First we should discuss the traffic regulations governing the hanging of a traffic sign. The official word comes from the Manual on Uniform Traffic Control Devices, or the MUTCD in the sign biz lingo. The MUTCD basically says “count up.” The bottom of the sign has to be seven feet from the ground. In keeping with the desire to keep people from hitting their head, the minimum height when hanging a sign over traffic is 17 feet.
Practically, what does this mean you ask? Suppose you were going to hang a standard 30 inch stop sign. You might be targeting 24 inches for burial, add the seven feet and now add the 2.5 feet to fit the sign and you are at 12 feet. Twelve feet is a very standard post size. What if you are simply doing a handicap sign in a parking lot? Well, a handicap sign is usually 12×18, depending upon the rules in your area. This would only require a ten foot post, another very common sized post. Let’s step through the math: Again, 24 inches for burial, add the seven feet and now add the 1.0 foot to fit the sign and you are at 10 feet.