These symbols are from the FHWA proposed figures for the 2009 edition of the MUTCD manual. I used CorelDraw and PhotoImpact to generate the files needed for this slideshow.
"The Manual on Uniform Traffic Control Devices (MUTCD), by setting minimum standards and providing guidance, ensures uniformity of traffic control devices across the nation." Where it is necessary to indicate a prohibition of an activity or an item within a recreational or cultural interest area for non-road use, the appropriate symbol sign for prohibited activities and items shall be used.