Approccio teorico-sperimentale per l'analisi delle condizioni operative di rotatorie urbane non convenzionali

I metodi tradizionali di analisi delle rotatorie cadono in difetto in presenza di schemi organizzativi dell’intersezione, nei quali l’organizzazione geometrica, unitamente alla molteplicità delle situazioni di traffico ed all’inserimento in contesti vincolati, induce comportamenti di guida anche molto distanti da quelli osservabili in rotatorie convenzionali. Quanto evidenziato interessa, in particolare, l’ambito urbano, dove le intersezioni a circolazione rotatoria risultano particolarmente diffuse e si caratterizzano per requisiti geometrico-funzionali spesso distanti da quelli propri delle rotatorie convenzionali. Uno dei maggiori ostacoli che si incontrano nell’analisi delle condizioni operative di questo tipo di intersezioni dipende dalla stessa molteplicità dei possibili schemi e dalla conseguente difficoltà di inquadrarne il funzionamento in un unico modello teorico. A partire da queste considerazioni, il presente articolo mostra il percorso concettuale seguito per analizzare le condizioni operative di rotatorie non convenzionali di grandi dimensioni, per le quali i modelli basati sulla teoria del gap-acceptance non sono direttamente applicabili. Al contrario, le osservazioni di campo hanno suggerito un approccio teorico-sperimentale che, pur muovendo da una base empirica, non rinuncia alla possibilità di generalizzazione offerta dai modelli teorici. L’applicazione ad un caso studio permetterà di illustrare come derivare il modello analitico della capacità a partire dai dati osservati. I risultati ottenuti si dimostrano di particolare interesse per sviluppare lo studio delle rotatorie in contesti fortemente vincolati, per le quali la specificità dell’organizzazione infrastrutturale non permette l’analisi delle condizioni operative e la valutazione delle prestazioni a partire dai metodi proposti dalla letteratura, calibrati su sistemazioni solo parzialmente assimilabili a quelle in esame

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

Improving reliability of road safety estimates based on high correlated accidents 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