The Manual on Uniform Traffic Control Devices (MUTCD) is widely considered to be a dry and unemotional document. Published by the Department of Transportation, it outlines the basic specifications of all the street signs you could expect to see out on roads and highways across the United States. Most are familiar, but if you dive deeper into its pages, you can find some unsettling relics from darker times.

Back in the mid-20th century, America was tangling with the realities of nuclear war. Top generals contemplated targeting strategies, while engineers mused over whether there was anything to be done top stop a torrent of enemy missiles falling across the nation. These superweapons seemed to promise destruction on an overbearing scale, threatening the very existence of human civilization itself.

Against this bleak backdrop, government administrators turned to the concept of Civil Defense. The idea was to do whatever could be done to protect the citizens of the nation from the horrors of nuclear war and its immediate aftermath. In turn, the Department of Transport worked up some rather depressing road signs to help people get where they needed to be in these bleak and trying times.

Flip open the 1961 edition of the MUTCD, and you’ll find an important section on Civil Defense. It featured a handful of designs for traffic management in a post-nuclear world. Perhaps most interesting was the “MAINTAIN TOP SAFE SPEED” sign, designated CD-4. It’s purpose was highly unique:

The “MAINTAIN TOP SAFE SPEED” sign may be used on highways where radiological contamination is such as to limit the permissable exposure time for occupants of vehicles passing through the area. Since any speed zoning would be impractical under such emergency conditions, no minimum speed limit can be prescribed by th