Subhourly Variations in Traffic Flow Rate 7. Author(s)

14. Sponsoring Agency Code The rate of flow measures demand on a highway facility and is defined as "the equivalent hourly rate at which vehicles pass over a given point or section of a lane or roadway during a given time interval less than one hour, usually 15 minutes. " The Highway Capacity Manual (HCM) states that flow rates calculated for periods less than 15 minutes should be avoided since they are "statistically unstable. " This assertion is problematic since traffic conditions can change dramatically in the course of 15 minutes. An investigation of this concept using a large set of data from freeways in the urbanized Hampton Roads region of Virginia led to the following conclusions: • flow rates may be determined using measurement intervals shorter than the IS-minute interval suggested by the HCM. In some cases, stable flow rates were achieved using measurement intervals of 10 minutes. • There is not a clear breakpoint in measurement intervals in which the flow rate measure will become significantly more stable. It was determined that in all cases, increasing the measurement interval by 2 minutes resulted in more stable flow rates. The following recommendations were made