The MAINTAIN TOP SAFE SPEED sign may be used on highways where radiological contamination is such as to limit the permissible 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 the sign in numerical terms. Where traffic is supervised by a traffic regulation post, official instructions will usually be given verbally, and the sign will serve as an occasional reminder of the urgent need for all reasonable speed.
The sign should be erected at random intervals as needed, in the same manner as other standard speed signs. In rural areas it shall be mounted on the right-hand side of the road with its lower edge not less than 5 feet above the crown of the roadway, 6 to 10 feet from the roadway edge. In urban areas the height shall be not less than 7 feet, and the nearest edge of the sign shall be not less than 1 foot back from the face of the curb. Where an existing Speed Limit sign is in a suitable location, the Top Safe Speed sign may conveniently be mounted directly over the face of the older sign, which it supersedes.