§ 393.91 Buses, aisle seats prohibited.

PART 393—PARTS AND ACCESSORIES NECESSARY FOR SAFE OPERATION, last amended 2016-12-12

No bus shall be equipped with aisle seats unless such seats are so designed and installed as to automatically fold and leave a clear aisle when they are unoccupied. No bus shall be operated if any seat therein is not securely fastened to the vehicle.

Official text from the eCFR. The eCFR is an authoritative but unofficial editorial compilation; the Federal Register is the legal source.