Mid-sized and smaller cities have frequently chosen to provide protected left turn phasing on main street approaches of signalized intersections, citing a belief that it provided safer operations. Recently however, this traditional belief has been challenged for a variety of reasons. An option that combines protected phasing with permitted phasing emerged and is starting to see deployment. Also, federal research identified a new approach in the signal display, using a flashing yellow arrow during the permitted portion of the signal cycle. W-Trans completed a design at several intersections in Petaluma which is currently in construction.
As part of the project a literature search was conducted for the best signage to accompany the new phasing sequence. NCHRP report 493 presents the summary of the federal research with a brief mention of recommended signage. The report can be found at
http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_493.pdf. The included picture shows a sign developed for a project in Torrance, CA