Given this, a point of diminishing returns is reached with the addition of phases as the efficiencies gained by separating traffic movements eventually become outweighed by the inefficiencies of increased lost time. Thus, a primary concern in signal timing is to keep the number of phases to a minimum. Because protected-turn phases add to lost time, they should be used only when warranted. Because of opposing motor vehicle traffic, left-turn movements typically require a protected-turn phase much more often than right turns. There are no nationally established guidelines on when protected left-turn phasing should be used, so local policies and practices should be consulted before a decision is made about whether to provide a protected left-turn phase. In general, decisions on whether to provide a protected left-turn phase are based on one or more of the following factors:
• Volume (just left turn or combination of left turn and opposing volume)
• Queuing (spillover)
• Traffic progression
• Opposing traffic speeds
• Geometry (number of left-turn lanes, crossing distance, sight distance)
• Crash experience (which may also be related to any of the above factors) More specific guidance on this issue can be found in several references, including the Highway Capacity Manua, the Traffic Control Devices Handbook, and the Manual of Traffic Signal Design.
One of the more common guidelines is the use of the cross product of left- turn volume and opposing through and right-turn volumes. The Highway Capacity Manual offers the following criteria for this guideline: The use of a protected left-turn phase should be considered when the product of left-turning vehicles and opposing traffic volume exceeds 50,000 during the peak hour for one opposing lane, 90,000 for two opposing lanes, or 110,000 for three or more opposing lanes.