The terrorist of the 21st century will not necessarily need bombs, uranium, or biological weapons. He will need only electrical tape and a good pair of walking shoes. Placing a few small pieces of tape inconspicuously on a stop sign at an intersection, he can magically transform the stop sign into a green light in the eyes of a self-driving car. Done at one sleepy intersection, this would cause an accident. Done at the largest intersections in leading metropolitan areas, it would bring the transportation system to its knees. It’s hard to argue with that type of return on a $1.50 investment in tape.