There is no single value that you can use to make this model. I can point you in the direction of the IRI (International Roughness Index) which is a measurement calculated from the RTRRMS (response type road roughness measuring system) values. You can use this as an index to find various frequencies of vibration at a given speed for various conditions and types of roads. Hope this helps.

In fact you need to describe the terrain profile. Most researchers adopt a linear linear Gaussian and stationary process. However, actual terrains may not fit these conditions. They can be nonlinear, non-stationary and non-Gaussian. In general, you need to do some statistical tests on the terrain to determine the terrain topography and then choose the time series to model it. This is the most feasible method to model a terrain. Otherwise, just for testing your model, you may adopt any similar train model, from the published literature, that is very close to your application and type of terrain.