Sub-problem 1c: Westbound Peak Hour

The first task is to specify the conditions. The question again emerges: where should we do the analysis? Should we use the 6-7% upgrade, the 1-2% upgrade, or the 1-2% downgrade? The HCM says: use the section that will produce the most conservative estimate of the LOS. That is, worst case governs. So we’ll use the 6-7% upgrade section. In addition, since it is a mile long or more, we’ll assume it’s a constant grade, not a “rolling” or “mountainous” section.

For the other inputs, we’ll have the following values: 3 lanes, 3,240 veh/hr (the average PM peak hour volume), 55 mph as the free flow speed, 0.90 as the peak hour factor, 5% trucks/buses, and “local” drivers, i.e., ones that are familiar with the facility.

The results are as follows. A flow rate of 1,440 pcplph, a density of 26 pcplpm, a LOS of D, and an average passenger-car speed of 55 mph (see Dataset 5).

With this kind of output, we should see if two lanes would be enough. If we change the number of lanes to two, the flow rate becomes 2,160 pcplph, the density is 42 pcplpm, the LOS is E, and the average passenger car speed is 52 mph. The third lane has a huge impact on the results (see Dataset 6).

Let’s look at the issue about whether the truck-climbing lane is needed. While this specific issue is not addressed in the HCM methodology for basic freeway sections, we can analyze this by looking at the LOS for the section without the presence of the truck-climbing lane and comparing that to the LOS calculated if all trucks are removed from the passenger car travel lanes.

First, let's look at the truck-climbing lane. We effectively have 5% trucks, or 162 trucks per hour, in the peak hour volume. If the E_T is 5, as HCM Exhibit 23-9 suggests, the equivalent flow in passenger cars per hour is 810 passenger cars per hour. That illustrates the impact of the trucks. That’s almost a half lane’s worth of capacity.