Abstract
Road safety depends on humans, vehicles, and highway conditions. These factors influence road safety separately or in combination. Despite these facts, the police report indicated that only a single cause of accident has been reported for a given accident in Ethiopia and the number of accidents that had been caused by road defect accounted around 1% of total accidents per year, which underestimates its contribution to road accidents. Beside these facts, only a few studies were carried out in Ethiopia on this issue. Even though dangerous road segments were identified and ranked along the same study area of this paper in our previous publication, Hawassa-Shashemene-Bulbula two-lane rural highway, the influences of road geometric parameters on road safety were not correlated along this study area. Therefore, it is imperative to clarify the relationship between the expected number of traffic accidents and the geometric conditions of the road under study to reduce accidents and provide safe driving environment. The main objective of this study was to quantify the influence of road geometric parameters on road safety. Using historical accident data, traffic data, and road data, the expected numbers of accidents were estimated by using the empirical Bayes (EB) method. Then, the expected numbers of accidents have been correlated with one or more road geometric parameters at a time. Accordingly, the identified dangerous road segments in the previous publication (DOI: 10.4236/jtts.2018.83009) were further analyzed to estimate the influence of road geometric parameters on road safety based on expected accident frequency. The result of the analysis shows that the radius of horizontal curve, superelevation, transition curve length, lane and shoulder widths were negatively correlated with the expected frequencies of accidents. Whereas the numbers of horizontal and vertical curves per segment, grade of the road, the occurrence of left turn horizontal curves with down grade and right turn horizontal curves with upgrade were positively correlated with the expected number of accidents. Therefore, it has been concluded that the influence of road geometry has significant effect on the occurrence of accidents along this study area.
Highlights
The very high cost of highway accidents paid by societies around the world makes highway safety improvement an important objective of transportation engineering
The expected number of accidents (i.e. empirical Bayes (EB) safety estimate), which were estimated to be the weighted average of observed and predicted number of accidents, for the ranked dangerous road segments are presented in Table 1 [4]
The analysis showed that short and sharp (i.e., R ≤ 450 m) horizontal curves were associated with higher crash frequencies
Summary
The very high cost of highway accidents paid by societies around the world makes highway safety improvement an important objective of transportation engineering. Highway engineers are usually interested in identifying road and traffic factors (which includes pavement condition, road geometry and its environmental, lane and shoulder width, number of traffic and its composition) to improve roadway design for new roads and provide a safer driving environment for existing roads. The occurrence of road accident is usually caused by a single factor rather due to the convergence of a series of events. A framework for relating the series of events in a road crash to the categories of crash-contributing factors is the Haddon Matrix [1]. According to this matrix, the factors that may cause to road crashes are generally categorized into three types, such as: 1) Human Factors, 2) Vehicle Factors, and 3) Roadway/Environment Factors. The report indicated that the sum of the causes of accidents in UK only exceeds 100% (accounted 131%) owing to more than one identified causing factors of accidents
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