Recently my friend took up a job for the local highway construction that's been happening around my city. They gave him this information, that should work for nearly the whole U.S.
"DOTS" is 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!
Now you can do whatever you want.
The only problem is if they have a lock on it.