I whipped this up Friday afternoon after a costume party invite. It normally says "LANE ENDS MERGE RIGHT" and can be hacked to say "CAUTION! ZOMBIES! AHEAD!!!"
I used an RBBB arduino board from Modern Devices to control an 8x32 LED display from Sure Electronics. When you open the access panel, the blinking green light indicates normal operation. To hack the sign you must figure out which of six bolts is the key and clamp an alligator clip to it. When the correct bolt is found, the light will flash blue quickly and then continue flashing red while the hacked message is displayed. The key bolt is randomly picked each time the hack is disengaged.
and my wife was a zombie!! yeah!