Influence of Various Restrictions on Speed-Flow Models

The paper presents traffic speed models developed on the basis of numerous measurements. In studies of the free flow speed and speed reduction, the following research issues were analyzed: impacts of qualitative road and roadside features on free flow speed (cross-sections, road function, accessibility, intensity of development, roadside pedestrian traffic); and impacts of the speed limit, traffic volume, traffic flow composition and time of day on speed-flow relationships. The research included road sections with numerous constraints of traffic freedom. Such sections comprise sections of roads through built-up areas (small communities and towns) and streets in towns. Essential factors constraining free traffic flow on these sections are speed limits; however, the effect of numerous turning maneuvers and merging from access points is also significant. Often the speed limit alone does not decide on the speed level in traffic flow, since a degree of speed limits depends on many factors. From the recent research results, it can be seen, that mostly qualitative road characteristics, intensity of roadside development and potential police control influence drivers' behavior. Psychophysiological features of drivers, especially road image perception or motivation of journey cannot be disregarded. Quantitative assessment of influence of these factors is very significant for proper description of the speed-flow relationship on road sections through built-up areas. Assuming that drivers obey speed restrictions a hypothesis could be formulated that traffic volume has a limited influence on speed. A number of models, including the HCM model, suggest that for a very broad range of traffic volumes (up to 1200 veh/h/lane) average flow speed of more than 70 to 80 km/h is possible – depending on the rate of slow moving vehicles. It means that in built-up areas with speed limits at 60 km/h or 50 km/h and drivers actually respecting these limits, a constant driving speed close to the 60 or 50 km/h limit would be possible on road sections between intersections for volumes to about 1200 veh/h/lane at much lower speed dispersion. Verification of such hypothesis was one of the objectives of this study. Another aim was to assess possibilities of use of a universal model to describe the relation between speed and traffic volume when different traffic constraints occur. At the present stage, the authors have formulated the main research problems and presented examples of models for estimating speeds in built-up areas. Once calibrated and supplemented with data, the models can become a universal tool for traffic management, traffic forecasts and road safety assessments and for assessing the environmental impacts of traffic.


  • English

Media Info

  • Media Type: CD-ROM
  • Features: Figures; References; Tables;
  • Pagination: 19p
  • Monograph Title: Symposium on the Fundamental Diagram: 75 Years (Greenshields 75 Symposium), July 8-10, 2008, Woods Hole, MA. Preprints

Subject/Index Terms

Filing Info

  • Accession Number: 01109346
  • Record Type: Publication
  • Files: TRIS, TRB
  • Created Date: Aug 22 2008 9:31AM