Abstract: A visually detectable warning device particularly suited for use by cyclists in attracting the attention of others, particularly motorists. The device is characterized by a flexible staff adapted to be mounted on a bicycle, a transparent housing rotatably mounted on the staff, and a lamp and reflector mounted in the housing for directing therefrom a sweeping beam of light from the housing. The housing is characterized by an annular array of uniformly spaced vanes which serves to impart to the housing rotary motion in response to an impinging stream of air resulting from motion imparted to the bicycle.