Modeling traffic operations and drivers' behavioral parameters at not-conventional roundabouts. A theoretic-experimental approach

The inapplicability of current methods based on gap-acceptance theory for analyzing operationalconditions at not-conventional roundabouts (very frequent in urban areas) seriously hampers theperformance assessment; in practical applications, this also makes the choice of corrective designmeasures very uncertain.Starting from this considerations, the present paper shows the conceptual path followed for analyzingtraffic operations at multilane-large-diameter not-conventional roundabouts. The research follows atheoretic-experimental approach that intends to put in a fair equilibrium the need both to match fieldobservations and to have a general criterion to determine behavioral parameters, on which trafficperformances depend.The main idea of the proposed approach derives from field observations at not-conventional roundaboutswhich show traffic operations following a pattern of a consensus of right-of-way alternating betweenvehicles entering from the approach and those streaming in the circulating lanes. A generalized model,similar to that one characterizing All-Way-Stop-Controlled (AWSC) intersections, was implemented,accounting for peculiarities revealed by an in-depth exploratory analysis of field data at not-conventionalroundabouts.As basic behavioral parameters (e.g. saturations headways) elude direct observations, i.e. operationalconditions in which they are observable rarely occur, in this paper a procedure to draw them frommacroscopic observations of traffic conditions has been proposed. For this purpose, a regression analysiswas carried out starting from observational data; the presence of response correlation required theregression parameters to be estimated through Generalized Estimating Equations models (GEEs), i.e.developing a marginal model for the unobservable - unknown - parameters.