Answer:
As far as I know vehicle traveling vibration is random. It means that there is no one exact frequency. Of course if there are not regular cavities in the road. The dominating frequency range on the body (sprung mass) is for passengers cars approx 1-2 Hz. This is because of suspension natural frequency. This range is well tolerated by the human body. That is why this range is chosen. But I am not the specialist so anyone can correct me at will.