The effect of various factors on the capacity of a two-lane section of the M4 motorway is assessed by detailed analysis of speeds and headways between vehicles under capacity conditions. Car drivers in the nearside lane are found to leave a larger safety margin to the vehicle in front at any speed than car drivers in the offside lane. The mean time headways of queueing and non-queueing cars are approximately constant over the speed ranges 40-75 km/h in the nearside lane and 50-85 km/h in the offside lane. The mean time headway, t seconds, of all cars at speed V is given approximately by: t = V/35 in the nearside lane, and T = V/47 in the offside lane. Passenger car unit (pcu) values for different vehicle types in the nearside and offside lanes are obtained by forming the ratios of their mean time headways to that of cars. It is shown that it is not possible to obtain overall pcu values by taking the average time headways for both lanes combined unless, for each vehicle type, the proportion of all vehicles of that type on the motorway which are travelling in each lane is the same as the proportion of all cars on the motorway which are travelling in that lane. The following typical pcu values for the two-lane section of the M4 are obtained as weighted averages of pcu values in individual lanes: van 1.1; bus/coach, 1.6; non-articulated lorry, 2.0; articulated lorry, 2.9. The mean time headways between queueing cars give theoretical capacity estimates (attained when the traffic stream consists wholly of queueing cars) of 2400 car/h in the nearside lane and 2800 car/h in the offside lane. In terms of passenger car units, the possible capacity of this section of motorway under prevailing traffic conditions is approximately 2125 pcu/h in the nearside lane and 2525 pcu/h in the offside lane, or an average of 2325 pcu/lane/h.(a) /TRRL/

  • Availability:
  • Corporate Authors:

    Printerhall Limited

    29 Newmart Street
    London W1P 3PE,   England 
  • Authors:
    • Branston, D
  • Publication Date: 1977-6


  • English

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Filing Info

  • Accession Number: 00163953
  • Record Type: Publication
  • Source Agency: Transport and Road Research Laboratory (TRRL)
  • Report/Paper Numbers: Analytic
  • Files: ITRD, TRIS
  • Created Date: Dec 8 1977 12:00AM