I don't smoke anymore and it was over 20 years ago now, but I had a spliff before taking my driving test to relax me a bit and passed, so they obviously did'nt detect any eratic driving from me.
Also many years ago when I was foolish and reckless I drove back from a party once on LSD at 4am, down deserted country lanes in Wales in a snow blizzard, despite having seemingly heightened senses and awareness that was much harder! It was a 10 mile journey that seemed like the Millenium Falcon in hyperspace. It was'nt big or clever and I'm not proud of it, but just goes to show what goes on out on the roads. I doubt the Police would even consider anyone stupid enough to do this, and if there is a test for it, other than looking out for the eyes being out on stalks and the driver being transfixed by the lovely flashing blue lights.