When the U.S. President, the Pope, or any other high-target dignitary comes to New York City, the NYPD and other counter-terrorism officials place special locks on all of the area's USPS Mailboxes to prevent someone from placing a bomb inside. Area trash bins are removed, and even manhole covers are spot welded to prevent somebody from getting down there. Similar precautions are taken for the Macy's Thanksgiving parade, and several of our other large parades.
via reddit
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