Not later than January 1, 1992, the Administrator shall promulgate regulations under section
7521
(a) of this title applicable to urban buses for the model year 1994 and thereafter. Such standards shall be based on the best technology that can reasonably be anticipated to be available at the time such measures are to be implemented, taking costs, safety, energy, lead time, and other relevant factors into account. Such regulations shall require that such urban buses comply with the provisions of subsection (b) of this section (and subsection (c) of this subsection, if applicable) in addition to compliance with the standards applicable under section
7521
(a) of this title for heavy-duty vehicles of the same type and model year.