We knew hacking road signs was easy (and illegal), but they changed the control boxes in the latest models and nobody knew to handle those—until now. Here's the manual, so you can warn people of zombies and raptors ahead.
It contains complete instructions on how to change the messages, as well as the default passwords, which of course they never change. Or maybe—hopefully—they would change them after this article.
The previous instructions could only applied to very old signs. This 2004 manual covers all the new signs. [Manual (PDF in ZIP file) —Thanks James!]