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MnIDOT Traffic Signal Timing and Coordination Manual <br />Question 6does include athreshold volume ufZ4Ovphfor the subject left turn. However, if the opposing <br />through volume is low, apply engineering judgment to clotermirie if FYA operation could be used even if <br />the left turn volume exceeds 24Dvph. <br />If permissive FYA operation is allowed, protected/permissive operation may be investigated. The decision <br />to use protected/permissive operation should be based on a capacity analysis. <br />Definitions <br />Protected only left turn operation: signal phasing that allows left turn movements to only be made <br />unanexclusive phase (green orrnu). <br />Conflicting Left Turn Paths: At some locations geometric constraints at the intersection omooe the <br />paths of opposing left turn vehicles to unuso as overlap creating o conflict. An example is an <br />approach that crosses a divided roadway with a wide median. In these locations, itmay benecessary <br />to operate the left turns in a lead -lag sequence or a split phase sequence, not allowing simultaneous <br />opposing left turns. This operation will require protected left turna. <br />r/ Opposing through {ono (conflict); The opposing through lanes are the lanes across from, and in <br />conflict with, the left turning vehicle. Multiple lanes make itdifficult for adriver toevaluate gaps in <br />oncoming traffic. An opposing separate right turn lane will typically not be counted with opposing <br />through lanes unless engineering judgment indicates that the lane configuration and number of <br />right turns will cause conflicts with the /eftturnmovement, <br />v/ Limited Sight Distance /Degu/,enmentd/The minimum sight distance valuesnecessary for the design <br />vehicle volume tocomplete the turn movement. Distance should be calculated from the stop bar <br />for the mainline left turning vehicle. Measurement is based an travel path, speed, and acceleration <br />vehicle height. Both the sight dixtanceforpnasengurvehic/es and trucks should he checked using <br />heights and distance requirements pertheAASHTOSeonoetrioDesignGu/da. The current reference <br />at time this manual was prepared is the 2004 Guide, Chapter 9, Exhibit 9-67). <br />`/ Doo} Left Tomo Lanes: Multiple !eft turn lanes may consist of exclusive loft turn lanes or o <br />combination of exclusive left turn lanes and lanes that are shared by through and left turning traffic. <br />Both the duo/ lane and the left turn lane opposing this operation are suggested to operate with <br />protected phasing. Left turn lanes without opposing traffic, uu'ch as left turns off of one- Way <br />street, does not require protected only phasing based upon this criteria. It might also be possible to <br />run the FYAinpermissive mode during low volume times ofthe day. <br />Protected/permissive left turn operation, signal phasing that pnux1das an exclusive phase (green <br />arrow) followed by a permissive phase (flashing yellow arrow), time during the signal cycle where <br />left turning traffic may make aleft turn after yielding tooncoming traffic. <br />f Left Turn Related Collisions: These are Collisions that could be corrected by protected only phasing, <br />such as those between those involving a left turning vehicle and an opposing through vah/c|e. At <br />higher speeds the accidents collisions are likely to be more severe. Therefore, a lower number of <br />colfisions might be used as the parameter for consideration for high-speed approaches, Because of <br />the variations in collisions overtime, an average number of collisions per year ovef a 3- year period <br />sbbu|dbeused ifthe data /savailable. <br />`/ Speed/ Because it can be difflmult fora driver to accurately judge available gaps in traffic <br />approaching athigh speeds, the oogineermustexercise discretion when considering permissive or <br />protected permissive left turn phasing with opposing speeds of 45 MPH or above. <br />Page 13-30 Local Intersection Concepts <br />Moy2D11 <br />� <br />v <br />