You should never hack a road sign as part of a prank. But what if you know that there really are Zombies ahead? What then??
Apparently, while most road sign control pads are placed in a lock box, that box is rarely actually locked. And while most road signs are under password protection, that password is most generally just the default code "DOTS"—or you can easily reset the password by holding "shift" and "control" while typing "DIPY" (so that it just defaults to "DOTS" again).
Of course, it makes sense that road signs aren't all that protected. Most of us would hope that you wouldn't be such a jackass as to take swap useful information for some joke about ninjas and/or pirates. [iHacked via Geekologie]