Some say that all of those signs have the same default password. They rarely get changed, because so many people have to use them. And it is easy to reset the module to the default.
others say Utah DOT has control interfaces that a central dispatch uses to update these overhead signs. They run all up and down the I15/215 corridor. They're used to alert of accidents and as of lately, little catches phrases. All done remotely on a computer, they cannot be set by the side of the road. Updates have to be called in.
https://www.facebook.com/toshpoint0/photos/a.406757646907.201668.100834231907/10155695032006908/