I don't know the exact answer to your question, but we were stuck in Port Jarvis, NY once because all the bridges were signed as 12'6....called police for routing advice and were told that NY law requires bridges to be marked for 1 foot less than they actually are...which seems stupid at best.
Maybe your obstruction is just signage and the bridge is higher. Having said that, it looks like a weight restriction to me. I would check with local authorities. NY is the wild west of bridges....
http://www.landlinemag.com/Story.aspx?StoryId=24233#.V9P_vFQrJdg
https://en.wikipedia.org/wiki/U.S._Route_20A_(New_York)