At least over here, traffic lights fail and turn off on their own, no need for hackers :) . Now, fixing them (for example setting up a "green wave"... hmm that could be a more interesting use :)
And then the red light cameras start magically getting bullet holes in them.
http://www.knoxnews.com/news/2007/dec/12/police-man-shot-cam...
http://www.nj.com/news/index.ssf/2012/08/shoot_out_the_red_l...