PROBLEMS IN IMPLEMENTATION

• Failure to prevent 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. The yellow clearance intervals of the two circular green phases are set to different values.

• Failure to prevent green trap.

  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.

• Failure to provide flashing yellow signals for 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.

• Using a flashing yellow arrow in a shared signal face.

  This 6-indication signal face may be used only if flashing yellow arrow is used in combination with lane use control. The 6 aspects are:

  1. Steady circular red
  2. Steady circular yellow
  3. Steady circular green
  4. Steady yellow arrow
  5. Flashing yellow arrow (dual color section)
  6. Steady green arrow (dual color section)

  Different rules apply in cases where left turns are currently in exclusive or shared lanes:

  • In cases where all left turns are in exclusive lanes, the flashing yellow arrows behave normally. There are no restrictions on flashing yellow arrow use.
  • Where multiple lanes turn left exclusively, permissive left turns should be avoided.
  • Where left turns and through movements share a lane, left turn and through movements for that lane must receive stop signals simultaneously and receive go signals (permissive or protected) simultaneously.

  If lane use control is not being used, the flashing yellow arrow must NOT be used in a shared signal face.

• Replacing the circular green in the left turn signal with a flashing yellow arrow, but not changing the sequence to the correct one for a flashing yellow arrow.

  This is the worst 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.

• 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. But all such installations should have been removed by 2003.

• Using flashing yellow arrows at some intersections where geometries or other conditions make permissive turns dangerous.

  Any case (including acute angle turns) where it takes longer for the driver to make the turn than he thinks it will take must not have permissive turns. Exclusively protected signals must be installed.

  If the exit of the turn is not readily visible due to vertical alignment or obstructions, the turn must not be permissive.

• A misguided effort to protect pedestrians causes third yellow trap.

  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. They recommended inhibiting the flashing yellow arrow while a pedestrian phase is timing.

  This will cause third yellow trap if the phase is reserviced to serve a pedestrian pushbutton call with no calls on cross street phases.

  If the flashing yellow arrow is inhibited for pedestrians, reservicing the phase must also be inhibited. Otherwise, the flashing yellow arrow must not be inhibited.

  The flashing yellow arrow can be inhibited by time of day, provided that the time-of-day switching occurs while the left turn signal shows a red arrow.

• Using flashing yellow arrows at intersections where flashing yellow arrows can not be used.

  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 one permissive left turn can turn into multiple intersection legs at the same time
  • Where the flashing yellow arrow movement would cross more than one other movement that has a green light
  • Where conflicts develop in multilane left turns
  • Where a flashing yellow arrow face is installed on one leg of a road, but no flashing yellow arrow face is installed for the oncoming leg (except where the oncoming left turn is impossible).
  • When the left turn signal gets a steady yellow arrow while the oncoming circular green is shown
  • Any time more than one lens of the flashing yellow arrow signal face is lighted at the same time.
  • When the proper yellow and red change intervals are not displayed on each flashing yellow arrow face at the proper times:
    • The steady yellow arrow must accompany any change from flashing yellow arrow to red arrow.
    • The steady red arrow may follow the steady yellow arrow in the red change period.
    • The steady yellow arrow must not accompany any change from flashing yellow arrow to green arrow.
    • The flashing yellow arrow must continue to flash during the entire change interval before the green arrow.

  Even if the flashing yellow arrow is not allowed to be installed, the approach must still be protected against yellow trap. Other methods must be used to prevent yellow trap.

• The displays on the flashing yellow arrow face and the oncoming circular faces conflict with each other:
  • Steady yellow arrow shown while the oncoming circular green remains lit.
  • Steady green arrow shown while the oncoming circular green is lit.
  • Steady yellow arrow shown while an oncoming green right turn arrow is lit.
• The displays on the flashing yellow arrow face and the oncoming circular faces conflict with each other during a pre-emption:
  • Steady yellow arrow shown while the oncoming circular green remains lit.
  • Steady green arrow shown while the oncoming circular green is lit.
  • Steady yellow arrow shown while an oncoming green right turn arrow is lit.

  The pre-emption must not be designed to protect government employees in an emergency vehicle at the expense of safety to other drivers. The flashing yellow arrows must be displayed on the leg oncoming to the leg getting the priority green.

• A misguided effort to protect government employees causes yellow trap during an emergency vehicle preemption.

  Politicians tend to want to protect the government employees driving a police, fire, or medical emergency vehicle at the expense of the safety of other drivers.

  Because they want to favor their employees, they demand a design that cuts off three approaches and gives an exclusive green to the leg with the emergency vehicle. But this causes yellow trap if the phase oncoming to the exclusive phase is cut off by a yellow light for the preemption.

  Proper preemption design requires that a flashing yellow arrow must be shown to left turns that would otherwise be cut off, so they are not caught in yellow trap. Otherwise, an all-way-red clearance must precede the exclusive green.

  The flashing red lights on the emergency vehicle itself should protect it from the one movement that has the flashing yellow arrow. With the rest of the intersection cleared of traffic, the driver can easily see the emergency vehicle.

  If the yellow trap caused by an improper installation causes an accident, the emergency vehicle may be blocked from reaching its intended destination by the accident.

• Any time the flashing yellow arrow can end before the oncoming circular green ends

  Yellow trap can be prevented through other methods used with permissive turns, but the main advantage of having the flashing yellow arrows is lost. The purpose of the flashing yellow arrow is to prevent yellow trap, and it must be installed correctly to do this.