The fake signs in this example were projected/displayed in/on trees/billboards and would be ignored fairly easy because it would be the image embedded in a a shape that didn't match the shape of a sign. These types of signs shouldn't fool people, you could fool both human [0] and CV with a fake sign on a pole but it shouldn't fool either just projected onto a tree. With LIDAR you can tell if the image of the speed limit is on a speed limit sign or not.

[0] Though people would have an additional layer of reasoning about if the speed limit was reasonable for a particular set of road. Just putting up a 90 MPH speed limit on a random residential road even with a convincing sign should get people to do 90 on it. (Granted computers already have databases of posted speed limits they could draw from too)