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

The results of this analysis (critical v/c ratio = 1.23), shown in Exhibit 1-56, indicate that the existing geometric configuration and signal timing would not accommodate the projected volume. This could indicate the need for intersection improvements, but first we must be sure that the quick estimation method is representing the operation realistically.

We should begin by looking at the right turns. The quick estimation method assumes that the right turns will proceed only on the concurrent through traffic phases. The phasing plan presented in Exhibit 1-55 indicates that both the eastbound and westbound turns are served during other phases as well. Therefore, these movements may have some capacity that is not being recognized by the quick estimation method.

The HCM suggests that "when an exclusive right-turn lane movement runs concurrent with a protected left-turn phase from the cross street...the total right-turn volume for the analysis can be reduced by the number of shadowed left-turners." The shadowing movements for the EB and WB right turns are actually through movements in this case because of the peculiar intersection geometrics. The EB right turn is shadowed by the NB through movement, and the WB right turn is shadowed by the EB through movement. In both cases, the lane volumes of the shadowing  movements exceed the lane volumes of the shadowed right turns. Therefore, it would be reasonable to remove the EB and WB right turns from the analysis.