49 CFR §37.191

1. (a)For purposes of this section, a small mixed-service operator is a small operator that provides both fixed-route and demand-responsive service and does not use more than 25 percent of its buses for fixed-route service.
2. (b)An operator meeting the criteria of paragraph (a) of this section may conduct all its trips, including fixed-route trips, on an advance-reservation basis as provided for demand-responsive trips in § 37.189. Such an operator is not required to comply with the accessible bus acquisition/equivalent service obligations of § 37.183(b).