The number of whiplash accidents is an appreciable portion in the total automobile accidents. The purpose of this study is to measure the impact in very low-speed vehicle collisions and to clarify the mechanism of whiplash in low accelerations.In low-speed collisions, the change of total momentum of both vehicles before and after a collision cannot be considered to be nil but equals to the impulse of tire friction and rolling resistance. Consequently, the acceleration of struck vehicle depends on whether the emergency brake is applied or the engine stalls during a collision. The neck of a dummy was modified for the whiplash studies and the improved neck is found to well duplicate human passengers by comparing dummy's head motions with already reported human and cadaver experiments. It was found by the study that a head-rest and an elastic bumper stay are very effective to prevent whiplash for low speed rear-end collisions of few km/h.MORE THAN 40% of total injuries in automobile accidents in Japan are whiplash and more than 30% of total automobile insurence payments for injuries are against whiplash. Whiplash, in some cases, persists for years but usually no obvious symptoms show up with radiological or other quantitative diagnostic techniques. Since there is no externally visible evidences and the injuries are based solely on victim's complains, it is sometimes difficult to justify the causative relation of the whiplash and the particular accident. It is desirable to establish criteria and to clarify mechanisms of suffering whiplash and hence to suggest preventive measures for the injury.