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