, 4, 7, and 8 have no traffic waiting, and are skipped.
- Phase 5 receives a call, and phase 6 ends. Ring 2 moves backwards in its cycle, and phase 5 becomes active.
- Finally, phase 4 gets a call, and phases 5 and 2 end.
Time −−−>
| 2. Opposing circular | 2. | 2. | 2. | |||
| 1. Left turn arrow | 1. STOPPED | 1. STOPPED | 1. STOPPED | |||
| 6. Adjacent circular | 6. | 6. | 6. |
Yellow trap does not happen, because there are no permissive turns. Each Left turn is always controlled by only one ring.
DOGHOUSE** PROTECTED/PERMISSIVE
It is different when the 5-section permissive left signal face is used. An unexpected and unwanted event happens in this case. Left turn timing passes from ring to ring. The circular green the left turning driver sees and the protected left phase he sees are on different rings. The permissive lefts are the circular greens:
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This intertangles the timings for the left turns. It causes yellow trap by ending the permissive left turn at the WRONG TIME. If one side of the barrier has no cars waiting, the backing up of a timing ring can cause yellow trap. Note that this is caused by the use of the circular green as the permissive left turn indication AND the thru movement indication for the same approach. These movements are supposed to be on different rings. |
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This is the same scenario:
- Phases 1, 2, and 6 receive detector calls (traffic). Phases 1 and 6 are active (display green) first.
- When phase 1 runs out of traffic, phase 2 becomes active. Phases 3, 4, 7, and 8 have no traffic waiting, and are skipped.
- Phase 5 receives a call, and phase 6 ends. Ring 2 moves backwards in its cycle, and phase 5
becomes active.
Yellow trap ( YT ) occurs because phase 6 ended, but phase 2 did not end at the same time. - Finally, phase 4 gets a call, and phases 5 and 2 end.
Time −−−>
| 2. Opposing circular | 2. | 2. NOT STOPPED | 2. | |||
| 1. Left turn arrow | 6. Circular green | YT | 6. STOPPED | 6. STOPPED | ||
| 6. Adjacent circular | 6. | 6. | 6. |
Yellow trap ( YT ) occurs because the permissive turn ends too early. Because both phase 1 and phase 6 control this left turn, control jumps back and forth between the rings.
FLASHING YELLOW ARROWS PROTECTED/PERMISSIVE
With Flashing Yellow Arrows, control of each left turn is always confined to the same ring. The permissive lefts are the flashing yellow arrows.
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Yellow trap is not possible here, because the timings are controlled to always eliminate the unexpected conflict. Each movement is always controlled by the same ring. It does not matter if a ring decides to back up to an earlier phase, because the flashing arrows always start and end at the correct time when wired to operate with the opposing thru phase. |
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Here is the same scenario again:
- Phases 1, 2, and 6 receive detector calls (traffic). Phases 1 and 6 are active (display green) first.
- When phase 1 runs out of traffic, phase 2 becomes active. Phases 3, 4, 7, and 8 have no traffic waiting, and are skipped.
- Phase 5 receives a call, and phase 6 ends. Ring 2 moves backwards in its cycle, and phase 5 becomes active.
- Finally, phase 4 gets a call, and phases 5 and 2 end.
Time −−−>
| 2. Opposing circular | 2. | 2. | 2. | |||
| 1. Left turn arrow | 2. Flash yellow arrow | 2. | 1. STOPPED | |||
| 6. Adjacent circular | 6. | 6. | 6. |
Yellow trap is prevented by ending the permissive turn with the opposing circular green. Because phases 1 and 2 are in the same ring, this left turn is always controlled by one ring.
EXTRA TURN TIME WITH FLASHING YELLOW ARROWS
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Here is an animation of a dual lead signal using the three conditions mentioned above. The left panel of each case shows the signals for opposing traffic, as if seen in a mirror. |
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Notice the differences in the times the left turns are permitted: Case 1: SHORTEST TIME − exclusively protected* left turns. Left turns are permitted only when the green arrows are lit. Note that only one ring controls the left turns. Case 2: MEDIUM TIME − doghouse** protected/permissive left turns. Left turns are permitted on either the green arrow or the circular green. Note that both rings control the left turns. The gap between the end of the protected left turn and the beginning of the permitted left turn is caused by the different timings of the two rings. Case 3: LONGEST TIME − Flashing Yellow Arrows protected/permissive. Left turns are permitted on either the green arrow or the flashing yellow arrow. Note that only one ring controls the left turns. There is no gap between the end of the protected left turn and the beginning of the permitted left turn, because only one ring controls the timing. This provides a longer period to turn left. |
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Case 1: Exclusively Protected*
One interval
Time −−−>
| 2. Opposing circular | 2. | 2. | 2. | |||
| 1. Left turn arrow | 1. STOPPED | 1. STOPPED | 1. STOPPED | |||
| 6. Adjacent circular | 6. | 6. | 6. |
−−
Case 2: Doghouse** Protected/Permissive
Two separated intervals
Time −−−>
| 2. Opposing circular | 2. | 2. | 2. | |||
| 1. Left turn arrow | 6. STOPPED | 6. Circular green | 6. STOPPED | |||
| 6. Adjacent circular | 6. | 6. | 6. |
−−
Case 3: Flashing Yellow Arrows Protected/Permissive
Three contiguous intervals
Time −−−>
| 2. Opposing circular | 2. | 2. | 2. | |||
| 1. Left turn arrow | 2. Flash yellow arrow | 2. | 1. STOPPED | |||
| 6. Adjacent circular | 6. | 6. | 6. |
* The phrases "permissive-only" and "protected-only" violate the grammar rules on the proper placement of the word "only" for the intended meaning. Government often misuses the word "only" by placing it in the wrong place. Correct usage places the word "only" (which is not an adverb) immediately before the word modified. My new usage substitutes the phrases, "exclusively permissive" and "exclusively protected."
** The term "doghouse" refers to the 5-section cluster shared left-turn signal face. But its good and bad properties also apply to the 5-section vertical and 5-section horizontal arrangements for shared left-turn faces. In this document, assume that any mention of the doghouse signal face also includes the other 5-section shared left-turn faces.
# This application is not allowed by the 2009 MUTCD.
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