FLASHING YELLOW ARROWS
CAN MAKE LEAD-LAG TRAFFIC SEQUENCES SAFE

FIXING YELLOW TRAP

The above two signal sequences prevent yellow trap, even though permissive turns are used with lead-lag phasing. The flashing yellow arrow can continue to flash while the thru signals are red. This allows engineers to end the permissive turn at a safe time.

The traditional 5-section doghouse signal (with the yellow-trap circular green) is now being replaced with the 4-section signal (top animation) in many states, preventing yellow trap at intersections so equipped.

The difference between the circular green and the flashing yellow arrow:

The circular green and the flashing yellow arrow have the same meaning for left turning drivers.

But, for drivers who are not turning left, the circular green has a very different meaning from that of the flashing yellow arrow.

That difference is what cures the yellow trap.

NEW MEANINGS OF SIGNAL INDICATIONS

Note that the meanings of some signal indications have been redefined, so the flashing yellow arrow signal can work.

Also note that left and right turns must yield to vehicle and pedestrian conflicts for the following signal indications:

flashing circular yellow
steady circular yellow
steady circular green
flashing yellow arrow
steady yellow arrow

THE 5-SECTION VERSION

The 5-section version above is not allowed by the new 2009 Manual of Uniform Traffic Control Devices (MUTCD), because it does not match the defined signal face in the manual. It has been allowed as an experimental alternate in Idaho.

The 5-section signal face has the following advantages over the 4-section all-arrow face:

1. A separate signal section terminates the flashing yellow arrow. This distinguishes it more from the termination of the green arrow. A driver facing a flashing yellow arrow is also busy looking for gaps in traffic to make his turn through. Such a driver just glances at the signal briefly, and might not notice that the yellow arrow moved to a different lens and stopped flashing.
   See this perception problem yourself.
   Another way to make the signal change a more positive indication is to install black backplanes with yellow reflective borders, as shown in the 4-light signal example above. These not only locate the arrow in the signal face, but also locate the signal face itself, if the power fails.
   See this solution.
   If the signal face is painted a dark color, or is hard to see at night, this border should be used.
2. The 5-section version allows the use of existing signal faces. But an extra head must be added anyway where the left turn face is one of the two required faces for straight-ahead traffic, since the left turn signal no longer displays straight-ahead indications. Also, additional signal circuits are needed, since the old 5-section shared face has only two left-turn circuits, and five circuits are needed for this face. The signal heads may also have to be moved, because circular greens are not allowed over the left turn lane.
3. The longer 4-section signal face might require replacing the span wires or mast arms holding up the signals, to maintain the required vertical clearance. But a dual color indication arrow can be used to display both the flashing yellow arrow and the green arrow in the same section.

The 5-section flashing-yellow-arrow signal face shown above should be safer. But it technically violates the new MUTCD.

Germany is using a similar signal, but has a black arrow on a yellow background (the reverse of a normal yellow arrow). It also has a circular red with a similar black arrow on it. This removes some of the confusion. A normal yellow arrow is used for the flashing yellow arrow.

Switzerland uses a flashing yield triangle to the left of the green turn arrow to indicate that the green arrow is permissive, not protected. For the protected phase, the triangle does not light up. This also removes the confusion.

OTHER DESIGNS NOT APPROVED

The display of a circular yellow to terminate a flashing yellow arrow is a more positive indication of a signal change. A driver facing a flashing yellow arrow is also busy looking for gaps in traffic to make his turn through. Such a driver just glances at the signal briefly, and might not notice that the yellow arrow moved to a different lens and stopped flashing. A change to a circular indication (or a reversed arrow indication) removes this confusion. This distinction is especially important at night, when the driver can't see the entire signal face, but only the lighted indication.

Note that the different clearance indication in either of these signal sequences tells the driver that the yellow clearance is still a permissive turn through oncoming traffic, and not a protected clearance (as the yellow arrow is after a green arrow).

THE RED ARROW PROBLEM

Many states still prohibit the red arrow. Their legislators seem to need some brain transplants, but that is beyond the scope of this article. If they can't see that an arrow indicates a signal for only one direction of intended movement, rather than a "go" signal, they have serious mental blocks. Most drivers catch on to the red arrow indication after seeing one in actual operation. When the signal changes from green arrow to yellow arrow, and then to red arrow, the meaning usually becomes obvious. The light changed from green to yellow, and then to red.

The corresponding 3-section face for permissive left turns, but no protected indications also makes more sense. It is shown here.

YELLOW TRAP

The above signal causes yellow trap, because permissive turns allowed on circular green must be ended at unsafe times. When the circular green is ended to stop traffic moving straight, the permissive turns are stopped at the same time. But the opposing thru movement still continues, trapping left turn drivers waiting to turn through gaps in traffic. They are either trapped in the intersection with a red light, or they expect opposing traffic to have a yellow too, and turn across live traffic moving on a green light.

Meanwhile, opposing thru drivers see no change in signal indication until after the yellow trap is sprung. So they don't expect anything unusual. Suddenly, a car turns left across their paths.

The really dangerous part is that each driver expects the other driver to stop or yield.

The circular green is the cause of this trouble, because it cannot release permissive turns without also releasing traffic going straight. Thus, the permissive left turn is cut off at an unsafe time in the signal cycle, causing yellow trap, because the straight-ahead traffic must be stopped at that time.

The flashing yellow arrow can solve this problem. It can allow permissive turns to go while the straight-ahead signals are red. Thus, the permissive turns can be safely synchronized to the oncoming circular green, rather than the circular green facing those left turns.