We often watch this in movie but not in real life.
According to i-hacked.com, one electronic road sign has been easily hacked, with word "ZOMBIES AHEAD". In i-hacked.com blog post, the electronic road sign control panel normally locked with a small lock and often left unprotected. The programming is also very straight forward, even a newbie hacker could figure that out easily. Well if you think this is just a joke, Check this out, It's on video. I don't know who made this video. Could it be one if the hacker?