Abstract
In this study, capacity estimations with the incorporation of Highway Capacity Manual (HCM) 2010 method are evaluated. Parameter based sensitivity analysis on calculations with the new HCM formula and a comparative evaluation of the new methodology with two most common capacity analysis methods, i.e., the method of critical gap acceptance and the method of regression analysis, are performed. Maximum and minimum headway intervals of follow up time and critical gap parameters are alternated within the sensitivity analysis. The Transport Research Laboratory formula for regression and Australian formula for gap acceptance method are considered in comparison. Relative comparisons of predictions on capacity by HCM2010 method, regression analysis and gap acceptance method are presented considering field data obtained by observations at two roundabouts in Izmir, Turkey. The results of the study show that the HCM2010 formula led to lower capacity estimates than regression analysis and higher estimates than the gap acceptance method. Regarding the real capacity observations under high circulating flow-rates the HCM2010 method yielded to more appropriate results than the regression method. In addition to comparisons, studies on the sensitivity analysis show that entry capacity estimates possess sharper changes as smaller follow up headways are accepted.
Highlights
A modern roundabout is a type of intersection design that controls and diverts traffic flow around a central island
The case study in this paper presents a brief comparison of Highway Capacity Manual (HCM) 2010 formulation with regression and gap acceptance theories
Data obtained from one approaching leg of two roundabouts in Izmir are studied in order to make comparison between HCM 2010 default values, regression model an gap acceptance model
Summary
A modern roundabout is a type of intersection design that controls and diverts traffic flow around a central island. Various studies investigating the safety performance are made in Europe and United states. This paper discusses the parameter sensitivity of new Highway Capacity Manual (HCM) 2010 formula and comparison of new methodology with critical gap acceptance and regression models considering field observations. Capacity analysis used in comparison, HCM 2010 [4], gap acceptance and regression methods are introduced. HCM 2010 is compared with both methods and observed data. In the last part of the study sensitivity analysis of HCM 2010 on parameters follow-up time, Tf, and critical gap, Tc, are investigated. Considering the validity of applied methods it is useful to mention the design differences between investigated sites in Turkey and typical sites used to develop compared models in the US, Australia and UK. The findings represent application of those models to Turkish data only for the studied roundabouts and are not generalized for different sites
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