A highway sign warning of zombies has prompted CDOT to make sure all the electronic signs it uses are locked to prevent hacking.The issue arose early Monday morning when a Colorado Department of Highways electronic road-warning sign in Boulder flashed a warning of "ZOMBIES AHEAD."The sign, at Foothills Parkway near Baseline Road, was supposed to say "Shoulder work Feb. 28 through June," but someone was able to open the unlocked control panel and change the warning message.Rick Barron and his wife saw the sign while he was driving her to the airport."My first thought was, 'Did that really say Zombies Ahead?' And my second thought was 'I've been in Boulder almost 30 years. If there were naked zombies riding bicycles with pumpkins on their heads, I guess we'd be used to it," said Barron.There are instructions for changing the electronic signs on the Internet which explain what the default passwords for the signs are and explains how to reset the password if it has been changed.The control boxes for the orange signs are usually locked, but apparently some of the control boxes aren't.A project engineer for CDOT, Gerry Padilla, told the Daily Camera that the crew overseeing the sign on Foothills Parkway had left the control box unlocked. He said the crew was reprimanded.Other electronic signs operated by CDOT were being checked Monday to make sure they were all locked to prevent tampering. The signs don't belong to CDOT but to a contractor."I'm sure it's just a question of contacting the contractor and asking them to make sure their signs are locked," said CDOT spokeswoman Stacey Stegman. "There are no zombies that I know of in Boulder, but stranger things have happened."Stegman told 7NEWS that electronic Colorado traffic signs have been hacked about three times in the last 10 years. She said the fine for tampering with Colorado highways signs is $66.The "Zombies Ahead" warning has been seen across the United States by pranksters who access the control panels of the highway signs.