This information was copied from jalopnik.com
DO NOT under any circumstances run around hacking into electronic road signs using the information contained in this step-by-step guide of how to transmit hilarious messages to passing motorists.
How many times have you driven by an electronic road sign like one of these? 
This is the ADDCO portable sign. Today, you see what is on the inside, and how they are programmed to display important information.
*** WARNING YOU SHOULD NEVER TAMPER WITH THESE SIGNS ***
- The access panel on the sign is generally protected by a small lock, but often are left unprotected. Upon opening the access panel you can see the display electronics.
- The black control pad is attached by a curly cord, with a keyboard on the face.
- Programming is as simple as scrolling down the menu selection to “Instant Text”. Type whatever you want to display, Hit Enter to submit. You can now either throw it up on the sign by selecting “Run w/out save” or you can add more pages to it by selecting “Add page”
** HACKER TIPS **
Should it ask you for a password try “DOTS”, the default password. In all likelihood, the crew will not have changed it. However if they did, never fear. Hold “Control” and “Shift” and while holding, enter “DIPY”. This will reset the sign and reset the password to “DOTS” in the process. You’re in!