What you’re looking at is a salt circle, a traditional form of protection—from within or without—in magical practice. In this case it’s being used to arrest an autonomous vehicle—a self-driving car, which relies on machine vision and processing to guide it. By quickly deploying the expected form of road markings—in this case, a No Entry glyph—we can confuse the car’s vision system into believing it’s surrounded by no entry points, and entrap it.