FWIW, I go up and down 69 from 71st to 116th M-F for work, I drive the same speed as everyone else which is just as fast as before those warnings went up, and I haven't received any tickets. While I'm sure it would be simple enough for them to automate such a system and rack up fines, they don't seem to be doing so. I'm not sure they actually even have the necessary equipment in place.
Obligatory IANAL, this is not legal advice.