Sub-problem 6c: Planning Analysis at Sweet Avenue

The quick estimation method takes protected plus permitted phasing into consideration by subtracting two sneakers per cycle from the left-turn volume. The volume is not reduced at all for protected only operation, and it is not reduced below four vehicles per cycle in either case. The four vehicles per cycle lower limit was imposed to prevent unreasonably short left turn phases from occurring in the sequence. If the cycle length is not known, the HCM suggests using the maximum allowable cycle length to provide a conservative estimate.

For planning level analysis with projected volumes, many analysts prefer not to count on the additional permitted phase to provide capacity for left turns well into the future.  There are several reasons, including safety problems, that a protected left turn supplemented by a permitted phase today may not be able to retain that option twenty years from now.

Consider what would happen if the phasing for this example were modified to allow the left turn to proceed on the solid green for north-south traffic. The full operational procedure would assign some additional capacity to the left turn, thereby reducing the delay, and possibly improving the level of service. 

The quick estimation method, on the other hand, would see a much smaller difference in the operation. Based on the maximum cycle length of 120 seconds, four vehicles per cycle would create a volume of 120 vph.  So the left-turn volume entry would be reduced from 152 vph to 120 vph, but no other benefit of the permitted phase would be applied.