I don't know whether private signs have the force of law (I think not), but statutory Gun Free Zones must be identified by signs:
§ 22-4502.01. Gun Free Zones; Enhanced Penalty.
(a) All areas within,1000 feet of an appropriately identified public or private day care center, elementary school, vocational school, secondary school, college, junior college, or university, or any public swimming pool, playground, video arcade, youth center, or public library, or in and around public housing as defined in section 3(1) of the United States Housing Act of 1937, approved August 22, 1974 (88 Stat. 654; 42 U.S.C. § 1437a(b)), the development or administration of which is assisted by the United States Department of Housing and Urban Development, or in or around housing that is owned, operated, or financially assisted by the District of Columbia Housing Authority, or an event sponsored by any of the above entities shall be declared a gun free zone. For the purposes of this subsection, the term "appropriately identified" means that there is a sign that identifies the building or area as a gun free zone.
(b) Any person illegally carrying a gun within a gun free zone shall be punished by a fine up to twice that otherwise authorized to be imposed, by a term of imprisonment up to twice that otherwise authorized to be imposed, or both.
(c) The provisions of this section shall not apply to a person legally licensed to carry a firearm in the District of Columbia who lives or works within 1000 feet of a gun free zone or to members of the Army, Navy, Air Force, or Marine Corps of the United States; the National Guard or Organized Reserves when on duty; the Post Office Department or its employees when on duty; marshals, sheriffs, prison, or jail wardens, or their deputies; policemen or other duly-appointed law enforcement officers; officers or employees of the United States duly authorized to carry such weapons; banking institutions; public carriers who are engaged in the business of transporting mail, money, securities, or other valuables; and licensed wholesale or retail dealers.
§ 22-4502.01. Gun Free Zones; Enhanced Penalty.
(a) All areas within,1000 feet of an appropriately identified public or private day care center, elementary school, vocational school, secondary school, college, junior college, or university, or any public swimming pool, playground, video arcade, youth center, or public library, or in and around public housing as defined in section 3(1) of the United States Housing Act of 1937, approved August 22, 1974 (88 Stat. 654; 42 U.S.C. § 1437a(b)), the development or administration of which is assisted by the United States Department of Housing and Urban Development, or in or around housing that is owned, operated, or financially assisted by the District of Columbia Housing Authority, or an event sponsored by any of the above entities shall be declared a gun free zone. For the purposes of this subsection, the term "appropriately identified" means that there is a sign that identifies the building or area as a gun free zone.
(b) Any person illegally carrying a gun within a gun free zone shall be punished by a fine up to twice that otherwise authorized to be imposed, by a term of imprisonment up to twice that otherwise authorized to be imposed, or both.
(c) The provisions of this section shall not apply to a person legally licensed to carry a firearm in the District of Columbia who lives or works within 1000 feet of a gun free zone or to members of the Army, Navy, Air Force, or Marine Corps of the United States; the National Guard or Organized Reserves when on duty; the Post Office Department or its employees when on duty; marshals, sheriffs, prison, or jail wardens, or their deputies; policemen or other duly-appointed law enforcement officers; officers or employees of the United States duly authorized to carry such weapons; banking institutions; public carriers who are engaged in the business of transporting mail, money, securities, or other valuables; and licensed wholesale or retail dealers.
Emily Miller 
