Read the DMN article more closely and ask yourself why the violations went down. From the article:

"City records indicate Dallas has lengthened yellow-light intervals on 12 of its 62 monitored traffic signal."

and

"Dallas City Hall has idled more than one-fourth of the 62 cameras that monitor busy intersections"

Your presumption, and that of the DMN, which never prints a negative word about cameras, is that the ~15 cameras were idled because they were working too well. The alternative theory is that tickets went down when the yellow went up. *Side note: violations also go down when a camera goes out of service, there's construction at or near a camera intersection or the city/vendor just turn the thing off to goose the "success" numbers -- it's a worthless measurement.)

Did the yellow go up because Dallas City Hall was concerned about safety? Nope. Because they got busted using yellows shorter than allowed under a 2007 Texas law:
http://www.thenewspaper.com/news/20/2068.asp

A story, by the way, on which the DMN never reported.