Why does the 2009 MUTCD recommend against locating a signal face that displays a circular green indication above or in front of an exclusive left turn lane?
Section 4D.13, paragraph 09, contains this Guidance: “For new or reconstructed signal installations, on an approach with an exclusive turn lane(s) for a left-turn (or U-turn to the left) movement and with opposing vehicular traffic, signal faces that display a CIRCULAR GREEN signal indication should not be post-mounted on the far-side median or mounted overhead above the exclusive turn lane(s) or the extension of the lane(s).” This provision is based on research (reported in NCHRP Report 493) which found that the circular green permissive left-turn indication is confusing to some left-turn drivers who assume it provides right-of-way during the permissive interval. The placement of the circular green indication directly above or in line with an exclusive left-turn lane exacerbates the safety issues with this display. Research has found that displaying a circular green signal indication directly over an exclusive left-turn lane led to a higher left-turn crash rate than “shared” displays
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