PREVENTING YELLOW TRAP IN SIGNAL PREEMPTIONS

PRE-EMPTED SIGNALS ARE DIFFERENT

When a traffic signal is pre-empted for a railroad crossing, drawbridge, emergency vehicle, queue discharge system, or other special sequence, special care must be taken that the preemption sequence does not endanger people using the roads. One danger to the people is yellow trap. Yellow trap occurs when left turns are made on circular greens, and then one of the circular greens ends, but the other one does not.

Special considerations must be taken into account when designing signals that are pre-empted for special events. They can cause special hazards even when used with Flashing Yellow Arrows.

Two special words are used to differentiate two similar procedures:

  • Clearance - The display of a yellow signal indication to stop a traffic movement and clear the intersection.
  • Clearing - The removal of vehicles from a leg of an intersection as part of a preemption.

THE WRONG WAY

Too often, public officials value public employees more than the general motoring public. This is too often true when signal preemptions are designed. The preemption designs protect the emergency vehicle too much, by exposing other motorists to yellow trap.

Here is the wrong way to do it:

The pre-empted leg is the leg of an intersection with these properties:

  • The leg that is blocked by a railroad, drawbridge, or other use that stops traffic
  • The leg that priority is needed on for an emergency vehicle or a queue discharge.

In these diagrams, the pre-empted leg is on the north (the leg at the top of the diagrams).

The wrong sequence:

  1. The trouble happens only if the street with the pre-empted leg has green signals in both directions (first diagram).
  2. The second diagram shows what happens if one leg is cut off, while the other leg retains a circular green. Any driver waiting to make a left turn from the opposite leg (red) is faced with yellow trap.
  3. The third diagram shows the intersection cleared for the emergency vehicle.

There is a second reason why yellow trap must be removed from all preemption sequences. If the yellow-trap causes an accident, the accident will prevent the preemption from clearing out the road for the railroad crossing, drawbridge, or emergency vehicle. This endangers the lives of others, including anyone an emergency vehicle was hurrying to rescue.

The government officials in charge of traffic control need to know the following:

permissive left turns

preemption yellow-trap occurs

preemption discharge

THE RIGHT WAY

PREEMPTIONS FOR BLOCKED ROADS (RAILROAD CROSSINGS, DRAWBRIDGES, ETC.)

In the following table, the pre-empted leg, also called the blocked leg, is:

  • The leg that is blocked by a railroad crossing
  • The leg that is blocked by a drawbridge or other movable bridge
  • The leg that is blocked by any use that requires clearing all traffic out of a section of road
  • The leg that is blocked by any other use that stops traffic

Here is the sequence. Dark entries are not part of the sequence.

Signals Left Thru Left Thru Left Thru Left Thru Left Thru Left Thru
PREEMPTION SEQUENCES PRIORITY LEG SEQUENCE
Signal display type: Permissive Turns
on Circular Green		 Flashing Yellow Arrows
Exclusively Protected‡		 Any of These
Displays; Exclusively
Protected Shown
Signal color displayed to priority street: Any signals GREEN Any signals GREEN All signals RED
Direction on priority street: Priority Opposing Priority Opposing Priority Opposing
Signals Left Thru Left Thru Left Thru Left Thru Left Thru Left Thru
Active yellow clearance for conflicting traffic†



FYA
FYA
<-
<-
Red clearance for conflicting traffic



FYA
FYA
<-
<-
Clearing: Turn all signals facing pre-empted leg green. <-


<-
FYA
<-
<-
After enough delay, start warning signals at the blockage. <- YES

<- YES FYA
<- YES <-
Allow time for blocked area to empty out. <-


<-
FYA
<-
<-
Clearance for clearing traffic leaving blocked leg. <-


<-
<-
<-
<-
Red clearance for clearing traffic. Signal is red in all directions.



<-
<-
<-
<-
Leg blocked. Begin hold period. Red arrows for turns into blocked leg.


ROR <-
<- ROR <-
<- ROR
Leg blocked. Hold #1: cross street.


ROR <-
<- ROR <-
<- ROR
Leg blocked. Hold #1: clearance.


ROR <-
<- ROR <-
<- ROR
Leg blocked. Hold #2: turns from opposing leg.

<- ROR <-
<- ROR <-
<- ROR
Leg blocked. Hold #2: clearance.

<- ROR <-
<- ROR <-
<- ROR
Leg blocked. Repeat Hold #1 and Hold #2 until blockage ends.


ROR <-
<- ROR <-
<- ROR
Blockage removed. Preemption released.



<-
<-
<-
<-
Turn off signals at the blockage. (Signals shown at end of Hold #1.)



<-
<-
<-
<-
Allow all movements to operate. <-
<-
<-
<-
<-
<-
Signal operates without yellow trap or short green. <-
<-
<-
<-
<-
<-

† Shown clearing thru phases in both directions. If any left turn phases are active, they are cleared in a similar way. If the desired preemption phases are already active, no clearance takes place.

‡ Flashing Yellow Arrows shown. If Exclusively Protected is used, substitute red arrow for FYA in all positions.

FYA - Flashing Yellow Arrow

ROR - Either right turn on red, or free right turn.

  • The clearance sequence must actively clear all cars that are waiting in the intersection to turn into the road about to be shut off by the blockage. This must be done BEFORE the blocked street clearing phases are displayed (top left diagram).
  • After the blocked leg starts the clearing phase (middle diagram), red arrow or circular red must be displayed continuously to any movement that would enter the blocked leg, until after the obstruction is removed (bottom diagrams). This will require special logic (e.g. exclusively protected*), because the hold phases would normally display the flashing yellow arrow with the opposing circular green. A blank-out sign identifying the hazard and prohibiting the turn should also be displayed during this period.
  • If Flashing Yellow Arrows are used on the street that is to be shut off by a railroad, drawbridge, or other event, the standard Flashing Yellow Arrows display can be used.
  • If Flashing Yellow Arrows (or exclusively protected* left turns) are not used on the street that is to be pre-empted, flashing yellow arrows should be added, even if they are just the three-section permissive variety. Otherwise, the change from a two-way green on the street with the preemption to the clearing preemption phase (middle diagram) causes yellow-trap to occur.
  • If an approach has a lane that shares left turn and straight ahead traffic, Flashing Yellow Arrows can not be used. A special sequence that takes longer must be used.
  • The signal can cycle between phases that do not enter the blocked leg (bottom diagrams). The black T shapes on the diagrams are stopped movements that will not be released until the preemption ends.
  • When the preemption ends, enough time must be provided for those waiting to turn on the flashing yellow arrow to clear, before changing the signal back to standard operation. Also, make sure the signal returns to normal operation in a safe way, without causing another yellow-trap. It should return by enabling the missing phases, not by switching controllers.
  • There is a need to check for second yellow trap, because right turn signals are used. They must not be allowed to prevent left turns from safely clearing.
  • Also check for third yellow trap, if phases are reserviced during the preemption.

Either

permissive left turns

Or

permissive left turns

Clearing phase

Preemption discharge

Cycle between phases
until preemption ends:

Preemption hold cross

Preemption hold turns

PREEMPTIONS FOR PRIORITY (EMERGENCY VEHICLES, QUEUE DISCHARGE, ETC.)

In the following table, the pre-empted leg is:

  • The leg that priority is needed on for an emergency vehicle
  • The leg that priority is needed on for a queue discharge
  • The leg that mass-transit priority is needed on

Here are the sequences.

Signals Left Thru Left Thru Left Thru Left Thru Left Thru Left Thru
PREEMPTION SEQUENCES PRIORITY LEG SEQUENCE
Signal display type: Permissive Turns
on Circular Green		 Flashing Yellow Arrows
Exclusively Protected‡		 Any of These
Displays; Exclusively
Protected Shown
Signal color displayed to priority street: Any signals GREEN Any signals GREEN All signals RED
Direction on priority street: Priority Opposing Priority Opposing Priority Opposing
Signals Left Thru Left Thru Left Thru Left Thru Left Thru Left Thru
Active yellow clearance for conflicting traffic†



FYA
FYA
<-
<-
Red clearance for conflicting traffic



FYA
FYA
<-
<-
Turn all signals facing pre-empted leg green. <-


<-
FYA
<-
<-
If emergency vehicle, light white preemption grant signal. <- YES

<- YES FYA
<- YES <-
Wait for priority condition to end. <-


<-
FYA
<-
<-
Preemption release, turn off white grant signal. <- YES

<- YES FYA
<- YES <-
Allow all movements to operate. <-


<-
FYA
<-
<-
Signal operates without yellow trap or short green.



FYA
FYA
<-
<-

† Shown clearing thru phases in both directions. If any left turn phases are active, they are cleared in a similar way. If the desired preemption phases are already active, no clearance takes place.

‡ Flashing Yellow Arrows shown. If Exclusively Protected is used, substitute red arrow for FYA in all positions.

Criteria:

  • The clearing sequence must actively clear all cars that are waiting in the intersection to turn into the road about to receive priority preemption.
  • If Flashing Yellow Arrows (or exclusively protected* left turns) are not used on the street with priority preemption, flashing yellow arrows should be added, even if they are just the three-section permissive variety. Otherwise, the change from a two-way green on the street with the preemption to the preemption phase causes yellow-trap to occur.
  • If an approach has a lane that shares left turn and straight ahead traffic, Flashing Yellow Arrows can not be used. A special sequence that takes longer must be used.
  • For a queue-discharge preemption that gives the green to only one approach, Flashing Yellow Arrows faces or exclusively protected* left turns must be installed in both the direction of the street to be cleared and the opposite direction.
  • For an emergency vehicle preemption, care MUST be taken to prevent yellow-trap. A misplaced priority for protecting emergency vehicles must never be allowed to cause a signal to display a sequence that endangers other vehicles.
  • The flashing yellow arrow must continue to be displayed to the driver wanting to make the left turn across the path of the emergency vehicle. Otherwise, that driver will turn left across opposing traffic when his signal turns yellow. Any resulting crash will then delay the emergency vehicle further. Instead, let the emergency vehicle's own flashing red lights cause the turning driver to wait for it to pass.

Either

permissive left turns

Or

permissive left turns

Clearing phase

Preemption discharge

Phase continues
until preemption ends:
  • Because this preemption sequence clears the way for an emergency vehicle or a queue discharge, the signal remains in the clearing phase until the preemption ends.
  • When the preemption ends, enough time must be provided for those waiting to turn on the flashing yellow arrow to clear, before changing the signal back to standard operation. Also, make sure the signal returns to normal operation in a safe way, without causing another yellow-trap. It should return by enabling the missing phases, not by switching controllers.
  • There is a need to check for second yellow trap if right turn signals are used. They must not be allowed to prevent left turns from safely clearing.
  • Also check for third yellow trap, if phases are reserviced during the preemption.

* 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.

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