http://pec.lib.wvu.edu/record/P119311973A Recommended clearance interval signal indication for protected left turns.<p> Pamphlet. "WVaDH research project, 31." On cover: Engineering Experiment Station, West Virginia University. Sponsored by West Virginia Department of Highways in cooperation with U. S. Department of Transportation, Federal Highway Administration." </p>King, L. Ellis.Plummer, Ralph W.Traffic signs and signals.14256