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 Capacity and LOS: Professional Practice

 
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].