From page 271 in the new rest rules document.

For the high case the FAA backed away from a benefit outcome based on mean
fleet, flight hours, and occupant numbers because ultimately we were persuaded there
was information which could not be ignored by the three regional passenger accidents
occurring without a mainline passenger accident. For this reason, we selected an 88 seat
regional jet (like an ERJ-175) to be the representative airplane for the high case. This
size airplane is also consistent with the fact that regional operators are expected to fly
somewhat larger airplanes in the future.