If your free-flow speed calculation resulted in a speed other than 70, 65, 60 or 55 mph, you would construct the appropriate curve on the graph below. The curve would have the same general shape as those shown and would intersect the y-axis at the estimated (or measured) free-flow speed. The red line in the figure below presents an example of a curve drawn for an estimated free-flow speed of 63 mph.
Vehicle density is calculated by the following equation:
D = vp/S