Big Easy,
Once you get out of the town of greece there isnt any extremely low bridge underpasses. I think there are 4 low bridges and I believe that you are correct and they are marked 11' and change. I have seen box trucks (which are 12') and tractor trailers (which are 13'6") on the parkway (illegally) but I would call the town of greece DPW and see if they would be willing to answer your question. 585-225-2000. If not you can easily reroute yourself from 390 to rt 18 latta rd to rt 260 manitou rd and back to the parkway.