Sub-problem 1b: Analysis of the Eastbound Freeway Section

The basic freeway analysis methodology is shown in Exhibit 4-11. The inputs required include geometric data, a free-flow speed (FFS) (or a free flow speed derived from a basic free-flow speed) and volume information. The left-hand branch addresses actions you have to take to obtain the free-flow speed while the right hand branch focuses on the computation of a peak 15-minute flow rate. We’ll consider the left-hand branch first.

Free-flow speed (FFS) can be measured in the field or estimated using the procedure outlined in Chapter 23. We’ll consider both.

The benefit of having speed-flow data as shown in Exhibit 4-8 in this case study is that we can select a FFS and no adjustments are necessary. If you look at that figure, you can see that a value in the range of 55 mph is a good choice. This range reflects the average maximum speed when the flow rate relatively low. We’ll assume that 55 mph is the right value.

However, it’s useful to see what FFS estimate we would get if we trace through the branch labeled "if BFFS is input." We then take a basic free-flow speed (BFFS) and make adjustments to it to account for lane width, the number of lanes, the interchange density, and lateral clearances. Nominally, the basic free flow speed (BFFS) is how fast vehicles are traveling when the volumes are very light. The HCM assumes the BFFS is BFFS is 70 mph in urban settings and 75 mph in rural settings. Exhibit 4-8 shows that both of these values are too high for this facility. The maximum speed when the flow is almost zero is about 60 mph. The HCM allows us to use a local value rather than the defaults. We’re going to do that and assume the BFFS is 60 mph.