Study On Road Alignment Optimization Based On Road Network

Based on the theories of traffic planning and road design, the paper discusses on road alignment optimization and proposes road alignment optimization based on road network. The purpose of this study is to advance the worth of traffic planning theory applied to road alignment optimization, and provide a new idea and method for road engineers and related researchers, which find out the optimal road alignment in the process of optimization more efficiently, scientifically and exactly.This model is a mathematic optimization problem finding the road spatial alignment between the start and end points whose ratio of benefit to cost is optimal. In Geographical Information System (GIS), this model records road network, Digital Elevation Model (DEM) and natural surface characters in the target region as the vectorgraph layers. This model is solved by Genetic Algorithm (GA).In GIS, firstly, according to different control points, the candidates of road alignment are generated randomly, and then horizontal and vertical alignment of each road is designed automatically. On the one hand, new road can change the topology structure of original road network. Topology of the changed road network is constructed automatically firstly, and the travel times and flow characteristics of OD traffic on the network is obtained through user equilibrium traffic assignment, and then total travel time and traffic environment loads on the network are estimated. By comparing them with those of original network, reduced values of travel time and environment load of OD traffic on the network are calculated. The sum of two values is taken for total benefit of each road candidate. On the other hand, the construction of road itself can result in several costs. Various project costs including construction cost, earthwork cost, occupied of green land or wet land cost, removed building cost and construction cost of potential bridges or tunnels are calculated in GIS. The sum of above cost of every item is taken for total cost of each road candidate. In additional, road design criterion can not entirely be satisfied in the process of road design in GIS, and thus here the penalty functions are used to correct it in the model.In GA, all road candidates are taken for generation population, and control points in the each candidate are taken for genes in its chromosome. The fitness of each individual is designedamong the cost, the benefit and penalty function. Firstly, initial the population and calculate the fitness of each individual. Then evolve one by one generation through selection, crossover. and mutation and protect excellent strategy. At last, the algorithm does not stop until the terminate condition is reached. The optimal road alignment is obtained.This model is validated by the numerical test. The feasibility and effectiveness of this model is proved in solving the problem on road alignment optimization.