So you can just put a couple of cones across a highway and stop all traffic for a few hours? Cool! I'll bet you can even camouflage the transmitter so that no one can figure out why all traffic has stopped until they come out with a really sensitive detector and realize that there's something stuck 30 feet up in the tree branches instructing all vehicles to go into emergency stop mode.
You can stop opperations at London Heathrow within 15min with one or two phone calls. Yes, it is that easy. People regularly don't do it because (1) what is good for and (2) what ever it is good for, is it worth paying the damage caused for the rest of your life?
--- In Germany, a live TV-show with 10.000 spectators in the studio (hosted by Heidi Klum) was canceled on Tuesday because of a fake bomb-threat-call. Stuff like that remains the exception.
If the cars are stopping because of the device, I think you can say that they have successfully detected it, and expect that they would provide some explanation as to why they have stopped.
That doesn't solve the problem of localizing the device, but I think you would want the implementation of such a thing to have some robust distance bounding[1], so it might not be a careful, sensitive process, more of walk up to where your phone says it is thing.
