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.