Research On Highway Alignment Optimization Methods Of Long Steep Downgrades On Expressway

To meet the requirements of the rapid development of social and economic in China, the emphasis of expressway construction has moved from eastern plain areas to middle and western mountainous areas. For the limitation of topography, geology and economic development, the number of long steep downgrades on expressways increases significantly. Moreover, combination of long steep downgrades and tunnel group, bridge-tunnel-bridge sections, spiral tunnels and high pier bridges is very common. Influenced by the high altitude, bad weather condition, e.g. fog, ice, snow, strong crosswind, often exists in long steep downgrades. Therefore, operation safety risks of long steep downgrades are higher than other road sections. According to the survey results, most long steep downgrades on domestic expressways are accident-prone sections. Influenced by the fatal accidents on long steep downgrades, some expressways are called "dead road". Although much research on safety enhancement of long steep downgrades has been conducted, operation safety problems still exist on long steep downgrades. So much more in-depth and targeted research on geometric design and the optimization techniques for long steep downgrades on expressways should be conducted based on current research findings, in order to improve the ability of operation safety risk prevetion and control of long steep downgrades.The focus in this dissertation is long steep downgrades on expressways. Based on the analysis of main operation safety risk factors for long steep downgrades, research from the aspects of the relation between driving workload and geometric alignment, optimization techniques of vertical alignment, combination of horizontal and vertical alignment is conducted.(1) Analysis of operation safety risk factors for long steep downgrades. Considering the factors of driving behaviors, vehicle conditions, highway infrastructures and traffic environment, main influence factors of traffic safety are analyzed and summarized. Then operation safety risk factors checklist for long steep downgrades on expressways is developed.(2) Research on the relation between driving workload and geometric alignment for long steep downgrades. Driving experiences, characteristics of vehicle control, routing selection and the effects of traffic safety facilities on long steep downgrades are analyzed. Then relational model between geometric alignment and driving workload is developed based on the site and real vehicle tests of driving workload on long steep downgrades. Moreover, the geometric alignment evaluation techniques for long steep downgrades on expressways based on driving workload is put forward.(3) Research on the optimization of profile design parameters for long steep downgrades. Considering the factors of the longitudinal grade composition of long steep downgrades, characteristics of heavy trucks, operating speed, gross vehicle weight, etc., characteristics of brake temperature and brake torque of heavy trucks descending the grade are put forward, using brake temperature prediction model and vehicle dynamics simulation techniques based on TruckSim. The distribution characteristics of brake torque among different axles of heavy trucks are analyzed. Then the influence of gross vehicle weight and operating speed on brake temperature and brake torque of heavy trucks, as well as the influence of grade percentage of each section of long steep downgrades on brake torque is analyzed. Moreover, influence of two different profile design alternatives on brake temperature and brake torque of heavy trucks is discussed. The first design scheme is that unique longitudinal grade is used throughout the whole downgrade. The second one is that different grade percentages are used in different segments, e.g. a steep grade is preceded by a gentle slope. Finally, the quantified section classification criterion and recommendations for the profile design of long steep downgrades on expressways are provided. The research findings have been proved to support the amendment of article 3.0.17 of Technical Standard of Highway Engeering (JTG BO 1-2003).(4) Research on the optimization of geometric alignment combination for long steep downgrades. Considering the factors of three-dimensional (3D) highway alignment, drivers’ dynamic visual field, the illumination angle of vehicle headlamps etc.,3D dynamic sight distance is used to evaluate the combination of horizontal and vertical alignment of long steep downgrades on expressways. Firstly, the 3D highway alignment fitting methods based on cubic B spline curves are put forward. Secondly, the calculation model of 3D dynamic sight distance is developed with the constraint of the relation between the computed angle composed by the line from the position of drivers’ eyes to the sight distance checking point and the centerline of drivers’ visual field, and drivers’ maximum dynamic visual angle. Finally, the evaluation technique of geometric alignment combination of long steep downgrades on expressways based on 3D dynamic sight distance is provided.