The MUTCD requires that sign foundations, posts, and mountings be constructed to be able to hold signs in a proper and permanent position while offering reasonable resistance to wind and vandalism. While the authority doesn’t say much about sign bases specifically, it does provide some guidance about sign supports such as wood posts, U-channel steel posts, square or round tube steel posts, and I-beam steel posts. You can find information about the size, placement, and maintenance of these posts here.