Side impact accidents are the most severe and second frequent traffic accident configuration. By a comparison of results of tests with and without intrusion, this paper shows that the severity of side impact for the occupant seated near the impacted side is mainly due to intrusion of the side wall inside the car compartment of the struck car. Without intrusion, injury criteria recorded on a dummy are much lower than when there is intrusion.The influence of intrusion can be explained by the deformation speed of the side wall of the struck car. The velocity change of different parts of cars and of dummy have been determined in two tests conducted at 40 and 50 Kph. The diagrams of these velocity changes indicate that the door and the frame seat of the struck car sustain a velocity change higher than the car floor. The pelvis of the dummy seated near the impacted side sustains a velocity change in the same order of magnitude as the door, whereas its chest has a lower velocity change.This paper concludes that preventing from intrusion will decrease the severity of side impact and the seat can be used to participate in this stiffening. A padding should also be necessary but the determination of its characteristics needs a better knowledge of human tolerance to side impact.