MSNBC has a nice article about Dallas turning off quite a few traffic cameras because they were too effective. It turns out that motorists were obeying the red lights at the camera'd intersections in enough numbers that the cameras couldn't generate enough revenue to even pay for themselves. You'd think that that would be a good thing, as the stated justification for installing the cameras was to reduce the red light running.
You'd be wrong.
It turns out that the Dallas Police, like most traffic departments, treat motorists as rolling revenue opportunities. If they can't make money enforcing a particular traffic law they aren't going to bother enforcing it, even if it makes the roads safer.
(h/t to schneier)