Separate left-turn lanes may or may not be provided.The model also generates traffic forecasts for the A.M. Exceptions would be at those locations where turns are restricted by signs or other devices. Left-turns are permitted during a circular green when there are sufficient gaps in opposing traffic flow to allow a left-turning vehicle to safely make the turn. Unprotected (permissive) left-turn phasing occurs when there is no exclusive phase provided for left-turn vehicles. With a protected only left-turn, vehicles are only permitted to turn left when the left-turn green arrow is activated and turns are prohibited on the circular green indication. When a separate interval is provided to accommodate a left-turn without allowing any conflicting traffic a protected only left-turn phasing occurs. It is therefore emphasized that the protected/permitted left-turn phasing technique is an efficient concept opposed to a collision reduction concept, although it will probably offer safer operation than permissive operation only. One of the basic precepts of the protected/ permitted left-turn phasing is that the protected arrow is displayed only when needed in a traffic demand condition. 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. 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 a non-protected circular green interval. For both approaches to an intersection, the critical number is seven left-turn collisions in one year, or eleven in two years.For one approach to the intersection, the critical number is five left-turn collisions in one year, or seven in two years.Separate left-turn phasing may be considered if the critical number of reportable left-turn collisions has occurred. The average delay for all left-turning vehicles on the approach is at least 35 seconds during that same peak hour.There is a minimum left-turn volume on the approach of greater than two vehicles per cycle during the same peak hour and,.The total delay to all the left-turning vehicles on a crucial approach equals or exceeds two vehicle hours during a peak hour and,.Separate left-turn phasing may be considered if Further study of the intersection is recommended, including review of other potential solutions. Volumes meeting these levels are only an indication that a left-turn phase may be justified.
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