LEFT TURN SIGNAL PHASING

Left turn signal phases facilitate left turning traffic and usually improve the safety of the intersection for left turning vehicles. However, this is done at the expense of the amount of green time available for through traffic and will usually reduce the capacity of the intersection. Left turn arrows also result in longer cycle lengths which will in turn have a detrimental effect by increasing stops and delays. Pedestrian delays may be increased and due to the increased delay, pedestrians may elect to ignore the pedestrian signal. While phases for protected left turning vehicles are the most popular and most often added phases, other methods of handling left turn conflicts should be considered first. Potential solutions include prohibited left turns and geometric improvements.

LEFT TURN PHASE CRITERIA

The left turn phase criteria suggested below are a combination of left turning phasing used in several State in the United States and the result of considerable research and study. These warrants are not mandated by the Manual on Uniform Traffic Control Devices (MUTCD) and are provided for information purposes only. Suggested warrants are as follows:

Volume - Consider left turn phasing when the product of left turning and opposing volumes during peak hours exceed 100,000 on a four lane street, or 50,000 on a two lane street (1 approach lane). Also, the left turn volumes for 2 or more approach lanes should be greater than 2 vehicles per cycle during the peak hour period. Volumes meeting these levels indicate that a left turn phase may be justified and further study of the intersection recommended.Delay - Consider installing left turn phasing if a left turn total delay of 2 vehicle hours or more occur in a peak hour on a critical approach. Also, there should be a minimum left turn volume of greater than 2 vehicles per cycle during peak hour, and the average delay for left turning vehicle should be at least 35 seconds. Accident Experience - Install left turn phasing if the critical number of left turn accidents has occurred. For one approach, the critical number is 4 left turn accidents in one year, or 6 in two years. For both approaches, the critical number is 6 left turn accidents in one year, or 10 in 2 years.

PROTECTED/PERMITTED LEFT TURN PHASING

Protected/permitted left turn phasing is a left turn movement of traffic at a signalized intersection having a separate left turn phase in the signal cycle to provide a protected green arrow interval, as well as nonprotected circular green interval. Use of the protected/permitted left turn phasing technique is based on the assumption that the need for a protected left turn interval has been established. One of the basic precepts of the protected/permitted left turn phasing, is that the protected green arrow is displayed only when needed in a traffic demand condition. It is therefore emphasized that the protected/permitted left turn phasing technique is an efficient concept as opposed to an accident reduction concept although it will probably offer safer operation than permissive only operation.

PROTECTED ONLY LEFT TURN PHASING

When a separate interval is provided to accommodate a left turn without conflicting traffic, and left turns are prohibited during the rest of the cycle, protected only left turn phasing occurs. Although the MUTCD provides no left turn phasing warrants, the traffic control device handbook offers suggested guidelines for separate left turn phasing.

UNPROTECTED LEFT TURN PHASING

Unprotected left turn phasing occurs when an exclusive phase is not provided for left turn vehicles. Left turns are permitted to occur through gaps in the opposing traffic flow. Separate left turn lanes may or may not be provided.