In the 1985 Highway Capacity Manual (HCM), percentage time delay (PTD) is used as the main indicator of the level of service for two-lane, two-way highways. HCM defines PTD as the average percentage of time that all vehicles are delayed while traveling in platoons due to the inability to pass. For field measurement purposes, the HCM states that PTD is approximately the same as the percentage of vehicles traveling in platoons at headways less than 5 sec. The relationship between local platoon percentage, measured with a fixed 5-sec headway as the platooning criterion, and the previous estimate of PTD is analyzed. Theoretical considerations indicate that local platoon percentage is a biased estimate of PTD and must be corrected with a factor calculated as the ratio of space mean speed of all traffic and that of the platooned vehicles. If several measurements are made along a road, PTD can be estimated as a weighted average of local values using mean travel times and traffic flows as weights. Analysis of simulation and field data indicate that the local platoon percentage is usually about 3 to 5% lower than the corresponding PTD value. To make local measurements correspond more precisely to the basic definition of PTD, one could exclude from measurements situations in which a slower vehicle is behind a faster one inside the 5-sec headway. The effect of this exclusion, however, is not tested.


  • English

Media Info

  • Features: Figures; References;
  • Pagination: p. 58-65
  • Monograph Title: Traffic operations: highway capacity
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 00713575
  • Record Type: Publication
  • ISBN: 0309061237
  • Files: TRIS, TRB, ATRI
  • Created Date: Nov 21 1995 12:00AM