Yellow trap

The yellow trap is a potentially dangerous scenario relating to turns at a traffic light-controlled intersection across oncoming traffic without a protective turn signal; for right-hand traffic, this refers to a vehicle attempting to make a left turn without a permissive/protective left-turn signal.

The yellow trap occurs when the timing of the yellow lights is asymmetric for two-way traffic on a single road: when a vehicle is waiting to turn across oncoming traffic and receives a yellow light from the traffic signal, the driver may assume that oncoming traffic also has received a yellow light and proceeds with the turn, believing that oncoming traffic will yield the right-of-way on an assumed yellow. If oncoming traffic has an extended green light instead, this could lead to a crash.

Sequence
Illustration of the yellow trap at an intersection Top: The driver of the red car is stopped at the intersection, waiting to turn left against oncoming traffic once the blue car passes. The turn against oncoming traffic is not protected, i.e., there is no separate signal for the turn.
Middle: The driver of the red car has received a yellow light signal and mistakenly assumes that oncoming traffic also has received a yellow light signal and will yield. The driver of the red car proceeds to turn against oncoming traffic (in this case, a left turn).
Bottom: The blue car, which still has the right-of-way due to the green light, crashes into the red car. The red car's driver should have yielded the right-of-way but mistakenly assumed the blue car also had received a yellow, then red light and would yield.

When a vehicle traveling on a road with two-way traffic is preparing to turn at an intersection and the intersection does not have a protected turn across oncoming traffic, the yellow trap may occur when the vehicle preparing to turn is given a yellow light, while at the same time, traffic on the same road moving in the opposite direction still has a green light. The driver who is intending to turn, facing the yellow (then red) light, could assume that the traffic going in the opposite direction on the same road also has a yellow or red light, and that oncoming traffic will stop or yield the right-of-way.[1] This could potentially lead to a crash if the turning driver attempts to proceed with the turn across traffic, assuming mistakenly they can take the right-of-way, but it is not safe to do so. The turning driver may legally be at fault, for failure to yield, but the understandable mistake stems from the reasonable assumption that both lights were turning yellow simultaneously.[2]

The Manual on Uniform Traffic Control Devices (MUTCD) allows this asymmetric light timing only if the intersection has signs posted stating that Oncoming Traffic Has Extended Green or Oncoming Traffic May Have Extended Green.[3] However, many agencies in North America routinely allow the yellow trap, especially during emergency vehicle preemption, despite clear prohibition from the MUTCD. Some of this stems from difficulties programming older traffic signal control software to prevent the yellow trap, but much stems from traffic engineers or technicians not understanding the yellow trap hazard, or believing it is not a serious problem.

In left-hand traffic, the same scenario is applicable for drivers turning right.

Solutions
Protected-permissive traffic signals in the United States: (1) "Classic" doghouse signal (1971); (2) 4-light signal with flashing yellow arrow; (3) Doghouse with flashing yellow arrow

Solutions to the yellow trap include the flashing yellow arrow, Dallas phasing, Arlington phasing, simultaneously ending both through movements (circular green lights change to yellow, then red) before serving both left-turn movements (green arrows), or prohibiting one of the two left-turn movements.

References
  1. "Frequently Asked Questions – Part 4 – Highway Traffic Signals". Federal Highway Administration. Retrieved February 10, 2019.
  2. Vigmond, Karen. "Yellow Light and Left Hand Turn Accidents – Who's at Fault?". Oatley Vigmond. Retrieved February 10, 2019.
  3. "Interpretation Letter 4-320(I) - Steady Yellow Arrow After Flashing Yellow Arrow". Federal Highway Administration. Retrieved February 10, 2019.
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