A THIRD YELLOW TRAP HAZARD

WHAT CAUSES THIRD YELLOW TRAP?

Third yellow trap occurs when any of the following is allowed to happen:

The flashing yellow arrow is delayed for a few seconds (with a red arrow) after the oncoming thru phase is given a green.

The oncoming phase is reserviced.

If the left turn yellow arrow change and red arrow display at this time, yellow trap occurs.

 no left turns permissive left turns third yellow-trap occurs no left turns

The flashing yellow arrow is inhibited when the pedestrian phase associated with the oncoming circular green is serviced.

The phase is initially green without the pedestrian phase (the flashing yellow arrow is displayed).

If the phase is reserviced for pedestrians, yellow trap occurs.

 permissive left turns third yellow-trap occurs clearing turns pedestrians free of conflict

The pedestrian and flashing yellow arrow have nonconcurrent phases. The oncoming circular green overlaps both phases.

Pedestrians usually go first, followed by flashing yellow arrows.

If the pedestrian phase is called again with no intervening cross street phases, yellow trap occurs.

 no conflict pedestrians permissive left turns third yellow-trap occurs no conflict pedestrians

Third yellow trap can also occur if a flashing red arrow signal is used instead of a flashing yellow arrow.

Third yellow trap occurs only under the following conditions:

  • The flashing yellow arrow has one of the three restrictions listed above that prevents its display with the oncoming circular green.
  • No traffic is present on the cross street.
  • The pedestrian signal is set on a recycle recall (it cycles between Walking Man and Flashing Hand when no other traffic is present), or it receives a call while the associated circular signals are already green.
  • The re-entry into the pedestrian phase causes the flashing yellow arrow to clear to yellow arrow and then red arrow.
  • The left turn is yellow trapped, because the oncoming traffic does not stop.

Third yellow trap does not occur if any of the following is true:

  • Flashing Yellow Arrows are not used at the intersection.
  • There is nothing preventing the appearance of the flashing yellow arrow while the oncoming circular green is on.
  • The pedestrian phase is prevented from being reserviced while the associated phase is already green.
  • The phase is not allowed to be reserviced without first changing to all-red for at least 5 seconds.
  • The delay or inhibition occurs only at the first display of the flashing yellow arrow, not during any reservicing of the oncoming through phase.
  • If pedestrians and flashing yellow arrows have separate phases, backup prevention is employed on the flashing yellow arrow phase.

The diagrams show the sequence of third yellow trap. Two third yellow traps on opposite approaches are shown:

  1. The first diagram shows the active phases before third yellow trap. The straight-ahead signals have circular green, the left turns have flashing yellow arrow, and the pedestrian signals are showing Flashing Hand.
  2. In the second diagram, the left-turn signals receive a steady yellow arrow indication. The left turns (red) are trapped by the oncoming traffic, even though flashing yellow arrows are used.
  3. The third diagram shows the recycle of the pedestrian phases, with red arrow shown to the left turns.

The problem is in the delayed or prohibited appearance of the flashing yellow arrow, intended to give pedestrians a safe chance to cross before the permissive turns begin. If the flashing yellow arrow signals turn red without the circular signals also turning red when the phase is reserviced (to give another Walking Man indication), it causes yellow trap, because the permissive left turn is stopped before the oncoming through traffic is stopped.

permissive left turns

third yellow-trap occurs

pedestrians free of conflict

A steady yellow must NEVER be shown to traffic making permissive turns unless oncoming straight and right traffic gets a steady yellow at the same time.

Notice in the following timing diagram how this third yellow trap happens. The letters " YT " show where each third yellow trap happens:

Flashing Yellow Arrows Protected/Permissive

Flashing yellow arrow - Left turn movement is still yellow trapped.
Time --->

2. Opposing circular
 2.
 2.
 2.
 2.
5. Opposing left turn
 6. Flash yellow arrow
 6.YT 5. STOPPED
 6. FYA again
1. Delayed left turn
 2. Flash yellow arrow
 2.YT 1. STOPPED
 2. FYA again
6. Adjacent circular
 6.
 6.
 6.
 6.

 

CURING THIRD YELLOW TRAP (METHOD 1)

Any of the following modifications will cure third yellow trap:

  • Remove the delay before the flashing yellow arrow begins flashing.
  • Prevent the reservicing of the phase from invoking the delay again.
  • Don't recall the pedestrian phase.

The diagrams show how the modified sequence prevents third yellow trap:

  1. The first diagram again shows the active phases before third yellow trap. The straight-ahead signals have circular green, and the left turns are permissive turns with flashing yellow arrows.
  2. In the second diagram, nothing changes, except that the pedestrian signals have changed to Walking Man. The left turns are not trapped, because the flashing yellow arrow never ended.

Permissive left turns

Permissive left turns

Flashing Yellow Arrows Protected/Permissive

Flashing yellow arrow - No yellow trap.
Time --->

2. Opposing circular
 2.
 2.
 2.
 2.
5. Opposing left turn
 6. Flash yellow arrow
 6.
 6.
 6.
1. Delayed left turn
 2. Flash yellow arrow
 2.
 2.
 2.
6. Adjacent circular
 6.
 6.
 6.
 6.

 

CURING THIRD YELLOW TRAP (METHOD 2)

Any of the following modifications will cure third yellow trap:

  • Clear out the recycled phase, and start it over from red (inefficient).
  • Clear out the entire intersection, and start it over from red (inefficient).

The diagrams show how the modified sequence prevents third yellow trap:

  1. The first diagram again shows the active phases before third yellow trap. The straight-ahead signals have circular green, and the left turns are permissive turns with flashing yellow arrows.
  2. In the second diagram, all traffic stops. Yellow trap has been prevented.
  3. In the third diagram, the pedestrian phase has restarted safely.

Permissive left turns

all traffic stops

no conf;lict pedestrians

Flashing Yellow Arrows Protected/Permissive

Flashing yellow arrow - No yellow trap.
Time --->

2. Opposing circular
 2.
 2.
 All Red 2.
 2.
5. Opposing left turn
 6. Flash yellow arrow
 6.
 All Red 5. STOPPED
 6. FYA again
1. Delayed left turn
 2. Flash yellow arrow
 2.
 All Red 1. STOPPED
 2. FYA again
6. Adjacent circular
 6.
 6.
 All Red 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.

Links

  1. All Yellow Trap Links are here:
    YELLOW TRAP INDEX PAGE
  2. Preventing yellow trap in signal pre-emptions.
  3. Preventing Second Yellow Trap.
  4. Unusual Traffic Problems, Hazards, and Solutions
  5. Home