On the fundamental diagram for freeway traffic: A novel calibration approach for single-regime models

Xiaobo Qu, Shuaian Wang, Jin Zhang

Research output: Journal article publicationJournal articleAcademic researchpeer-review

124 Citations (Scopus)


The speed-density or flow-density relationship has been considered as the foundation of traffic flow theory. Existing single-regime models calibrated by the least square method (LSM) could not fit the empirical data consistently well both in light-traffic/free-flow conditions and congested/jam conditions. In this paper, first, we point out that the inaccuracy of single-regime models is not caused solely by their functional forms, but also by the sample selection bias. Second, we apply a weighted least square method (WLSM) that addresses the sample selection bias problem. The calibration results for six well-known single-regime models using the WLSM fit the empirical data reasonably well both in light-traffic/free-flow conditions and congested/jam conditions. Third, we conduct a theoretical investigation that reveals the deficiency associated with the LSM is because the expected value of speed (or a function of it) is nonlinear with regard to the density (or a function of it).
Original languageEnglish
Pages (from-to)91-102
Number of pages12
JournalTransportation Research Part B: Methodological
Publication statusPublished - 1 Mar 2015
Externally publishedYes


  • Density
  • Fundamental diagram
  • Speed
  • Traffic flow theory
  • Traffic volume

ASJC Scopus subject areas

  • Transportation
  • Management Science and Operations Research

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