3M AUTHORIZED. Official 3M reflective films and inks on durable, rustproof aluminum ensures that our signs meet all DOT requirements for outdoor parking and traffic signs. OFFICIAL SIGNS. Buy the same signs sold to our governmental customers. Over 1, 000 municipalities buy their signs from us. The DOT requires 18 x 24 and larger signs for public roads. This 18 x 24 sign is our most popular size for speed limit signs. 12" x 18" speed limit signs are not covered by the MUTCD standard. DURABLE ALUMINUM. Heavy-duty rustproof aluminum has a 10 year outdoor durability rating backed by a 3M . LAMINATED. To achieve maximum durability, sign graphic are protected with a 3M traffic sign laminate. This provides enhanced weather and graffiti resistance. INSTALLATION. All signs come with pre-punched and pre-cleared mounting holes for easy installation. Signs mount to U-channel posts, square or round galvanized posts, wooden posts and chain link fences. SAVE LIVES. You are 7x less likely to die if you are hit at 20mph than 30mph. And this increases to 10x if you are over 60 years old. In the distance a 20mph car can stop, a 30mph car will still be doing 24mph.