47 U.S.C. 541(a)(1). https://www.law.cornell.edu/uscode/text/47/541. Part of the 1992 Cable Act: http://www.lib.niu.edu/1994/im940207.html.

"A franchising authority may award, in accordance with the provisions of this subchapter, 1 or more franchises within its jurisdiction; except that a franchising authority may not grant an exclusive franchise and may not unreasonably refuse to award an additional competitive franchise. Any applicant whose application for a second franchise has been denied by a final decision of the franchising authority may appeal such final decision pursuant to the provisions of section 555 of this title for failure to comply with this subsection."

Theoretically, a pre-1992 contract negotiated before the law was passed might still be in effect. In practice, these contracts had 5-10 year terms and expired long ago.