Passenger car units (pcus) are traditionally used to represent the effects of changes in traffic composition (the mix of cars, goods vehicles, buses, and so on) on the saturation flows at traffic signal junctions. This paper describes differences in the results obtained by the two main methods of derivation of pcu values, regression analysis of asynchronous vehicle counts (asynchronous regression) and headway ratio methods, when applied to data from two large public road studies. The relationship between the various methods of derivation used previously is investigated. Regression analysis of synchronous vehicle counts, Webster's method, and headway ratio methods are seen to agree, but asynchronous regression necessarily gives low results so long as there is variability in the headways of vehicles of a given class (e.g. In car-to-car headways). An alternative method of regression analysis of asynchronous counts is investigated but found to be biased, although less so than the existing method. Conventional asynchronous regression gives unbiased saturation flow estimates if unbiased prior pcu values are used. The effects of assumed pcu values on signal settings and consequent delays in subsaturation conditions are examined. Values corresponding to delay-minimising settings are close to the ratio of the mean headways of the appropriate vehicle classes.(a)

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  • Corporate Authors:

    Pergamon Press, Incorporated

    Headington Hill Hall
    Oxford OX30BW,    
  • Authors:
    • KIMBER, R M
    • McDonald, M
    • Hounsell, Nick B
  • Publication Date: 1985-2

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Filing Info

  • Accession Number: 00451737
  • Record Type: Publication
  • Source Agency: Transport Research Laboratory
  • Report/Paper Numbers: HS-039 069
  • Files: HSL, ITRD, TRIS
  • Created Date: Dec 31 1985 12:00AM