The signs in California aren't useless because the signs are useless; they're useless because of the specific implementation. If you do have an area with known

The signs in California aren't useless because the signs are useless; they're useless because of the specific implementation. If you do have an area with known-dangerous substances, and don't put up the sign, you get hit with a penalty significant enough to hurt. If you don't have anything dangerous, but do put up a sign, nothing bad happens to you. So rather than actually check for what's there and put up the sign only when relevant, everyone errs on the side of "better put up the sign just in case".