The US State Department's visa bulletin for October 2015 announced a new policy.  Starting on October 1, 2015, foreign nationals seeking permanent residency in the United States can file their adjustment of status earlier than previously allowed.  Although applicants can't get their "green cards" before immigrant visas are available,  this new policy provides benefits such as employment and travel authorization.

The USCIS has provided additional information regarding these revised procedures, with the USCIS website announcement.