Those signs are stupidly easy to hack. Often the only security measure is a lock on the box housing the interface – and most of the time it's not even locked.
I've seen a few where they require a log in, but the user name and password are usually written on the inside cover of the operator's manual inside box, right next to the keyboard.
Just be sure to do it late at night and change it back once you've had your fun, those signs are important.
That's outrageous! When there will be a real zombie outbreak we won't believe those signs! Those who made this surely haven't read the fable named The Boy Who Cried Wolf.