Lewis & Short

bĭrŏtus, a, um, adj. [bis-rota], twowheeled, with two wheels (post-class.): vehiculum, Non. p. 86, 30.
More freq. subst.: bĭrŏta, ae (gen. plur. birotūm, Cod. Th. 6, 29, 2, § 2; 8, 5, 9), f. (sc. raeda), a cabriolet, Cod. Th. 8, 5, 8, § 1 sqq.