Free-Flow Speed and Flow Rate The following excerpt was taken from Chapter 3, page 3-3, of the 1997 revision of the Highway Capacity Manual published by the Transportation Research Board. All recent freeway studies indicate that speed on freeways is insensitive to flow if the flow is low to moderate. This is reflected in Figure 3-2 [reproduced in the Theory and Concept section entitled Free-Flow Speed and Flow Rate], which shows speed to be constant for flows up to 1,300 pcphpl for a 70-mph free-flow speed. For freeways with a lower free-flow speed, the region over which speed is insensitive to flow extends to even higher flow rates. Thus, free-flow speed is easily measured in the field as the average speed of passenger cars when flow rates are less than 1,300 pcphpl. Field determination of free-flow speed is easily accomplished by performing travel time or spot speed studies during periods of low flows. Note that although Figure 3-2 shows only curves for free-flow speeds of 75,70, 65, 60, and 55 mph, curves representing any free-flow speed between 75 and 55 mph can be obtained by interpolation. Also, the speed-flow curve representing a 75-mph [not shown in the figure included with this chapter] free-flow speed, which corresponds with the recent increase in the posted speed limit on many rural freeway sections throughout the United States, shown by a dashed line, is not based on empirical field research but was created by extrapolation from the 70-mph free-flow speed curve. Capacity at free-flow speeds greater than or equal to 70 mph is considered to be 2,400 pcphpl.