And there are few vehicles that have that configuration any more, now being only
two high/low dual filament fixtures per vehicle, or four headlights per vehicle,
with discrete low beam outboards and inboard high beams. The official
justification for the low beams going off when the highs were engaged was that
high levels of foreground light from the low beams being left on with the highs
diminished the effectiveness of the high beams, but I still think it's the heat
from 4 bulb filaments burning at the same time in those plastic housings with
plastic reflectors and plastic lenses.
You are probably correct that the plastic housings and reflectors wouldn't
stand up to the heat produced by both filaments, but the old glass sealed
beams wouldn't have had that problem, yet they only burned one filament at a
time. I suspect filament life would be drastically reduced if both were on
at the same time.
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