The deviation angle for one-lane roundabouts: A general mathematical formulation and application

Abstract Properly designed roundabouts may lead to safety improvements based on both reducing approaching speeds and controlling traffic. Measurements of deflection of vehicle trajectories are commonly used to estimate roundabout speed control. One of these measurements is the deviation angle, which is mentioned in both the Italian and Swiss road standards and, in specific conditions, can be more effective than other methods. This article presents a general mathematical formulation for linking several geometric parameters with the deviation angle in different rural and urban one-lane roundabout configurations, which is currently missing in the literature. For urban roundabouts, refuge islands for pedestrians and cyclists were considered. Based on the proposed formulation, a sensitivity analysis of the influential geometric parameters was conducted. Results suggest that an insufficient deflection of trajectories (deviation angle less than 45°) is always present for roundabouts with inscribed circle diameter less than 25 m; for urban roundabouts with refuge islands for pedestrians and cyclists having inscribed circle diameter less than 34 m and orthogonal legs; and for roundabouts with angles between opposite legs smaller than 140°. The main parameters which are responsible for a decrease in the deviation angle are: a decrease in the inscribed circle diameter; a decrease in the angle between opposite legs; and an increase in the width of the circulatory lane. Some optimized procedures for roundabout design, the generalized application of the deviation angle method and alternative speed control measures in cases of small deviation angles are discussed

Spatial competition and efficiency : an investigation in the airport sector

This paper analyses the potential impact of airport competition on technical efficiency by applying the spatial stochastic frontier approach (SSFA) rather than traditional model (SFA). The SSFA allows to isolate the cross-sectional spatial dependence and to evaluate the role of intangible factors in influencing the airport economic performance, through the inclusion of the distance matrix and the shared destinations matrix, calibrated for different distances. By analysing statistical differences between the traditional and the spatial model, it is possible to identify the competition effects. This study includes 206 airports at worldwide level. First, the results show the existence of the spatial component, that could not be otherwise captured by the traditional SFA. Moreover, airport competition is found to affect the efficiency level with either a positive or a negative effect, depending on the distance considered in the spatial model

Competition among airports at worldwide level: a spatial analysis

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Investigating the Deviation Angle Method for Ensuring Deflection at One-Lane Rural Roundabouts

Roundabouts developed as a road intersection design option has resulted in a series of nonuniform design guidance criteria in Europe, as well as in the United States and other Countries. In addition to different design specifications about the geometry of the elements constituting a roundabout (width and lanes of the circulatory roadway, entry and exit legs, splitter island), the methods for guaranteeing that vehicle paths deflect through the roundabout are also different. These methods ensure proper travel speeds between conflicting traffic flows. Currently, the main parameters used by standards to control the deflection are the deflection radius, the entry path radius, and the deviation angle. After a comparison between International deflection methods for roundabouts, this study checks the geometric requirements of the deviation angle for more than 7.000 hypothetical one-lane rural roundabouts. The Computer-Aided Design (CAD) drawing of the roundabouts takes into account the range of variability of their main geometric parameters, according to the Italian Standard. Subsequently, a number of the considered roundabouts checked with both the entry path radius and the German methods. Some results showing the greater effectiveness of the less popular deviation angle method are discussed. The main aims of this paper are:1) to promote the deviation angle method, which is only used in Switzerland and Italy;2) to improve standards, as regards the applicability and validity of the deflection angle method;3) to help practitioners to know in advance the outcome of the deflection checks at the beginning of the iterative design process, once the boundary conditions are known

Road design influence on driving behaviors: The influence of curve design, a case study

Road geometry has always been a key feature for road safety concerns. It will become more crucial in the context of future transportation, especially with the advent of Connected and Autonomous Vehicles (CAVs). In fact, recognizable and intuitive road alignments would simplify the driving tasks for both humans and CAVs (independently from the rate of automation). Thus, not only building consistent and self-explaining roads is fundamental for new and old vehicles, but also adjusting the existent ones, operation that seems even harder. Since most of the existing roads would need massive interventions, policy makers and road designers might choose between making adjustments being compliant with the current regulations in toto or adopting countermeasures supported by specific safety assessments to make existing roads safer, also in the perspective of future changes. In this optic, the present study tries to investigate a typical geometric design issue of existing roads, i.e., the presence of a long segment followed by a sharp curve without transition curves on undivided two-way two-lane rural roads. This alignment does not reflect the current recommendations for road alignment, so it was investigated the effect of such a design on users and safety for a specific testbed. The users’ behavior was investigated recording the kinematic parameters of the traveling vehicles. This data collection was run using radar traffic counters, placed on the roadside throughout the entire layout of the investigated segment-curve, to get speed and acceleration. The data were elaborated to investigate driving behavior in free-flow conditions. A K-means cluster analysis was run to characterize the users’ behaviors in terms of speed and acceleration. Hard braking was found to be strongly related to high speed, as well as ongoing deceleration on curve was detected for all the vehicles with high speeds on the segment. Results about users’ behaviors were compared to the available crash dataset to understand the possible implications of human factors on occurred crashes and to simulate the decision process of safety-related adjustments of existing roads

Integrated American-European protocol for safety interventions on existing two-lane rural roads

Abstract Purpose The main purpose was to integrate two strategies for road safety analyses (qualitative: audits, inspections; and quantitative: accident predictions) and develop a possible protocol for the safety interventions on existing two-lane rural road segments. Those road sections do not typically belong to the TEN-network, to which the 2008/96/EC Directive is mainly oriented. Hence, they could lack of methods for designing safety-based interventions. The main research questions were:Which possible problems can arise from the application of this protocol to real cases?Which data are practically needed?Which possible solutions can be provided for the highlighted problems? Methods The integrated protocol, including: 1) the HSM predictive method, 2) the EU Regulations, 3) the local road design standards, 4) some research developments; is applied to real two-lane rural road segments requiring safety-based interventions. Its application is divided in the typical road safety analysis stages. Results A wide list of possible problems was highlighted and addressed: 1) lack of data, 2) difficult comparison with current road standards in order to identify safety problems, 3) lack of methods for evaluating the skidding risk along the layout, 4) setting speed limits, 5) need for optimizing the selection of countermeasures based on their aims and their timely application, in different recurrent situations, 6) availability and comparison of predictive methods. Conclusions Based on the problems and solutions discussed, main advantages (1) the systematic approach, 2) the quantitative assessment of benefits, 3) the possible transferability) and disadvantages (difficulties in overcoming the lack of data and calibrated accident prediction models) of the method were remarked

Optimal planning of safety improvements on road sites belonging to different categories within large networks: An integrated multi-layer framework

Planning road safety interventions on large road networks implies several layers of complexity in the decisionmaking process. In fact, the following simultaneous problems should be addressed: estimating safety performances on the different road elements of the network, identifying sites showing high potential for improvement with respect to reference values, defining the possible types of safety measures to be implemented and their anticipated effect on traffic safety, limiting the number of interventions given fixed budget constraints. This study proposes an integrated multi-layer framework which takes into account the above-defined problems into a single optimization procedure which provides the number and type of safety interventions to be implemented over a wide road network composed of different categories of road elements. The proposed framework is based on the following peculiar aspects: the potential for safety improvement is quantitatively assessed based on the estimation of safety performances for each road category, a bi-level thresholding process integrated in the optimization process is used to highlight sites for interventions, the anticipated outcome of safety measures is quantitatively assessed as well through available crash reduction factors. The proposed methodology is applied to a case study which analyzes a sample of real roads belonging to a province-wide road network composed of various road elements (i.e., different categories of segments and intersections), under different budget constraints. Results demonstrate the applicability and flexibility of the proposed approach, which could be used for planning purposes, independently of the particular geographic location. Clearly, the approach is valid at the planning stage, given that several details of the different layers of analysis are necessarily simplified, while they should be studied in detail at the single intervention project stage

Geometric Design Issues and Safety Analysis of Two-way Rural Road Tunnels

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Rethinking the main road design concepts for future Automated Vehicles Native Roads

Road design standards/guidelines are based on internationally valid concepts. They include requirements concerning alignment, sight distance, speed, friction, cross-sections. Those requirements are based on three main factors related to: drivers, physics, and comfort. The introduction of automated vehicles in the market will likely have a great impact on the road and transport design, management, operation and safety. In particular, the concept of road design and the related standards and guidelines may be modified, since some driver-based requirements may lose their significance in case of self-driving vehicles.In this article, basic International road design concepts are reviewed, with specific focus on rural roads. The review aims at classifying the design concepts into the three categories: driver-, physics-, and comfort-based. Based on this classification, the applicability of traditional road design concepts to Automated Vehicles Native Roads -AVNRs- (roads specifically designed for automated vehicles) is discussed, by also taking into account results from simulations performed for the possible design of AVNRs.As a result of the study, concepts/requirements concerning road design consistency and discrepancies between design, operating and posted speeds, may be not more necessary in case of AVNR design. Concepts/requirements related to tangent and curve lengths and sight distances may significantly change as well. Some minor modifications were discussed for cross-sectional elements (e.g. reduction of lane widths), and design speeds. The essentially physics-based design elements and requirements, such as curves, grades, road friction, sight distance (based on road geometry), transition curves and shoulders, should still be considered for the AVNR design instead, as well as comfort-based requirements