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 regions of Virginia led to the following conclusions: Flow rates may be determined using measurement intervals shorter than the 15-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.


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    Title :

    Examination of Core Concepts in the Highway Capacity Manual Part I: Subhourly Varations in Traffic Flow Rate. Final Contract Report


    Contributors:
    B. L. Smith (author)

    Publication date :

    2002


    Size :

    32 pages


    Type of media :

    Report


    Type of material :

    No indication


    Language :

    English







    Highway capacity manual

    National Research Council, Transportation Research Board, USA | TIBKAT | 2000


    Highway Capacity Manual

    Transportation Research Board, National Research Council | TIBKAT | 1985