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Capacity and LOS: Theory and Concepts

Free-Flow Speed and Flow Rate

An understanding of the relationship between speed and flow rate is the key to determining capacity and LOS for a specific freeway section. In general, freeways are designed to accommodate relatively large numbers of vehicles at higher speeds than other roadways.

Free-flow speed is the term used to describe the average speed that a motorist would travel if there were no congestion or other adverse conditions (such as bad weather). The "highest" (ideal) type of basic freeway section is one in which the free-flow speed is 70 mph or higher. Flow rate is defined as the rate at which traffic traverses a freeway segment, in vehicles per hour or passenger cars per hour.

Free-flow speed is actually defined as the speed that occurs when density and flow are zero. Of course, observing zero density and flow doesn’t make much sense. The following scenario illustrates the relationship between Free-flow Speed and Flow Rate.

Imagine that you are the only motorist on a section of freeway that you travel frequently, the weather is good and you are driving at a speed that is comfortable for that particular section, say 70 mph. Studies have shown that as long as the number of vehicles traveling per hour per lane on your section of freeway does not exceed a flow rate of 1300, you will likely continue traveling at 70 mph. (This assumes all passenger cars - no trucks, buses or recreational vehicles). Your speed will start to decrease once the flow rate exceeds 1300 passenger cars per hour per lane (approximately 22 cars per minute, or about 1 car every 3 seconds). If you were traveling at 65 mph, your speed wouldn’t decrease until a flow rate of 1450 passenger cars per hour per lane (pcphpl) has been reached. The relationship is shown below.

Graph of Average Passenger-Car Speed versus Flow Rate