“Urban area” means either of the following:
(a) The central portion of a city or a group of contiguous cities with a population of 50,000 or more, together with adjacent densely populated areas having a population density of at least 1,000 persons per square mile.
(b) An urbanized area as defined in paragraph (2) of subdivision (b) of former Section 21080.7 of the Public Resources Code, as enacted by Section 6 of Chapter 1130 of the Statutes of 1993.