Estimation Of Gap Acceptance Parameters For
HCM 2010 Roundabout Capacity Model
Applications
Author(s)
A. Gazzarri, M. T. Martello, A. Pratelli & R. R. Souleyrette
Abstract
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 behaviour 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
Keywords
gap acceptance theory, critical headway, follow-up headway, HCM2010 roundabout capacity model, maximum likelihood, Raff’s method
Related Book
Other papers in this volume
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