"Speed Limit 20" Sign By SmartSign | 12" x 18" 3M Engineer Grade Reflective Aluminum Signs use 3M's reflective film. Visible at night, these signs are durable and comply with standard quality guidelines for parking signs. Signs have predrilled holes for easy installation. Signs can be installed onto a post, fence or wall. These municipal quality signs are great, also, for parking lots, garages or visitor areas.