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Mn/DOT Traffic Signal Timing and Coordination Manual Question 6 does Include a threshold volume of 240 vph for the subject left turn. However, if the opposing through volume is low, apply engineering judgment to determirie If Fi'A operation could be used even if the left turn volume exceeds 240 vph. if permissive FYA operation is allowed, protected/permissive operation may be investigated. The decision to use protected/permissive operation should be based on a capacity analysis. Definitions ✓ Protected only left turn operation: signal phasing that allows left turn movements to only be made on an exclusive phase (green arrow). ✓ Conflicting Left Turn Paths: At some locations geometric constraints at the intersection cause the paths of opposing left turn vehicles to cross as overlap creating a conflict. An example is an approach that crosses a divided roadway with a wide median. in these locations, it may be necessary to operate the left turns In a lead -lag sequence or a split phase sequence, not allowing simultaneous opposing left turns. This operation will require protected left turns. ✓ Opposing through lane (conflict): The opposing through lanes are the lanes across from, and in conflict with, the left turning vehicle. Multiple lanes make it difficult for a driver to evaluate gaps in oncoming traffic. An opposing separate right turn lane will typically not be counted with opposing through lanes unless engineering judgment Indicates that the lane configuration and number of right turns will cause conflicts with the left turn movement. ✓ Limited Sight Distance (Requirements): The minimum sight distance values necessary for the design vehicle volume to complete the turn movement. Distance should be calculated from the stop bar for the mainline left turning vehicle. Measurement is based on travel path, speed, and acceleration vehicle height. Both the sight distance for passenger vehicles and trucks should be checked using heights and distance requirements per the AASHTO Geometric Design Guide. The current reference at time this manual was prepared is the 2004 Guide, Chapter 9, Exhibit 9-67). ✓ Dual Left Turn Lanes: Multiple left turn lanes may consist of exclusive left turn lanes or a combination of exclusive left turn lanes and lanes that are shared by through and left turning traffic. Both the dual lane and the left turn lane opposing this operation are suggested to operate with protected phasing. Left turn lanes without opposing traffic, such as left turns off of a one- way street, does not require protected only phasing based upon this criteria. It might also be possible to run the FYA in permissive mode during low volume times of the day. ✓ Protected/pennlssive left turn operation: signal phasing that provides an exclusive phase (green arrow) followed by a permissive phase (flashing yellow arrow), time during the signal cycle where left turning traffic may make a left turn after yielding to oncoming traffic. ✓ Left Turn Related Collisions: These are Collisions that could be corrected by protected only phasing, such as those between those involving a left turning vehicle and an opposing through vehicle. At higher speeds the accidents collisions are likely to be more severe. Therefore, a lower number of collisions might be used as the parameter for consideration for high-speed approaches. Because of the variations In collisions overtime, an average number of collisions per year over a 3- year period should be used if the data Is available. ✓ Speed: Because it can be difficult for a driver to accurately judge available gaps In traffic approaching at high speeds, the engineer must exercise discretion when considering permissive or protected permissive left turn phasing with opposing speeds of 45 MPH or above. Page 3-30 Local Intersection Concepts May 2011 139