An engineering study should be conducted to determine the appropriate left-turn signal control mode for signalized intersections. The study should consider left-turn volumes, crash history, 85th percentile (posted) speed, sight distance, number of left-turn lanes, number of opposing through lanes, pedestrian volumes, and opposing through volumes. Additionally, intersection geometry can affect the selection of the left-turn signal control mode. Intersections with wide medians, conflicting turning paths, non-standard alignments, etc., are also factors in determining left-turn signal mode.

There are many sources for determining left-turn control mode. Two flow charts obtained from recent research projects that may be used as an aid in determining the appropriate left-turn control mode are shown below.