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Abstract
International crash data indicate that roadside characteristics contribute to more than half of all roadside accidents involving serious injury or death. Therefore, research on roadside safety is urgently needed. Based on the vehicle departure speed, pavement height (i.e., the difference between pavement elevation and ground elevation), slope gradient, and horizontal curve radius, this study uses PC-Crash simulation software to carry out tests of trucks and cars exiting a road. A chi-squared automatic interaction detection (CHAID) decision tree is used to explore the causative mechanism of vehicle rollover, and the concept of a “safe slope” to ensure that vehicles do not roll over is proposed. Aiming at straight and curved sections, discriminant functions of vehicle rollover and nonrollover are fitted through Bayesian discriminant analysis, and safe slope calculation models for trucks and cars are then constructed. Based on the obtained safe slope models, calculation methods for the safe slope and the roadside clear zone width involving different traffic compositions are proposed by calibrating the lateral distance from the final position of nonrollover vehicles to the road edge. The results show that the factors affecting vehicle rollover are, in descending order of importance, the slope gradient, pavement height, vehicle type, departure speed, and horizontal curve radius. For a section with a large proportion of cars, the slope gradient should not be steeper than 1:3.5. The horizontal curve radius should not be less than 600 m for a section with a large proportion of trucks and a slope gradient steeper than 1:3.5 or shallower than 1:2.5. Additionally, for a section with a pavement higher than 0.5 m and a slope gradient steeper than 1:2.5, the operating speed limit should be lower than 60 km/h. These research results have theoretical value and practical significance to improve the driving safety level and reducing the risk of roadside accidents.
Highlights
According to the Roadside Safety Research Program of the Federal Highway Administration (FHWA), roadside accidents account for more than 50% of all traffic fatalities [1]
Considering that roads host a mix of cars and trucks, this paper introduces the proportion of trucks W to further obtain calculation models of the safe slope and roadside clear zone (RCZ) width for different traffic compositions of straight and curved sections, as shown in models (20)–(23)
By recording vehicle motion states and motion tracks, safe slope calculation models corresponding to different operating speeds, pavement heights, and horizontal curve radii for trucks and cars are constructed. e proposed safety slope can be applied to a section with limited land area and high subgrade and ensure that vehicles entering the roadside do not roll over. e fourth edition of the Roadside Design Guide (RDG) stipulates that a slope of less than 1:4 is a recoverable slope for runaway vehicles, while China’s Specifications for Highway Safety Audit (JTG B05-2015) stipulate that the slope should be less than 1:6
Summary
According to the Roadside Safety Research Program of the Federal Highway Administration (FHWA), roadside accidents account for more than 50% of all traffic fatalities [1]. Referring to the fourth edition of the RDG, China’s Specifications for Highway Safety Audit (JTG B05-2015) divide the RCZ width into computed and actual values and provide a graphical method for calculating the RCZ width for a straight section based on operating speed and one-way annual average daily traffic (AADT). Taking the fourth edition of the RDG as an example, the graphical method can visually show how to determine the RCZ width by considering the design speed, AADT, slope form, and slope gradient This method has the following problems: (1) the AADT is related to the probability of roadside accidents but should not be applied as the basis for determining the RCZ width; (2) the influences of the vehicle type, presence of a shoulder, and adhesion coefficient are not considered; (3) the effectiveness of the graphical method is limited, which hinders the accurate calculation of RCZ width; and (4) the quantitative relationship between the
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