Abstract
Passenger Car Equivalents (PCEs) are used to convert a mixed traffic flow into an equivalent passenger car flow and to analyze capacity and level-of-service of roads and intersections. Most roundabouts guidelines propose constant values for PCEs but a single PCE value can result improper under heterogeneous traffic conditions. PCEs should be vary with traffic and road conditions so that PCEs used under undersaturated conditions may underestimate the effect of heavy vehicles or be not sensitive to the traffic level or characteristics of heavy vehicles. Compared to other at-grade intersections, the interaction between the operational performances of the heavy vehicles and the geometric features at roundabouts can produce significant impacts on the heavy vehicle paths and traffic operations due to the curvilinear nature of the roundabout design. Literature presents various estimation methods to obtain PCE values for heavy vehicles. The focus of this paper is to review statistical methods and traffic simulation studies based on microscopic approaches used to calculate PCEs for heavy vehicles driving roundabouts. Effects on capacity and PCE estimations based on models currently employed in roundabout analysis are also compared. The results obtained in this study aim at providing an overview of the existing knowledge concerning the estimation of PCEs at roundabouts and can represent a guideline for transportation engineers in the design and capacity analysis of roundabouts where mixed traffic conditions occur.
Highlights
Empirical evidence shows that the structure of traffic patterns makes operational conditions on highways, roads and intersections far from ideal conditions such as the Highway Capacity Manual (2000; 2010; 2016) describes
Geometric characteristics and traffic conditions, site and context of installation, users and driver behavior can affect equivalent factors, but further study should be done to consider the presence of heavy vehicles in mixed traffic, especially when high volumes move from one direction on the circulatory roadway and traffic streams can be highly saturated
Literature that has been mentioned above reports studies that included only a part of heavy vehicles used to estimate the values of equivalent factors for roundabouts proposed by the Highway Capacity Manual (2010); in any case the results showed in Figures 2, 3 are consistent with the results already highlighted by Lee (2015), namely that the equivalent factors depend on traffic conditions
Summary
Empirical evidence shows that the structure of traffic patterns makes operational conditions on highways, roads and intersections far from ideal conditions such as the Highway Capacity Manual (2000; 2010; 2016) describes. These factors include both properties of geometric design as outer diameter, width of the circulatory roadway and so on, and traffic properties as the amount of circulating and entering flows, as well as roundabout location, environment, and driver behavior, whose greater effects may be expected when unlimited traffic conditions and high degrees of saturated traffic occur It follows that traffic analysis and the design process must take account of the vehicular composition and the influence of heavy vehicles operating in mixed traffic due to their influence on choices relating location, alignment of the approaches, shape and size of different types of roundabouts.
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