Anna Grana

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 modelling. 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 modelling must contain both the analytical construction and then solution of the model, and the implementation of driver behavior. Thus, issues on a realistic modelling of driver behavior by the parameters that are included into the models are always of interest for practioners 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.

Traffic simulation models calibration using speed-density relationship

Calibration of traffic models.Calibration as optimization problem.Use of genetic algorithm. This paper presents the first results of a research which applied a genetic algorithm to calibrate a microscopic traffic simulation model based on speed-density relationships. A large set of traffic data collected from the A22 Freeway, Italy, was used and a comparison was performed between the field measurements and the simulation outputs obtained for a test freeway segment by using the Aimsun microscopic simulator.The calibration was formulated as an optimization problem to be solved based on a genetic algorithm; the objective function was defined in order to minimize the differences between the simulated and real data sets in the speed-density graphs. For this purpose, the genetic algorithm tool in MATLAB?was applied.Keeping in mind the objective to automatize this process, the optimization technique was attached to Aimsun via a subroutine, so that the data transfer between the two programs could automatically happen. An external script written in Python allowed the MATLAB?software to interact with Aimsun software.A better match to the field data was reached with the optimization parameters set with the genetic algorithm. In order to check to what extent the model replicated reality, model validation was also addressed. Results showed that a genetic algorithm is usefully applicable in the calibration process of the microscopic traffic simulation model. Beneficial effects are expected by applying the suggested optimization technique since it searches for an optimum set of parameters through an efficient search method.

Improving Reliability of Road Safety Estimates Based on High Correlated Accident Counts

Calibrating a safety performance function (SPF) with many years of accident data creates a temporal correlation that traditional model calibration procedures cannot deal with. It is well known that generalized estimating equations (GEE) models are able to incorporate trends into accident data and thus overcome difficulties in accounting for correlation; the usual application of GEEs to safety analysis uses robust (or sandwich) estimates of regression coefficients under the independence hypothesis for the working correlation matrix. This practice is justified by the robustness of the GEE procedure against misspecification of the response correlation structure. Nevertheless, with this method, one has to renounce the entirety of the advantages of GEE estimates, and–especially when correlation within the subject is high–significant losses in efficiency and misleading conclusions in model interpretation can occur. In such a case, losses in efficiency of the estimates will be transferred to the reliability of the final safety estimation, for example, by the empirical Bayes method. On the basis of these considerations, the main idea of this study is that, in safety modeling, additional effort to obtain the true data correlation structure will result in better precision in the estimation of SPF parameters. An example to illustrate the methodological aspects of the proposed approach is included.

Handling Underdispersion in Calibrating Safety Performance Function at Urban, Four-Leg, Signalized Intersections

Poisson basic assumption of equidispersion is often too much restrictive for crash count data; in fact this type of data has been found to often exhibit overdispersion. Underdispersion has been less commonly observed, and this is the reason why it has been less convenient to model directly than overdispersion. Overdispersion and underdispersion are not the only issues that can be a potential source of error in specifying statistical models and that can lead to biased crash-frequency predictions; these issues can derive from data properties (temporal and spatial correlation, time-varying explanatory variables, etc.) or from methodological approach (omitted variables, functional form selection, etc.). This article focuses on the potential of the Conway-Maxwell (COM-Poisson) model in handling underdispersion that arose in the development of a Safety Performance Function for urban four-leg signalized intersections; other issues, as temporal data correlation, have been intentionally eluded to test the best way of handling underdispersion. Results confirmed that the COM-Poisson model properly handled crash data set for which neither Poisson nor negative binomial model were able to account for dispersion phenomenon; they also showed that the COM-Poisson model provided a good statistical performance and a better goodness-of-fit than the quasi-Poisson and the traditional Poisson model.

Microsimulation-based passenger car equivalents for heavy vehicles driving turbo-roundabouts

AbstractDue to its geometric design, turbo-roundabouts impose greatest constraints to the vehicular trajectories; by consequence, one can expect a more unfavourable impact of heavy vehicles on the traffic conditions than on other types of roundabouts. The present paper addresses the question of how to estimate Passenger Car Equivalents (PCEs) for heavy vehicles driving turbo-roundabouts. The microsimulation approach used revealed as a useful tool for evaluating the variation of quality of traffic in presence of mixed fleets (different percentages of heavy vehicles). Based on the output of multiple runs of several scenarios simulation, capacity functions for each entry lane of the turbo-roundabout were developed and variability of the PCEs for heavy vehicles were calculated by comparing results for a fleet of passenger cars only with those of the mixed fleet scenarios. Results show a dependence of PCEs for heavy vehicles on operational conditions, which characterise the turbo-roundabout. Assuming the value...

Emission factors related to vehicle modal activity

Polluting emissions depend on vehicle characteristics and on traffi c conditions expected after the construction of a road project. Nevertheless, in the assessment of new projects, the task of road designer becomes uncertain when the reduction in polluting emissions has to be evaluated. Moreover, vehicle emissions are highly linked to modal vehicle activity, but current emission rate models do not properly predict on-road vehicle emissions produced by modal traffi c events, as those occurring at intersections. Modal emission models require the analysis of modal activity at a microscale level in order to evaluate emission factors by a single mode (idle, acceleration, deceleration, and cruise). This evaluation can be standardised with reference to the type of road, volume-to-capacity ratio and fl eet composition. On the contrary, the mesoscopic level for vehicle modal activities, as it is usual in traffi c analysis, will result appropriate to reach correct emission estimates. In order to explain factors affecting polluting emissions, a research programme targeted to the development of a methodology to be applied to traffi c studies has been undertaken. This paper reports the results of an exploratory analysis, based on examples of driving patterns, with the specifi c purpose to measure and to interpret vehicular polluting emissions in road situations different for geometric and traffi c conditions. Results referred to in this paper show that, for a specifi c traffi c condition, estimates of vehicle polluting emissions can be obtained from emission factors proper to each elementary modal activity and from proportion of time spent by vehicles in each modal activity as defi ned at mesoscopic level.

How to Derive the Analytical Capacity Model for Not-Conventional Urban Roundabouts

This paper on unconventional urban roundabouts (traffic circles) is from the proceedings of 14th international Conference on Urban Transport and the Environment in the 21st Century, which was held in Malta in 2008. The authors contend that the current methods for analyzing the operations of unconventional urban roundabouts are inadequate to assess their operational performances. In addition, local constraints and the road network structure have produced many geometric layouts that complicate any framework used to describe the performances of unconventional roundabouts. The authors then describe a method used to analyze the operations of unconventional roundabouts along an arterial of Palermo City, Italy. Their method balances the need to match field observations and to have a general criterion to determine performances of these road structures, including multi-lane, large diameter roundabouts. Other complexities considered include vehicles that disregard the priority rule, all-way stop situations, and vehicle movements when they are the only vehicle on the roundabout. They also discuss how to derive the analytical capacity model from field data. They conclude that the proposed method can easily be adapted to specific intersection layouts and to account for factors that may impact operations, such as the lane being occupied by vehicles at approaches, type of vehicular movements, and the presence of heavy vehicles.

Conversion of Existing Roundabouts into Turbo-Roundabouts: Case Studies from Real World

Compared with roundabouts the main advantages of turbo-roundabouts are the reduction in the number of potential conflicting points and the lower speed of vehicles passing through the intersection, they both can improve safety conditions at the junction. Moreover, the physical delimitation among lanes limits the side-by-side accident risk. These aspects make turbo-roundabouts more appropriate than roundabouts when a higher level of safety has to be guaranteed, particularly in presence of relevant pedestrian and two-wheels traffic volumes. The present paper has three main objectives: (1) to discuss general design criteria and functional principles of turbo roundabouts, (2) to give the geometric design principles of the central island and circulating lanes and (3) to present three case studies from real world concerning the conversion of existing roundabouts into turbo roundabouts.

SAFETY VALUATION METHODS AT URBAN ATYPICAL INTERSECTIONS. ANALYSIS OF INFRASTRUCTURAL SCENARIOS

The definition of decision support tools for identifying ex ante evaluation issues for road safety measures is a new challenge for policymakers and designers managing road networks with the intention of improving traffic safety for roads of their competence. This is consistent with explicit strategies on sustainability in the transport sector established at European level, considering road safety as an essential prerequisite to warrant a sustainable mobility. Based on these considerations, the need for evaluation instruments of safety conditions both for existing infrastructural schemes and for new road installations is recently arisen so that the identification of risks involved in a particular road infrastructure is easier, as well as the definition of the priorities of intervention by means of technical measures. This is true in particular for road junctions characterized by a configuration not conforming to those ones recognized as safer. On this regard, the results of safety evaluations at roundabouts are not an exhaustive reference for geometric design where organizational schemes for circular intersections are similar to roundabouts but resulting from compromise choices as regards one or more geometric features of the roundabout. For this kind of atypical intersections specific safety problems are found, particularly in urban areas, due to existing constraints of different kind and the great variety of geometric layouts they take on. Moreover, in these cases the possibility of applying the safety methods proposed in the literature regarding traditional roundabouts, as well as the road safety audit procedures, is also compromised. Based on these considerations, in this paper ten case studies of this kind of intersections have been examined in order to verify the effectiveness of the risk analysis proposed by means of an infrastructural scenarios method. From the methodological point of view, the application of the accident delocalization procedure has allowed to highlight in relative terms the riskiness of different infrastructural scenarios arranging the organizational schemes of intersections. Moreover, the proposed method can represent a useful tool for safety diagnostic analysis being able to provide informations for policymakers and designers in order to address the infrastructural scenarios and the road schemes that need to be tested. Keywords circular intersection, infrastructural scenario, risk analysis, road safety, roundabouts Language: en

Road safety audit effectiveness at urban junctions

In accordance with the Road Safety National Plan recently adopted in Italy and the initiatives subsequently undertaken by the National Infrastructure and Transport Department, this paper contains proposals for implementing an urban road safety audit in Italy, Our research will aim at evaluating the possible effects of such a practice. Our more specific aim is to determine the real or potential risk on roads by examining road infrastructure and observing road user behaviour in the absence of accident data. Four urban junctions, situated on one of the main thoroughfares of urban traffic in .Palermo, were studied and a comparison was made between the data obtained by observing road user behaviour and that, which would have been supplied by expensive accident data analysis,