Silver pillars on the bridges

Okay, I’ll bite. What are these things? I first saw them being installed at the Manhattan end of the Brooklyn Bridge. One of them rose smoothly and silently from a tube in the pavement, then stared with the beady little red eyes set around its upper rim.

More recently, I saw more of them at the Manhattan end of the Manhattan Bridge, including this one—already decorated.

Are they to prevent terrorists in motorized vehicles from accessing the bridges? Do the little red eyes count the number of people who pass them? Do they detect radiation? I am puzzled.