Actual Driving Data Analysis for Design Consistency Evaluation

To improve design consistency of existing and new roads, several studies developed operating speed prediction models and investigated drivers' speed behaviour. Most of the existing models are based on spot speed data assuming constant operating speed throughout the horizontal curves and occurrence of acceleration and deceleration only on tangents. To overcome limitations associated with these hypotheses, this study investigated continuous speed profiles of individual drivers by a driving simulator experiment carried out on a two-lane rural highway using the VERA high-fidelity dynamic-driving simulator, operating at the Road Safety Laboratory of University of Naples Federico II (Italy). The experimental route consisted of the succession of 20 tangents with length equal to 1,000 m and curves with radius equal to 400 m. The tangent-to-curve transition was carried out by spiral curves with parameter equal to 150 m and length equal to 55 m. Study results show that speed is not constant along the curve and deceleration rates are considerably higher than acceleration rates. These findings question the traditional hypotheses used to develop operating speed profiles. The analysis of individual drivers' behaviour showed that: (a) 85 th percentile of the speed reduction experienced by individual drivers is more than twice as the operating speed difference in the tangent-to-curve transition, and (b) deceleration and acceleration rates experienced by the individual drivers are approximately double than deceleration and acceleration rates used to draw the operating speed profiles. Study results emphasize that the analysis of the individual speed profiles adds important information to the analysis of the operating speed profile and has the potential to identify supplementary design inconsistencies and safety issues.

American Journal of Traffic and Transportation Engineering, 2021

Different studies conducted, regarding the development of statistical models used to predict crash occurrences along two-lane rural highways, around the globe and particularly in Ethiopia, rely on geometric characteristics than explicitly considering measures of design consistencies. Recently, the issue of geometric design consistency in highway design is emerging as an important criterion. This study is therefore carried out to evaluate the design consistency on horizontal alignments using measures of design consistency and develop safety functions that exclusively include design consistency measures. Elements of all road sections considered in the study were rated as good, fair or poor design using design evaluation criteria. Poisson regression and Negative Binomial regression modeling approaches were used to assess the relationship between measures of design consistency and road safety. Using information criteria goodness-of-fit, Poisson regression model was found better fit to the data than NB regression models. Three different crash prediction models which explicitly incorporate design consistency measures were successfully developed. The models can be used to predict crashes of two-lane rural highway having similar characteristics with the road considered under this study. Finally, based on the results of this research, further detailed study on effect of highway design consistency on road safety and current design guides related to design speed selection to account for design consistency revision was recommended.

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 vehicle's 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 carri ed out o n 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 empirica! Bayes (EB) procedure. Spearman's rank correlation was used to determine the leve! of agreement between the rankings obtained with the two techniques. The results from the Spearman's rank-correlation analysis validate the SI, indicating that the ranking from the SI scores and the EB estimates agrees at the 99.9% leve! of significance with a correlation coefficient of 0.87. Road safety evaluations on low-and medium-traffic rural two-lane highways can raise concems due to both the general defìciency of reliable data on road accidents and the circurnstance that few accident data can always give enough information on accidents to be prevented. O n the basis of these considerations, a methodological approach was defìned for the safety evaluation of two-lane rural highways that uses both analytical procedures referring to alignment design consistency models and safety inspection processes. The research was performed as part of the project ldentifìcation of Hazard Locations and Ranking of Measures to Irnprove Safety o n Local Rural Roads (ltalian acronym IASP), funded by the European Comrnission (Directorate Generai) and the Province of Catania in Italy (1-5). Many studies show that safety evaluations based on the analysis of alignment design consistency can be effective in identify-136 ing hazardous road locations (6, 7). The proposed approach uses theoretical-experimental models for the evaluation of alignment design consistency. However, the resulting analyses, although effective in addressing alignment inconsistencies, do not highlight all the potenti al accident contributory factors. Hence, the IASP methodology integrates the results ofthe models with those deriving from the safety issue evaluation made during the safety inspection process. Road safety inspections (RSis) are aimed at identifying potential hazards, which are assessed by measuring risk in relation to those road features that may Iead to future accidents so that remedial treatments may be implemented before accidents happen (8). Safety inspections are recognized as an effective tool andare becoming an accepted practice in many agencies around the world (9-14). Recent research performed in British Columbia (15) and in ltaly (8, 16) has shown that road safety impact assessments based on RSis can be effective. T o use safety inspections as part of a quantitative safety evaluation process, the IASP project defìned procedures and criteria for identifying and ranking safety issues (2). The ranking criteria take into account the road safety effects of the identifìed issues. The RSI carried out according to these procedures showed that there is a statistically signifìcant level of agreement (measured through the k-test) between the safety issue evaluations produced by different inspectors for the majority of the safety issues. A systematic and replicable safety inspection process allows a quantitative safety index (SI) to be assessed based on the data obtained from the RSI combined with information from theoretical-experimental models. FORMULATION OF SI The SI measures the relative safety performance of a road segment. l t does not take into accountjunctions, and i t refers to two-lane rural highways. The SI is formulated by combining three components of risk: the exposure of road users to road hazards (exposure factor) , the probability of a vehicle' s being involved in an accident (accident frequency factor), and the resulting consequences, should an accident occur (accident severity factor). The genera! formulation for the SI is as follows: SI = exposure factor x accident frequency factor x accident severity factor Exposure Factor (l) The exposure factor measures the exposure of road users to road hazards and is assessed as follows :

Publication No. FHWA-RD- …, 1995

The state of the practice in highway geometric design consistency was determined through a review of U.S. and foreign geometric design policy, practice, and research. Models, and a menu-driven microcomputer procedure for their use, were developed for operating-speed and driver-workload consistency evaluations of rural two-lane highway horizontal alignments. The operating-speed model was calibrated based upon speed and geometry data for 138 horizontal curves and 78 of their approach tangents in 5 States. The driver workload model was calibrated based upon 2 occluded vision test studies on a total of 55 subjects. The operating-speed data suggest that 85th percentile speeds generally exceed the design speed of horizontal curves whose design speed is less than drivers' desired speed (i.e., 85th percentile speed on long tangents). A preliminary evaluation comparing model-estimated operating-speed reductions versus degree of curvature as predictors of accident experience was conducted using a data base of 1,126 curve sites in 3 States. The evaluation suggests that accident experience increases as the required speed reduction from an approach tangent to a horizontal curve increases.

This consistency assessment of highways’ geometrical design has the objective of providing safer roads. There are two types of models for consistency assessment: aggregated and disaggregated. The first one considers the difference between design and operating speed at the middle point of isolated horizontal curves. The second one considers the spatial variation of the operating speed profile along the horizontal curve. This paper compares the two types of consistency assessment models, using naturalistic speed and geometry data obtained in 34 horizontal curves of two-lane rural roads in Chile, using a 10 Hz GPS. Results obtained showed that in only 19 cases both methods are equivalent. This equivalence occurred only when operating speed profiles have the lowest spatial variance along the curves. If the operating speed profile has a high variance the consistency level obtained using both methods is different and the better option is combine it.