In 2006, the Federal Highway Administration issued an Interim Approval for the use of a flashing yellow arrow signal indication in a separate signal face as an optional alternative to a circular green signal indication for permissive left-turn movements, and this display is now in use at 500 or more intersections in 30 states across the U.S. The provisions of the Interim Approval have now been incorporate into the Manual on Uniform Traffic Control Devices.
Research found that a flashing yellow arrow is the best overall alternative to the circular green as the signal display for a permissive left-turn movement. The flashing yellow arrow has a high level of understanding and correct response by left-turn drivers and a lower fail-critical rate than the circular green. The flashing yellow arrow display in a separate signal face for the left-turn movement also offers more versatility in field applications. It is capable of being operated in any of the various models of left-turn operation by time of day, and it is easily programmed to avoid the "yellow trap" that is associated with some permissive left turns at the end of the circular green display.