When side slip (β) is small, the first term of Equation 3 is very small thus the acceleration of vehicle has a magnitude of V(β ̇+γ) and is perpendicular to the vehicle longitudinal direction (x-direction). Therefore the direction of acceleration is in the lateral direction (y-direction). In addition, when the side slip is small, direction of vehicle lateral direction (y-direction) and direction perpendicular to traveling direction (n-direction) is almost coincide.