Estimation of gap acceptance parameters for HCM 2010 roundabout capacity model applications

A. Gazzarri, M. T. Martello, A. Pratelli, R. R. Souleyrette

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

4 Scopus citations


This paper deals with the field data observation and successive application of estimation procedures in order to estimate follow-up headway and critical headway at roundabouts. Average follow-up and average critical headway are two critical parameters in the new roundabout capacity model presented in the 2010 edition of the Highway Capacity Manual (HCM). The HCM 2010 capacity model was developed as an exponential regression model with parameter estimates based on gap acceptance theory. Gap acceptance models are strongly affected by driver behavior and local habits. Follow-up headways can be field measured, while critical headway cannot be obtained directly. The paper mainly aims to check if general suggested values are well-suited also for Italy or not. There have been numerous techniques developed for estimating critical headway. In order to perform a better check of a sample of experimentally observed values, three different procedures are chosen and applied. The first is a quite popular mathematical method based on maximum likelihood technique. The second is a statistical method based on the median of the observed sample distribution. The third is a graphical method known as Raff's method. All these three methods require information about the accepted headway and the largest rejected headway for each driver. Therefore a sample of field data was recorded by digital camera and processed following the instructions suggested by NCHRP. The sample data of critical headway and follow-up headway are gathered in seven selected roundabouts located in Northern Tuscany (Italy). Our first obtained results indicate that the average critical headway is significantly lower than the values recommended by some international references. However, the average follow-up headway is only higher than that recommended for the State of California and it is lower than that recommended by all other international references. Finally, conclusions drawn along with insights for further research developments are suggested.

Original languageEnglish
Title of host publicationUrban Transport XVII
Number of pages12
StatePublished - 2012
Event18th International Conference on Urban Transport and the Environment, UT12 - A Coruna, Spain
Duration: May 15 2012May 17 2012

Publication series

NameWIT Transactions on the Built Environment
ISSN (Print)1743-3509


Conference18th International Conference on Urban Transport and the Environment, UT12
CityA Coruna


  • Critical headway
  • Follow-up headway
  • Gap acceptance theory
  • HCM2010 roundabout capacity model
  • Maximum likelihood
  • Raff's method

ASJC Scopus subject areas

  • Architecture
  • Civil and Structural Engineering
  • Building and Construction
  • Automotive Engineering
  • Transportation
  • Safety, Risk, Reliability and Quality
  • Arts and Humanities (miscellaneous)
  • Safety Research
  • Computer Science Applications


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