The sign Friday briefly flashed: "OMG The British R coming. They R watching you."
KCBD-TV reports the electronic sign was in a construction area in southwest Lubbock.
The Texas Department of Transportation says the mobile sign belongs to a contractor.
A statement from Austin Bridge & Road says someone "with a questionable sense of humor" accessed the password on the message board.
The company says its portable sign that was hacked has since been removed, since a permanent exit sign has now been installed, and locks have been added to the portable version.
No word yet on the culprits.
Local events | Contests | Headlines at a glance
