Abstract: A warning device adapted to be laid flat upon a roadway produces an audible warning sound when contacted by the wheels of a moving vehicle. The device is comprised of a mat of flexible resilient material having an elongated configuration, and a series of rigid panels upwardly positioned upon the mat. The warning sound is produced when the panels are downwardly deflected, and thereby contact each other in domino fashion. By virtue of its specialized construction, the device can be folded and rolled into a compact storage state which can be contained in the trunk of an automobile.