What is the overhead inside rearview mirror above a school bus windshield for, and what can it not be relied on to show?

Explanation

The overhead mirror inside is for watching the passengers, and it is no substitute for the outside mirrors when it comes to traffic behind the bus. It hangs from the windshield header on the driver's side of the cab and looks back down the aisle. On a bus whose rear emergency door carries a glass panel low down, it may catch a sliver of the view straight back, but that is incidental rather than the point of it. Two unseen pockets rule it out as a rear traffic mirror: one immediately behind where the driver sits, and a much larger one opening at the back bumper that can run 400 feet or further. Set correctly it frames the top edge of the rear window along its own top edge and takes in every child aboard, down to the tops of the heads in the row directly behind the driver. The wrong options all reassign the mirror: two of them trade its job with the outside mirrors or strip it of the interior view that is its entire purpose, another hands it the work the crossover mirrors do at the bumper, and the last treats a glazed rear door as a reason to stop setting an outside mirror that still needs setting. 49 CFR 383.123(a)(2)(i) makes external mirror systems a required knowledge area; it says nothing about a mirror inside the bus and nothing about which mirror covers which ground, so the division of labor here comes from the manual.

Source: 49 CFR 383.123(a)(2)(i)