The sign on Auburn Street in Portland, Maine - which previously read "Night work 8pm-6am. Expect delays" - was altered on Wednesday (October 10), reports the Maine Sunday Telegram.
It is not the first time horror fans have used transport technology to warn of undead attacks.
In 2009, a road in Austin, Texas carried a similar message, and the text has also appeared on signs in Washington, Illinois and New York.
According to the Telegram, the pixel-lit signs are relatively simple to programme, though the controls are usually locked and password protected.
Portland's acting Police Chief Vern Malloch admitted he didn't know whether the local department was trying to track down the sign hijacker.
"On a serious note, when we pay too much attention to these kinds of things, it just encourages more behaviour like this," he said.
"While everyone would agree it was humorous, there are certainly instances when it could have put people at risk."
"They're not a toy," said Portland city spokeswoman Nicole Clegg of the signs. "I now have to talk to a New York radio station about why we shouldn't be alerting the public to zombies."