In this study, a computer simulation model of a rigid impactor loading laterally a roadside W152x13.4 post has been developed. The interaction of cohesionless soil with a post was studied and compared to an existing dynamic test results from a published literature. Two approaches to simulate the soil have been studied: the continuum method where the soil is modeled as a solid element with Drucker and Prager material law, and the subgrade method where the soil reaction is simulated by a series of nonlinear springs. An improved method of the subgrade approach has been developed where the soil is modeled as a system of parallel springs and dampers with a lumped mass attached to the post. A simple procedure to calculate the lumped soil mass and the damping coefficient is presented.