Nagai Hideyuki uses two sketchbooks to create 3D illusions. I was thinking this may be possible to do on the street/buildings with chalk, but in order to view it correctly everyone would have to be the same height, so that probably wouldn't work.
You have got to hand it to him, these 3-D drawings really are amazing: Artist makes his drawings leap off the page
Now you get an idea how they plan those sidewalk chalk drawings! >>>> I just drew this and im really surprised with the results! I did better than I thought I would but its still not even close to as good as the real thing