Does the MUTCD now require agencies to provide a flashing yellow arrow indication for a left-turn movement for the permissive left-turn phase?
No, this is not mandatory, but the flashing yellow arrow is now allowed in the 2009 MUTCD as an optional alternative display for PPLT or “permissive only” left turn movements (see Sections 4D.17 through 4D.20). NCHRP Report 493 describes the results of a comprehensive research project to evaluate the use of different permissive displays for protected-permissive mode left turns (PPLT), including the flashing yellow arrow (FYA). The research found that the most easily understood and most effective permissive display is the FYA in an all-arrows separate turn signal face for the left turn. An NCHRP follow-up study found that converting circular green permissive left-turn displays to flashing yellow arrow improved safety.