Reflective Signs Are Hard to Ignore. Easy to spot by day, and reflective at night, these Do Not Block Mailbox and Driveway signs can be used to deter drivers from parking their vehicle in front of your driveway and mailbox. 3M Engineer Grade [“EG”] Reflective 3M Engineer Grade reflective signs are the most common material found on our public roads. USA's Oldest Sign Manufacturer Family owned since 1918, we take immense pride in our signage. Being one of the oldest certainly means we know this business in and out. Our South Dakota factory is the oldest manufacturer of metal signs in the USA. We have been an integral part of safety and convenience our signs have provided over the years. but most signs last much longer. Our signs are laminated for enhanced performance. You can clean off graffiti and your graphics are protected. Unlike many other signs, our Do Not Block Mailbox and Driveway signs use industry-leading 3M inks and not off-brand or other low-durability ink jet pigments that fade in a year or two. Signs Give a Warning – So You Don't Have To Let this sign do the talking! This unambiguous message is not open to interpretation and, importantly, avoid a personal confrontation with the violator.