There's two versions of this: a hardware hack and a software hack.
The hardware hack (otherwise known as DIrtY MIRT, aka "Do It Yourself Mobile Infra Red Transmitter") is definitely doable and possible, as they are both being built, sold, and even having laws passed prohibiting their sale/usage.
As far as software hacks, this was also done:
officials now allege that two engineers, Kartik Patel and Gabriel Murillo, figured out how to hack in anyway. With a few clicks on a laptop computer, the pair -- one a renowned traffic engineer profiled in the national media, the other a computer whiz who helped build the system -- allegedly tied up traffic at four intersections for several days.
Transportation bosses in the Netherlands are trying to find out who hacked the traffic-light computer system to show a couple making love when the lights turned green.
The altered lights in Nijmegen, Holland, stopped traffic when passersby kept pressing the pedestrian buttons to see the couple having doggie-style sex on the green light, the Austrian Times reports.
* Dead link. Dutch regional news item on YouTube: https://www.youtube.com/watch?v=qewWW-mSua0