If a public entity operates a demand responsive system, it shall be considered discrimination, for purposes of section 12132 of this title and section 794 of title 29, for such entity to purchase or lease a new vehicle for use on such system, for which a solicitation is made after the 30th day following July 26, 1990, that is not readily accessible to and usable by individuals with disabilities, including individuals who use wheelchairs, unless such system, when viewed in its entirety, provides a level of service to such individuals equivalent to the level of service such system provides to individuals without disabilities.