I'm not sure if they're for security (like car bombs, etc) or just to keep cars out for pedestrians, but they're basically two lowerable barricades manned by a security guard. There are a couple of them in the area blocking passage to the upper parts of Wall. I forgot to snap a picture today, but there are generally half a dozen or so NYPD with M16s near the top of Wall as well. It's pretty cool when a politician comes in (Bush and his helicopter entourage land a few blocks away from my office. We can watch him go up the FDR. He comes in with about 4 helicopters and the military has a little base set up under the FDR.)