Abstract: In order to reveal the influence rule of spiral transition curve length on the path of a vehicle under free flow traffic states, a driver direction control model for highway alignment safety evaluation was put forward on the basis of the preselect optimal curvature method and by introducting the horizontal curve length-curvature alignment model. The simulation results show that when a vehicle enters a curve, its path has a tendency to shift inward the curve, while its path tends to shift outward the curve as it exits the curve. The length of a spiral transition curve has a significant impact on the path of a vehicle, the longer the spiral transition curve is, the smaller the lateral deviation of vehicle path is, whereas the greater it will be. To a symmetrical basic horizontal curve of second-class highways, the length of a spiral transition curve is suggested to be 0.4 to 0.6 times the radius of the curve.