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--- Technical Procedures <br /> <br /> As indicated in Table 8, the saturation flow rates varied by movement type. <br /> Exclusive left-turn saturation flow rates were approximately 10 percent less than those for <br /> through lees. Saturation flow rates for shared left and through lanes were 18 percent lower <br /> than for through lanes. Sufficient data was not collected to provide statistical accuracy for <br /> these averages. They were consistent, however, with the passenger car equivalent (PCE) <br /> values adjustments provided in Circular 212. <br /> <br /> 7.2 OPTIONAL CAPACITY REDUCTION <br /> <br /> The effect of vehicle mix, intersection geometries and other factors on <br /> intersection capacity is well documented. These factors, however, are not considered directly <br /> in the Circular 212 Planning Methodology. This was why a lower capacity (1,500 vph) was <br /> originally selected for use in Circular 212. <br /> <br /> The CCTA methodology, which uses a higher capacity (1,800 vph), may <br /> underestimate existing or future congestion at some locations. The reductions in the capacities <br /> provided in Table 9 are therefore optional, provided that measurement of saturation flow rates <br /> justify the lower capacities. Once an intersection's capacity is reduced, it cannot be increased <br /> unless intersection geometries are improved and higher saturation flow rates have been <br /> measured in the field. Under no circumstances can a signalized intersection capacity above <br /> 1,800 vph be used under the CCTA methodology. Saturation flow rates must be measured <br /> using the technique described in Chapter 9, Appendix IV of the 1985 Highway Capacity <br /> Manual. (A copy is provided in Appendix A of these Technical Procedures). <br /> <br /> The saturation flow rates must be adjusted to establish the capacity for the <br /> traffic movement considered. Adjustment of the saturation flow rates should be performed as <br /> described in equation 9-1 of the 1985 HCM: <br /> <br /> Where (for lane group or approach i) <br /> <br /> c~ = capacity in vehicles per hour; <br /> s, = saturation flow rate in vehicles per hour; <br />__ g = effective green time in seconds; and <br /> C = intersection cycle length in seconds. <br /> <br />" a:~sea~m.10 45 ~ <br /> <br /> <br />