They were pretty determined to hack? What are the chances it was set to the default DOT password?
I'm guessing by "more difficult in the future", they mean that in 8 months they'll change the default password to something that's available on the internet two minutes later.
Brilliant. Both as a statement on how easy it is to fuck up many of the everyday elements that keep our society running and as a generally good prank.
That being said, you could seriously institute some minor chaos if you fucked with these signs right. Instead of ZOMBIES LAWL you change it to "ROAD CLOSED AHEAD. EXIT IMMEDIATELY, TAKE SURFACE STREETS" and you've just created a motherfucker of a traffic jam. Much less if you put something people were actually SCARED of on there. A chemical spill or what-have-you.