18” + 6” = 24 inches. So, you have a total sign height of 24 inches or let’s say 2 feet. You can combine two 6 feet sign posts to give you 7 feet height between the bottom of supplemental sign and the ground.  An 8 feet post will give you about 5 feet of distance between the ground and the bottom of a parking sign because at least 3 feet goes under the ground.