These signs are ADDCO portable signs. They're used absolutely everywhere, and you can replicate the hilarious results in the twitter link above relatively easily.

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

The black control pad is attached by a curly cord, with a keyboard on the 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"

Should it will 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!

I can't personally vouch for these instructions, but considering how many signs have been changed by bored hackers, I'm guessing they're pretty fucking accurate.

The quote block above is from this jalopnik article . It's kinda shitty, but it shows a lot altered signs.