When I do a decent shot of tar I'll get blurry vision, but only when I'm driving home. I don't do enough to nod off or anything, but driving in traffic sucks with the blurry vision. I would describe it more as a double vision though and my solution for it while driving is to close one eye to judge my position in the lane, then, when I'm nearing another vehicle, I'll open both eyes briefly to regain depth perception, then back to one eye once I've comfortably judged the distance. Since my tolerance has gone up, I can rarely afford to do enough to get this affect anymore, but it used to scare the crap out of me driving home in the dark.