NHTSA mandated school-bus roof-crush resistance beginning in 1977.
The requirement came through Federal Motor Vehicle Safety Standard 220, formally titled School Bus Rollover Protection. It became effective on April 1, 1977, requiring covered school buses to withstand a roof-loading test without excessive roof deformation and to preserve emergency-exit operation during and after the test.
The standard followed the Motor Vehicle and School Bus Safety Amendments of 1974. NHTSA developed a group of school-bus standards in response: FMVSS 220 addressed rollover protection, FMVSS 221 addressed body-joint strength, and FMVSS 222 addressed passenger seating and crash protection.
The wording can cause a minor mix-up with FMVSS 216, the general roof-crush-resistance standard for certain vehicles. For school buses, the dedicated rollover and roof-strength requirement is FMVSS 220, effective in 1977. Thus, 1977 is the relevant year rather than the year the broader federal safety law was enacted.