1. A method for modeling a roadway as a structure having a plurality of rectangular plates linked to each other, the modeled structure being expressed as the following matrix equation (1): EQU P.sub.i =[w.sub.i l.sub.i .alpha..sub.i .beta..sub.i .gamma..sub.i ].sup.T( 1) wherein it is assumed that a vehicle is placed at the center of the bottom line of a plate P.sub.i having a width of w.sub.i and a length of l.sub.i the Z.sub.i axis is the traveling direction of the vehicle, the X.sub.i axis is lateral to the vehicle, the Y.sub.i axis is perpendicular to the plate P.sub.i, .beta..sub.i and .alpha..sub.i represent error angles of the X.sub.i and Z.sub.i axes of the plate P.sub.i with respect to the X.sub.(i-1) and Z.sub.(i-1) axes of the plate P.sub.(i-1) linked immediately before the current plate P.sub.i, respectively, and .gamma..sub.i represents error angle between the Y.sub.i axis perpendicular to the roadway and the direction perpendicular to the vehicle.