Abstract: A ground-engaging emergency brake and anti-jackknife system for a vehicle comprises a brake skid which has at least one elongated substantially planar rigid base member, and upper and lower planar surfaces. A replaceable traction member is removably affixed to the base member lower planar surface. A linkage mechanism positions the brake skid in a ground-engaging deployed plane which is parallel to the plane of the vehicle, and when retracted, in a position adjacent to the underside of the vehicle. An actuating mechanism actuates the brake skid between the deployed and retracted positions. The brake skid is centered axially with respect to the longitudinal axis of the vehicle between the rear wheels of the vehicle. The actuating mechanism and linkage mechanism are effective to place a substantial amount of the vehicle weight on the brake skid when the brake skid is in its deployed position.