Roberta Marino

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.

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.

ESTIMATING THE SAFETY PERFORMANCE FUNCTION FOR URBAN UNSIGNALIZED FOUR-LEGGED ONE-WAY INTERSECTIONS IN PALERMO, ITALY

Abstract Starting from consideration that urban intersections are sites with promise for safety and operational improvements, the paper describes the steps taken to develop a crash predictive model for estimating the safety performance of urban unsignalized intersections located in Palermo, Italy. The focus is on unsignalized four-legged one-way intersections widespread in Italian downtowns. The sample considered in the study consist of 92 intersections in Palermo, Italy. For the study were collected crashes occurred in the sites during the years 2006-2012, geometric design and functional characteristics and traffic flow. Results showed that data were overdispersed and NB1 distributed. In order to account for the correlation within responses Generalized Estimating Equations (GEE) were used under different working correlation matrices.

Flower roundabouts and double-lane roundabouts: a comparative performance analysis

Abstract In the paper, a choice criterion between flower roundabouts and double-lane roundabouts is proposed, focusing on operational benefits that can derive from one scheme over the other, and outlining a general framework for benefit-cost analysis. In order to assess operational benefits of innovative roundabouts over modern roundabouts, a comparative analysis was made. Capacity was estimated using gap-acceptance models. In detail, assuming the dichotomic shifted negative exponential distribution to model headways in circulating streams, the Hagring formula was adjusted to obtain entry capacity estimations at roundabout approaches where entering vehicles face one or two conflicting flows. Based on the control delay, the suitability domains and indifference areas were constructed. Thus, a sensitivity analysis to changes in traffic demand for operational benefits of flower roundabouts over double-lane roundabouts was carried out and discussed. At last, evidence for new installations and conversion of existing roundabouts can be found.

Modeling traffic safety at urban four leg-signalized intersections

According to the state-of-the-art of the methodologies, the development of safety performance functions (SPFs) for road sections and intersections requires the employment of statistical models to predict expected crash frequencies on the basis of traffic volumes and site characteristics to be surveyed and used as input to models. Nevertheless, literature reports several studies on issues deriving from data features or methodological approaches that may invalidate the efficiency of the models and the accuracy of the estimates. Drawing inspiration from the above mentioned considerations, the objective of this study is to develop safety performance functions for a sample of urban four leg-signalized intersections on the basis of 8 years of crash data in Palermo, Italy. Applications of the Conway- Maxwell model are presented for analyzing traffic crash data exhibiting underdispersion. Results comforted authors on the potential of the Conway- Maxwell model to account for dispersion phenomenon and to provide a good goodness-of-fit, as long as the temporal correlation in the data is not considered. In this regard, the GEE model, incorporating the time trend, allowed to gain further methodological insights compared to models that do not accommodate the temporal correlation in crash data.