I am against hacking in principle, and I suppose there was a chance this could have led to traffic problems and, worst-case, perhaps injury. But funny is
funny:
Over the weekend, Austin, Texas drivers received some important
warnings from their road signs about the impending zombie invasion. And
the hackers who did it may know something we don't about the undead.
The road signs, which normally warn drivers about traffic conditions, displayed these warnings: "Zombies
ahead . . . the end is near . . . run for cold climates!" Some signs
also warned of Nazi zombies.
While city officials claimed to FOX News
that the tampering could lead to jail time, nobody is going to get in
trouble for warning the world about zombies. The company that owns the signs, Sterling Construction, would have to
file a complaint with police for any legal action to be taken. Sterling
owner Wayne Haggard told local KVUE-TV, "It's Austin. We have a sense
of humor. Let it go."

According to the io9.com article, reprogramming road signs doesn't exactly take mad hacking skillz:
Most of these signs, including the ones owned by Sterling, have a
default password. Anyone can walk up to the sign, type the default into
the control panel, and reprogram it.
Even, presumably, the undead.