Salvatore Cafiso

Safety Index for Evaluation of Two-Lane Rural Highways

The methodological approach to safety evaluation of two-lane rural highway segments that is presented uses both analytical procedures referring to alignment design consistency models and safety inspection processes. A safety index (SI) that quantitatively measures the relative safety performance of a road segment is calculated from the procedure. The SI is formulated by combining three components of risk: the exposure of road users to road hazards, the probability of a vehicles being involved in an accident, and the resulting consequences should an accident occur. This systematic and replicable procedure integrates two different, complementary approaches–one based on design consistency evaluations and the other on safety inspections–and makes it possible to address a wide variety of safety issues effectively. A further advantage of the procedure is its applicability on highways where crash data are either not available or unreliable. Validation of the procedure was carried out on a sample of roads by a comparison of the risk rank obtained by using the SI and accident history. The SI was assessed in 30 segments chosen from a sample of two-lane rural highways in Italy, and the actual accident situation was obtained with the empirical Bayes (EB) procedure. Spearmans rank correlation was used to determine the level of agreement between the rankings obtained with the two techniques. The results from the Spearmans rank-correlation analysis validate the SI, indicating that the ranking from the SI scores and the EB estimates agrees at the 99.9% level of significance with a correlation coefficient of 0.87.

MULTICRITERIA ANALYSIS METHOD FOR PAVEMENT MAINTENANCE MANAGEMENT

Multicriteria analysis (MCA) provides a framework for breaking a problem into its constituent parts. MCA provides a means to investigate a number of alternatives in light of conflicting priorities. By structuring a problem within the MCA framework, alternatives may be ranked according to preestablished preferences to achieve defined objectives. A method developed for providing an MCA framework within the Highway Development and Management Tools, Version 4 (HDM-4) is described. Factors such as social benefits, environmental effects, safety impact, strategic importance of roads, and so forth have been included, within a unified decision-making framework. The analytic hierarchy process method was selected for HDM-4 because it transforms the analysis of competing objectives to a series of simple comparisons between constituent elements. The approach does not require a definition of trade-offs between the possible values of each attribute (i.e., it is not necessary to build utility functions), and it allows users to understand how outcomes are reached and how weightings influence outcomes. A pilot implementation checked the applicability of the analytical hierarchy process (AHP) method to road management and analyzed the effectiveness of outputs. Results obtained using HDM-4 economic criteria were compared with those produced by AHP. For the pilot implementation, the HDM-4 strategic planning application was chosen because it is the level in which, generally, medium- or long-term programs must be defined along with economic and other social and environmental aspects to justify budget allocations. Five main criteria were identified with regard to road maintenance budget allocations that constrain the decision-making process: comfort, environment, safety, road agency costs, and road user costs. Prioritization based on MCA more evenly distributes the available budget when compared with prioritization based on economic criteria, which tends to favor roads with high volumes of traffic.

Safety evaluation process for two-lane rural roads: A 10-year review

A practical procedure was developed that explicitly considers the driving behavioral and safety rules of a horizontal alignment for the evaluation of roadway new designs; redesigns; and projects of rehabilitation, restoration, and resurfacing. Design classes were developed to classify, from a traffic safety point of view, roadway sections as good, fair, or poor; these design classes are associated with three safety criteria to develop an overall quantitative safety evaluation procedure for two-lane rural roads. The safety criteria are introduced to analyze and evaluate by (a) Safety Criterion (SC) I: the difference between design speed and actual driving behavior as expressed by variations in observed 85th percentile speed; (b) SC II: the difference between observed 85th percentile speeds on successive design elements; and (c) SC III: the difference between side-friction assumed and side-friction demanded at the 85th percentile speed level on curves. Furthermore, the issues discussed include design speed, operating speed, sound tangential, and side-friction factors, as well as the application of tangents in the design process. A comparative analysis of the actual accident situation with the results of the safety criteria reveals a convincing agreement. Thus, the great advantage of the new concept is that as early as in the design stages, safety criteria can predict the endangerment (low, medium, and high) for new alignments or allow statements about the safety conditions of existing (old) roadway sections or whole road networks.

Driving Performance, Alignment Consistency, and Road Safety: Real-World Experiment

Road infrastructure improvement is one of the three pillars in the European Union policy aimed at halving the number of fatalities in road accidents by 2010. Increased knowledge and experience have proved that highway consistency analysis is an effective tool in the evaluation of road performance in terms of safety. Recent studies have attempted to provide information relating to the geometric consistency of road alignment by using different approaches based on geometric relation design, differential between design and operating speed, driving performance with instrumented vehicles, and human workload evaluation. This paper presents the results of a naturalistic driving experiment carried out with the University of Catanias Driving Instrumented Vehicle Acquisition System–instrumented vehicle. Representative variables for describing alignment consistency were defined and collected by direct measurement during real-world driving tests. From the seven parameters taken into consideration, the maximum driving speed differential between two successive elements and between the average section speed and the minimum single element speed were chosen as driving performance indicators (DPIs), since they are not correlated and agree statistically with the accident history. On the basis of actual driving behavior by means of DPIs, design inconsistencies were identified along different test courses, and their agreement with safety evaluation criteria and relation design parameters was verified. Comparison analyses among DPIs, speed consistency variables, and relation design parameters have evidenced the effectiveness of the indirect measurement of design consistency and the possibility of defining threshold values for the identification of those elements characterized by acceptable (good), reasonable (fair), or intolerable (poor) alignment inconsistencies.

Texture analysis of aggregates for wearing courses in asphalt pavements

It is well known that the wear of road pavement skid resistance is associated with a change in the geometrical characteristics of the micro- or macro-texture of aggregates of pavement surface. So, a study of the relationship between texture and skid resistance, to find suitable correlations, is of interest for research in road pavement materials. In this work the relationship between adherence and aggregate texture is analyzed, using superficial geometrical profiles, by means of both aggregate descriptors and the power spectral density function (PSD). In particular, a methodology for a surface texture analysis of aggregates for pavement wearing courses is presented which demonstrates the support given by both micro- and macro-texture, resulting in some indicators that can be related to pavement adherence. The results are, also, useful in order to explain the basic physic phenomenon between friction and the surface profile proprieties of aggregates. For research purposes, a procedure was established for the preparation and analysis of aggregates in order to carry out a survey of their geometrical profiles using laboratory samples which underwent an accelerated polishing control test (CLA).

Actual Driving Data Analysis for Design Consistency Evaluation

In recent years, researchers have proved that a consistent highway design ensures that successive elements are coordinated to produce harmonious and homogeneous driver performances and does not provoke unexpected events. Knowledge and practice show that drivers make fewer errors near geometric features that conform to their expectations. On this basis, the importance of identifying inconsistencies on highways for its significant contribution to road safety is emerging as an important feature in highway design. Although several techniques and models for evaluating the consistency of a design in a quantitative way have been identified and some countries have implemented the design consistency concept in their road design guidelines mainly in a qualitative way, there have been only a few efforts to measure actual driving behavior. The aim of this paper was to determine design inconsistencies on existing two-lane rural roads with the use of actual driving behavior by means of field data measurements and to verify their agreement with a consistency evaluation model. Furthermore, suitable equipment and a procedure for surveying driving dynamics and driver workload have been developed. In particular, driving behavior was assessed through direct measurements and parameters taken from data collected on a selected sample of test drivers by using a purposely designed instrumented vehicle. The vehicle, named the driver instrumented vehicle acquisition system (DIVAS), was driven under real traffic conditions on a two-lane rural road. The design classes of consistency of the test courses also were evaluated with a well-known safety criteria model. Data collection and treatment procedures are presented, and data analysis and results from this first experiment are given.

Safety Inspections as Supporting Tool for Safety Management of Low-Volume Roads

Road safety inspections (RSIs) are becoming an accepted practice in many agencies around the world. A safety assessment procedure based on safety inspections that can be used as a supporting tool in the safety management of low-volume rural roads is presented. From the procedure, a safety index (SI) that quantitatively measures the relative safety performance of a road segment is calculated. The RSIs carried out according to the defined procedures showed that, for the majority of the safety issues, there was a statistically significant level of agreement on the ranking of the issues produced by different inspectors. Further, the SI was assessed in 30 segments of two-lane rural roads, and rankings performed according to the SI scores and according to the empirical Bayes (EB) safety estimates were compared. This comparison showed a good correlation between SI and EB estimates. The results from the Spearmans rank correlation analysis provide additional validation of the procedure, indicating that the rankings from the SI scores and the EB estimates agreed at the 99.9% level of significance. The SI can be assessed as part of the safety inspection process without relevant supplementary work. The low cost and applicability in road networks where geometric and crash data are not available make the procedure very attractive for low-volume roads.

New approach to defining continuous speed profile models for two-lane rural roads

Despite a large body of research on operating speeds, there is still much to learn about the factors that affect free-flow speeds, especially with regard to the development of a continuous speed profile for two-lane rural highways. This paper presents a new approach to modeling driver speed profile along two-lane rural roads. Speed data regarding individual drivers traveling on selected two-lane rural roads were sampled during a naturalistic experiment that used an onboard Global Positioning System (GPS). GPS devices facilitate the collecting and processing of continuous speed data. Unlike spot speed measurements, which have typically been collected in similar studies, continuous speed data are useful for studying the actual speed profile with regard to acceleration–deceleration behavior and maximum speed on tangents and curves. New variables are used in the regression equation. These variables refer to the horizontal curvature and the vertical grade as weighted values of the geometric features of the alignment preceding and following the vehicle position. To predict the operating speed, the model takes into account the geometric alignment of the road preceding and following the vehicle, as this information is used by drivers to establish the driving task, which is based on memory expectation, vehicle dynamics, and visual perception. The final result is an algorithm for modeling a continuous speed profile without previous assumption of acceleration–deceleration rate and constant speed on tangents and curves.

Assessing the stochastic variability of the Benefit-Cost ratio in roadway safety management

Road Agencies set quantitative targets and adopt related road safety strategies within the priorities and the available resources at the time of an economic crisis. In this framework, benefit-cost analyses (BCA) are carried out to support the decision making process and alternative measures are ranked according to their expected benefit and benefit-cost ratio calculated using a Safety Performance Function (SPF) and Crash Modification Factors (CMFs) as predictors of future safety performances. Due to the variance of CMFs and crash frequency we are uncertain what the benefits of some future actions will be. The chance of making wrong decisions depends on the size of the standard deviation of the probability distribution of the considered stochastic variables. To deal with the uncertainty inherent in the decision making process, a reliability based assessment of benefits must be performed introducing a stochastic approach. In the paper the variability of the CMFs, the predicted number of crashes and the crash costs are taken into account in a reliability based BCA to address improvements and issues of an accurate probabilistic approach when compared to the deterministic results or other approximated procedures. A case study is presented comparing different safety countermeasures selected to reduce crash frequency and severity on sharp curves in motorways. These measures include retrofitting of old safety barriers, delineation systems and shoulder rumble strips. The methodology was applied using the Monte Carlo simulations to calculate the probability of failure of BCA statements. Results and comparisons with alternative approaches, like the one proposed in the HSM, are presented showing remarkable differences in the evaluation of outcomes which can be achieved.

Revisiting Variability of Dispersion Parameter of Safety Performance for Two-Lane Rural Roads

Safety performance functions (SPFs) are commonly calibrated with negative binomial regression in which a dispersion parameter that represents extra-Poisson variation is estimated. The primary use of this parameter is in empirical Bayes estimation for safety management applications such as treatment evaluation and network screening. It stands to reason that the importance of precise estimation of the dispersion parameter should be established. Recent research has suggested that the dispersion parameter is not constant but actually varies from site to site, depending on site characteristics such as segment length. In revisiting the dispersion parameter issue in this empirical investigation, previous research on this issue is reinforced by filling a number of knowledge gaps. First, light is cast on the dispersion parameter variation for SPFs for two-lane rural roads, an important entity type for which there is little or no knowledge in this regard. In this study, more precise model forms are investigated to represent the variation with respect to the key variable, segment length. This investigation confirms that the dispersion parameter is inversely related to segment length but reveals that it is not inversely proportional to segment length, as suggested in other research. Second, it was found that the dispersion parameter is smaller and the variation less pronounced with better-specified models. Finally, additional evidence is provided to suggest that dispersion parameter variation matters more for shorter segment lengths.