Determining LOS and Density The following excerpt was taken from Chapter 3, page 3-22, of the 1997 revision of the Highway Capacity Manual published by the Transportation Research Board. The level of service on a basic freeway section can be determined directly from Figure 3-4 (this figure is presented in Theory and Concepts, Determining LOS and Density) on the basis of the free-flow speed and the flow rate. The procedure is as follows: Step 1. Define and segment the freeway section as appropriate. Step 2. On the basis of the measured or estimated free-flow speed on the freeway segment, construct an appropriate speed-flow curve of the same shape as the typical curves shown in Figure 3-2. The curve should intercept the y-axis at the free-flow speed. Step 3. On the basis of the flow rate, vp, read up to the free-flow speed curve identified in Step 2 and determine the average passenger-car speed and level of service corresponding to that point. Step 4. Determine the density of flow as D = vp/S where D = density (pc/mi/ln), vp = flow rate (pcphpl), and S = average passenger-car speed (mph). The level of service can also be determined using the density ranges provided in Table 3-1 [this table is presented in Theory and Concepts, LOS Criteria].