Left-turn adjustment for permitted turns from shared lane groups. Another look

Elena Shenk Prassas, Roger P. Roess

Research output: Chapter in Book/Report/Conference proceedingChapter


Initial research on the left-turn adjustment factor for shared, permissive left-turn lane groups resulted in the recommendation that the theoretical model presented in the 1985 Highway Capacity Manual (HCM) be replaced by regression models. After several meetings and spirited debates, the Signalized Intersection Subcommittee of the Highway Capacity and Quality of Service Committee recommended that the regression-based models not be adopted for the HCM. The weaknesses in the 1985 HCM are discussed. Illustrations are provided to show how improved accuracy can be achieved with a hybrid model that uses parts of regression-based models inserted into the theoretic framework of the HCM model. The hybrid model is compared with the 1985 HCM and with other suggested methods. The hybrid model was found to be best for predicting saturation flow on the basis of field data for 267 data periods of 15 min each. At a workshop in summer 1992, the committee concluded that the hybrid model will be incorporated into the HCM.

Original languageEnglish (US)
Title of host publicationTransportation Research Record
Number of pages7
StatePublished - 1993

ASJC Scopus subject areas

  • Civil and Structural Engineering
  • Mechanical Engineering


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