Estimating traffic operations at multi-lane roundabouts: a case study

This paper addresses traffic modeling issues at urban multi-lane roundabouts where, despite circulating vehicles have priority, negotiation of the right-of-way can occur between antagonist traffic flows, as a result of minor drivers’ failing to obey the nominal operating rule (stop or yield control). Existing models for the estimation of operational performances have the shortcoming of not representing the interdependencies between entering and circulating vehicles at multi-lane roundabouts. An analytical capacity model derived from field observations was developed for this kind of intersections in a previous study. The complexity of the model lies in the difficulty of observing the behavioral parameters which are needed to implement the model. A procedure to get unknown behavioral parameters from traffic surveys is here proposed. This concerns saturation headways, often eluding direct observations due to rare occurrences of traffic conditions in which they can be observed. The unknown parameters were estimated through a regression model using on field data collected at a multi-lane roundabout. The presence of data correlation within a cluster of observations required the estimation of the regression parameters through a generalized estimating equation model. Results gave insight into the analysis of operations at multi-lane roundabouts, containing evidence to support assumptions made for the estimation of unobservable parameters

Methodological Frontier in Operational Analysis for Roundabouts: A Review

Several studies and researches have shown that modern roundabouts are safe and effective as engineering countermeasures for traffic calming, and they are now widely used worldwide. The increasing use of roundabouts and, more recently, turbo and flower roundabouts, has induced a great variety of experiences in the field of intersection design, traffic safety, and capacity modeling. As for unsignalized intersections, which represent the starting point to extend knowledge about the operational analysis to roundabouts, the general situation in capacity estimation is still characterized by the discussion between gap acceptance models and empirical regression models. However, capacity modeling must contain both the analytical construction and then solution of the model, and the implementation of driver behavior. Thus, issues on a realistic modeling of driver behavior by the parameters that are included into the models are always of interest for practitioners and analysts in transportation and road infrastructure engineering. Based on these considerations, this paper presents a literature review about the key methodological issues in the operational analysis of modern roundabouts. Focus is made on the aspects associated with the gap acceptance behavior, the derivation of the analytical-based models, and the calculation of parameters included into the capacity equations, as well as steady-state and non-steady-state conditions and uncertainty in entry capacity estimation. At last, insights on future developments of the research in this field of investigation will be also outlined

Passenger car equivalent for heavy vehicles crossing turbo-roundabouts

Turbo-roundabouts represent an innovative scheme of modern roundabouts which provides a spiraling traffic flow and requires drivers to choose their direction before entering the intersection, since raised lane separators mark the lanes on the ring. The configuration of the turbo-roundabout makes that patterns of conflict at entries with one and two conflicting traffic streams can coexist. This paper presents research efforts aimed at measuring quantitatively the effect of heavy vehicles on operational conditions of a turbo-roundabout. The study starts from the initial belief that the greatest constraints to the vehicular trajectories imposed by the turbo-roundabout necessarily imply that the impact of heavy vehicles on the quality of traffic flow is more unfavorable than on other modern roundabouts. Microsimulation revealed as a useful tool when the variation of the traffic quality in turbo-roundabouts should be evaluated in presence of mixed fleets, each having different percentages of heavy vehicles; indeed, it allowed to isolate traffic conditions difficult to observe on field and replicate them to have a number of data as much as possible numerous. Entry capacity values for each entry lane of the turbo-roundabout were obtained by microsimulation, varying the percentage of heavy vehicles for entering flows. Nonlinear regression analysis of simulation data allowed to derive the behavioral parameters for heterogeneous populations of users and, ultimately, composed exclusively of heavy vehicles. The capacity functions thus obtained allowed us to determine how the passenger car equivalent (PCE) varies with the percentage of heavy vehicles and circulating flows for each entry lane of the turbo-roundabout. The results of this study indicate that there is a need to distinguish the impact of heavy vehicles when analyzing the capacity of a turbo-roundabout. When the traffic stream contains a significant number of heavy vehicles, a larger PCE effect would be expected. This effect should be accounted for in the estimation of the turbo-roundabout capacity. Lastly it should be emphasized that an important aspect of the research consists in having identified a methodology for assessing the impact of heavy vehicles on the quality of traffic flow, that can be applied to different patterns of intersection

Gap-accepteance parameters for roundabouts: a systematic review

Purpose The critical and follow-up headways are the two gap-acceptance parameters which explain the traffic interaction of a minor street vehicle when enters the roundabout, merging into or crossing one or more circulating (major) streams.Severalstudiesandresearchesprovidemeasurements of critical and follow-up headways from real data at roundabouts. The objective of our research is to synthesize the data from the series of selected studies to interpret variation across the studies. Methods In order to match the research goal, a systematic literaturereviewonestimationsofcriticalandfollow-upheadways at roundabouts was undertaken. Since several studies andresearchesdevelopedworldwidewereexamined,wewere able to note that the effect size varied from study to study. Thus the meta-analysis of effect sizes was performed as part of the literature review through the random-effects model. Results After discussing the assumptions of this model, the dispersion in effects across studies was assessed and the summary effect for eachofthe parametersunder examination was computed. Calculations were made both for single-lane roundaboutsanddouble-laneroundabouts,aswellasforturbo roundabouts. Conclusions Compared to the results of individual studies, the single (quantitative) meta-analytic estimate provides an accurate and reliable synthesis on the specific issue here addressed, and gives, with greater power of the individual reviewed studies,a comprehensive measure for the parameters of interest

Crash modeling for urban roundabouts: a case study

In many cities and towns, a large number of intersections are considered sites with promise for safety and operational improvements. Several studies have been carried out in many countries to establish relationships between crashes and flow and non-flow explanatory variables, using statistical tools to investigate factors critical to road safety. Starting from a brief review of existing information and analysis on the issue, this article summarizes the findings of an exploratory analysis aimed at modeling injury crashes for a sample of urban roundabouts. The methodological path followed in this research allowed to handle issues associated with the estimation of a safety performance function, and also introduce concerns related to the crash model transferability. At last, results can supply methodological insights that may be useful in the subsequent quantifying of benefits obtainable by engineering measures aimed at enhancing traffic safety in built up areas

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 operational conditions at not-conventional roundabouts (very frequent in urban areas) seriously hampers the performance assessment; in practical applications, this also makes the choice of corrective design measures very uncertain. Starting from this considerations, the present paper shows the conceptual path followed for analyzing traffic operations at multilane-large-diameter not-conventional roundabouts. The research follows a theoretic-experimental approach that intends to put in a fair equilibrium the need both to match field observations and to have a general criterion to determine behavioral parameters, on which traffic performances depend. The main idea of the proposed approach derives from field observations at not-conventional roundabouts which show traffic operations following a pattern of a consensus of right-of-way alternating between vehicles 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-conventional roundabouts. As basic behavioral parameters (e.g. saturations headways) elude direct observations, i.e. operational conditions in which they are observable rarely occur, in this paper a procedure to draw them from macroscopic observations of traffic conditions has been proposed. For this purpose, a regression analysis was carried out starting from observational data; the presence of response correlation required the regression parameters to be estimated through Generalized Estimating Equations models (GEEs), i.e. developing a marginal model for the unobservable - unknown - parameters

Managing Greenhouse Gas Emissions for Airport Inventories: An Overview

For some years now, problems relating to gas emissions that affect climate and result from human activities have assumed a global dimension of large importance. The climate is, indeed, influenced by the concentrations of some pollutants in the atmosphere; these pollutants trap the long wave radiation emitted by the Earth and alter the energy balance, causing an accentuation of the natural greenhouse effect. In this view an inventory of greenhouse gases can become the benchmark against which to measure the achievement of quantitative targets set at the political level in the fight against climate change. Contrary to a general inventory of emissions related with air quality objectives and local effects of pollutants, a greenhouse gas inventory does not set objectives purely local, but it directs efforts towards the quantification of the liability of an area compared to a global problem as climate change, also in view of possible improvements. The inventory is thus the reference for the evaluation of pre-and post-operam actions aimed at reducing the greenhouse effect, as well as for their monitoring over time. The article in an overview of the main issues of air pollution associated to airport operations and a review of the airport sources and components contributing to greenhouse gas emissions. After a brief description of the most recent international and national standards and recommendations on this topic, the article focuses on reasons for developing a greenhouse gas emission inventories and traces the essential elements in the methodology to be followed by airport operators for preparing an airport-specific inventory of greenhouse gas emissions. Methods for calculating emissions are also summarized and reviewed. Finally, the new references to the environmental sustainability at airports, as well as the main measures to be taken to reduce emissions are also identified and commented o