Collision Sentry Collision Warning Device. Color: Yellow, Material: Plastic. Collision Sentry® helps reduce the probability of "blind corner" accidents by sending both an audio and visual warning well in advance of a pending collision. When the system detects motion on BOTH SIDES of the corner, creating conditions for an accident, it alerts approaching traffic. Integrated magnets on back allow for easy installation to a metallic corner or rack. Patented, easy to install, self-powered, compact, highly visible and reliable. Powered by six standard D-cell alkaline batteries (included). Audio level is pre-set with multiple setting options.