Rick Barron, his wife and his sister-in-law weren’t expecting much excitement on their drive to the airport before dawn Monday, so you could imagine their surprise when they spotted an electronic road-construction sign on Boulder’s Foothills Parkway that gave an unnerving warning.

“Zombies ahead.”

“We all three just cracked up — it was very funny,” said Barron, 66, of Boulder. “We decided that we ought to be especially alert since it was 4 a.m.”

Boulder became the latest in a long list of cities hit by zombie-loving hackers early Monday when someone broke into the messaging box on a sign stationed along the shoulder of Foothills Parkway near Baseline Road and changed the message to warn of the undead. An underground group of zombie-loving pranksters has been reprogramming electronic road signs across the country for the past few years.

Some come with exclamation marks and some include additional instructions such as, “Run!” The movement even has made its way to T-shirts and mugs.

Barron and his passengers kept an eye out for the walking dead along the way to Denver International Airport, but Barron said, “I couldn’t verify that I saw any.” When he returned to Boulder about 6 a.m., Barron said, the zombie warning still glowed in orange text.

“Someone decided to create a little prank,” he said. “And it was pretty good.”

When asked about the signs, Boulder spokeswoman Jody Jacobson said, “The city has received no reports of zombies.”

This is the first time the Foothills Parkway sign has been hacked, said Gerry Padilla, project engineer with the Colorado Department of Transportation. He said a project inspector noticed it when he arrived for work early Monday, and immediately turned it away from traffic while reprogramming it to say, “Shoulder work Feb. 28 through June.”

Crews working at the scene were reprimanded for not locking the message-entry box on the sign to keep pranksters away, Padilla said.

“They know better,” he said. “We don’t want people messing with it.”

Padilla said that motorists won’t see any more zombie warnings at projects he’s overseeing.

“We are putting locks on them as we speak,” he said.

Stephen Jones, who teaches a zombie class at the University of Colorado and has written a book and several stories on zombies, said zombie-sign hackers have been making their mark for about eight years. He said the sign changing seems to be done in good fun.

“It’s a victimless crime — it’s just changing something that can be undone really quickly,” Jones said. “And I don’t think anyone’s going to drive their car off the road when they see the sign.”

In fact, he said, a “zombies ahead” sign most likely will brighten the day of those who pass it.

“There’s a thrill when you see that because it makes the world a more possible place,” he said. “Like maybe there are zombies.”

Monday morning’s zombie alert might be related to the upcoming Humans vs. Zombies game at CU, said game organizer Scott Serafin. The weeklong, semi-annual event is scheduled to start March 28, and Monday was the first day for registration at the University Memorial Center on the Boulder campus.

Serafin said he didn’t think people actually hacked into the signs.

“I’ve seen a picture of it, but I thought it was fake,” he said.

Now that he knows the pranks are for real, Serafin said he’s wondering whether the prankster knew about CU’s upcoming zombie battle.

“But it wasn’t me,” he said.