Think of your last road trip. There were sights, destinations, but as much as anything else, there were signs. For businesses, public safety messages, accident memorials, threats directed at outsiders, advertisements for worms, eggs, and firewood, reminders of sin (83-FOR-TRUTH), and, of course, political campaigns. In his book Political Sign, author Tobias Carroll compares the yard sign to a “kind of coelacanth, something that reached its ideal form long ago and has contentedly stayed there, not evolving because it didn’t need to evolve any further.” Yet within this unevolving form…