This article assumes traffic drives on the right.
The yellow trap is a potentially dangerous scenario in traffic flow through a traffic light relating to permissive left turns. It occurs when a circular yellow light is displayed to a movement with permissive left turns, while at the same time, opposing through traffic still has a circular green light. Some drivers facing the circular yellow (then red) lights will assume the opposite direction faces the same color display, and that oncoming traffic will stop. This leads to disaster as drivers attempt to complete a left turn when it is not safe to do so. The left-turning driver may legally be at fault, for failure to yield, but made an understandable mistake.
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 (see List of variations in traffic light signalling and operation).
Many agencies in North America routinely allow the yellow trap, especially during emergency vehicle preemption, despite clear prohibition from the Manual on Uniform Traffic Control Devices. (The Manual allows this only if a sign reading '"Oncoming traffic has extended green"' or '"Oncoming traffic may have extended green"' is used.) 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.
There are extremely rare cases where creation of the yellow trap may be appropriate, such as where railroad preemption must begin as quickly as possible and traffic is likely to be queued on railroad tracks. A better long-term solution for such locations would be to improve the train detection system to provide more warning time of an approaching train. Another solution uses a flashing yellow arrow to allow the oncoming left turn to continue to turn through gaps in traffic. If a yellow trap collision occurs during the preemption, it blocks the intersection, preventing vehicles from getting out of the way of the train or the emergency vehicle.