We used to have a few telltales around DFW, but I haven't seen any in a while (maybe 15 years). Most are all either just a bunch of signs, or the warning lights.
I'm not sure how the lights are set up, but I know for a fact they are lower than the bridge. I have gone through a couple with a load that I knew would fit, and the lights still went off. Of course I put on the emergencies and crawled under the bridge just incase.
Casey