Stop it.

"The Manual on Uniform Traffic Control Devices (MUTCD) stop sign was altered eight times between 1935 and 1971, generally regarding mounting height or reflectorization. However, the most significant change was the 1954 alteration to white on red color. The modern US stop sign, white on red, mounted 2.1 meters (7 ft) above ground, 30" long with a 3/4" white line around the edge, was passed into law in 1971 – although 24" stop signs are also allowed."
- History of stop signs blog.