Yes, 20 mph might feel like one is crawling along --- but a driver might want to scroll down to section 1644 of article 39 of the New York State Vehicle and Traffic law before he or she decides section 1643 somehow invalidates such a posted speed. 1644 covers "Special speed limits on bridges and other elevated structures in cities and villages."
Good catch Morisot! I looked it up and it looks like the bridge speed limit is legal. However, the 20mph speed limit for several blocks north and south of the bridge still may not be.