[kazinator]

In British Columbia, Canada, the BC Motor Vehicle Act mentions interior lamps in connection with larger passenger vehicles. In addition to requiring that they have them, it has this requirement that they be on while driving at night:

  Interior lamp
    10.20 (1) A commercial passenger vehicle shall be equipped with a
              lamp or lamps within the vehicle so arranged to illuminate
              the aisle of the vehicle to the rear of the driver.

           (2) A person driving a commercial pass enger vehicle having a
               seating capacity including the driver of more than 12 occupants
               shall insure that the light or lights referred to in subsection
               (1) are illuminated at all times that passengers are being
               carried between sunset and sunrise.

[lhorie]

AFAIK, that's meant for buses, and it's the same rationale as airplane floor lights (i.e. illumination is necessary to avoid falls from tripping in the dark, e.g. if the passenger is going to the toilet, which some buses have).

[kazinator]

I can't find any other mention of interior lamps in the same traffic code document.