Impact attenuating devices (1) for vehicles (5) with frame side members (9) are used in e.g. roadwork areas to protect road-users, men at work and equipment. The attenuator (1) is connected at the rear or front end of a vehicle (5) which is placed behind or in front of a roadwork. A vehicle having not noticed the roadwork runs into the attenuator (1) which catches the vehicle. To be able to attenuate the great forces arising in collisions, the attenuator (1) must be long and fixedly connected to the vehicle (5). This makes it difficult to transport, and a long overhang easily sweeps outside the own lane when driving through curves. The object of the invention is to provide an attenuator (1) which is easy to transport between the roadwork areas. A vehicle (5), usually a lorry, with frame side members (9) is connected to an attenuator (1) comprising an attenuating part (3), wheels (4) and a connecting part (2). The connecting part (2) connects the device (1) to the vehicle (5) either in an operating position or in a transport position. To be able to transport the attenuator (1) between different roadwork areas in a manner safe for traffic, the impact attenuating device (1) is connected like a trailer to the vehicle (5).