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Fixing the Yellow Trap

A protected left-turn phase can lead or lag an opposing through movement. Traffic engineers often desire to increase operational efficiency on roadways by using lead-lag signal phasing but cannot necessarily safely do so if protected/permissive left-turn (PLT) control is used. The MUTCD does not preclude the use of lead-lag left turns with PPLT control; however, doing so can create what is known as the “yellow trap.” The yellow trap condition essentially leads the left-turning driver into the intersection when it is possibly unsafe to do so even though the signal displays are correct. During the signal change from permissive movements in both directions to a lagging protected movement in one direction, a yellow trap is presented to the left turning driver whose permissive left-turn phase is terminating. The animated file below illustrates how the yellow trap occurs.

Yellow Trap

There have been some innovative advances in signal phase sequence, such as the “Dallas Display,” which alleviate the yellow trap. The 2000 Manual on Uniform Traffic Control Devices (MUTCD) allows for the use of visibility-limited, left-turn signal faces that implement the Dallas Display.

The flashing yellow arrow eliminates the yellow trap without requiring louvers or other visibility-limiting devices. As demonstrated in the illustration below, logically tying the flashing yellow arrow indication to the opposing through green indication completely eliminates the yellow trap, even when lead/lag phasing is used or when side street phases are skipped and the leading left-turn phase now follows the through phase.

Yellow Trap Flashing Yellow Solution

Click the following links for more information on how the flashing yellow arrow works:


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