Trailer Reflectors are hermetically sealed to prevent moisture entry which can cloud or damage the reflector. Designed with a center hole for screw mounting (screws not included) ensures permanent installation and easy removal if desired. Large 3-3/16 inch size provides extra visibility vs smaller sizes but is still small enough to fit in most locations. We also offer Reflective DOT Legal Tape for commercial truck and trailer marking at low everyday prices. This is required by law for commercial trucks and trailers in the U.S. per Federal Motor Vehicle Safety Standard (FMVSS) 108. A study by the DOT's NHTSA confirms that the reflective tape is extremely effective in preventing highway crashe