Tullio Giuffrè

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.

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.

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.

Benefits of area-wide traffic calming measures: outcomes still to assess

The role of traffic calming schemes in urban network management is mainly to minimize the undesirable effect of traffic in residential areas; in this context accident reduction can be a realistic objective. Several studies, indeed, inform us that traffic calming is a major part of the treatments available to reduce road accidents in urban areas. Paradoxically the increase of the accident rate per kilometre travelled has been observed in the urban context as result of the wellknown accident migration phenomenon. In connection to these considerations, the effectiveness of traffic calming measures on road safety is discussed in this paper. Then, after a synthetic exposition of the conceptual formulation and the potential of the meta-analysis method in detecting the true safety effect of traffic calming measures, the paper also considers the role of road network planning and the characteristics of the urban network that have to be consistent with the traffic calming objectives. Lastly, a methodological framework to follow for the implementation of traffic calming schemes in residential areas, from the planning level to the road design level, is proposed.

Preliminary In-Situ Evaluation of an Innovative, Semi-Flexible Pavement Wearing Course Mixture Using Fast Falling Weight Deflectometer

In the last forty, years semi-flexible pavements have been successfully employed, especially in those areas subjected to heavy and slow-moving loads. They usually comprise a wearing course of Grouted Macadam, a composite pavement material that provides significant advantages in comparison to both concrete and asphalt pavements. On the other hand, the laying process of this material is a two-stage operation, and the realization complexity leads to long realization times and high initial costs. Therefore, the use of semi-flexible pavements has been limited to some fields of application and areas. Recently, an innovative material has been developed to be used as an alternative to Grouted Macadam for semi-flexible pavement wearing course realization. This material should provide similar or even superior characteristics compared to traditional Grouted Macadam. This will reduce semi-flexible pavement construction time and avoid the need for dividing the laying process. This paper presents an experimental program involving the use of FastFWD, as an APT device, to evaluate in-situ properties and performance of this material. The achieved results regarding the validation of this new material by means of FastFWD appear promising both in terms of the material’s properties and resistance to dynamic load repetitions.

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.

Evaluation of proneness to red light violation : Quantitative approach suggested by potential conflict analysis

Evaluation of proneness to red light running behavior, because it results from human and road factors, can aid in a proper selection of the sites to be treated and thereby increase the benefit of counter-measures to reduce the red light running (RLR) phenomenon. Starting from the conceptual framework of a model based on potential conflicts analysis, this paper shows that a quantitative evaluation of proneness to red light running behavior can be obtained from both the analysis of the effective operational characteristics of the intersection and the actual number of RLR violations. According to behavioral models referred to in the literature, which emphasize the influence that both human and road factors have on the users decision-making process at red lights, the proposed approach also accounts for the impact of the local (site) and general (population) characteristics on the phenomenon. Field observations for a case study in an urban area are discussed to illustrate the methodological approach. Results ...

Surrogate Measures of Safety at Roundabouts in AIMSUN and VISSIM Environment

This paper addresses issues on road safety analysis through microscopic traffic simulation models. The Surrogate Safety Assessment Model (SSAM) was applied to read vehicle trajectory files generated by two micro-simulators and then calculate surrogate measures of safety. Since safety assessment of any road entity can provide different results based on the micro-simulator which is used, the main objective of this research was to estimate the safety performance of three roundabout layouts and compare the conflict events simulated by AIMSUN and VISSIM. The two micro-simulators were used to build the calibrated models of the roundabouts, each fitting the corresponding empirical capacity function. The results provided insights on how to set the SSAM filters in order to have a comparable frequency of conflicts by simulation and examine conditions under which a safety analysis could be independent of the software used.

Ex-ante assessment of the implementation of an energy management policy in Northern Africa

Energy management policy is an important part of the environmental and climate change policies and permits to reach the goals of improved security of energy supply, economic efficiency and of greenhouse gases emissions reduction. In these years, the use of market mechanisms to meet environmental objectives is really growing, with the most important example to use of emissions trading schemes to control greenhouse gases emissions, e.g., EUs climate and energy policy sets a 20% reduction in energy consumption to be achieved by 2020 through energy-efficiency improvements. In this context, this paper aims to assess whether it is possible to transfer to the North African context the lesson learnt from the more representative experiences or if it is necessary to adopt an ad hoc system.