Sub-problem 6d: Planning Analysis at SH-8 Intersection

The NB right turn is shadowed by the WB left turn, which takes place from two lanes. From Exhibit 1-54, the total WB left turn volume is 254 vph or 127 vph per lane. So, it would be appropriate to reduce the NB right turn volume of 305 vph by 127 vph to give an effective volume of 178 vph.

The removal of the EB right turn and the reduction of the NB right turn volume do not affect the results, because the through movement is the critical movement that governs both of these approaches. On the other hand, the right turn is the critical movement on the westbound approach. So, the elimination of that movement will benefit the operation of the whole intersection.

Exhibit 1-57 indicates that the removal of the right turns from the WB approach will reduce the estimated critical v/c ratio to from 1.23 to 0.88, indicating a “near capacity” condition. We could conclude from this analysis that the intersection could probably accommodate the additional 20 year traffic volumes without the need for geometric improvements.

A final note on the importance of the specified cycle length range in the Quick Estimation Method:  note that the results for Sub-Problem 6a, Sub-Problem 6b and Sub-Problem 6c indicated a computed cycle length at the minimum of the specified range and a “below capacity” status.  Our first attempt at Problem 6d indicated a computed cycle length at the maximum of this range and an “above capacity” status”.  After the right turns were modified, a 120-second cycle length  was indicated in Exhibit 1-57 as the “specified cycle length.”  As a general rule, if the adequacy of the potential capacity of the intersection is being evaluated, the maximum allowable cycle length should always be used.  If a cycle range is specified, then the results could be misleadingly pessimistic because of the capacity reduction effect of shorter cycle lengths.