The "externality" most discussed is generally carbon emissions, for obvious reasons. It used to be particulates, for local health reasons. Noise rarely gets a mention.<p>But, if someone from an all electric future was dropped off in 2016 i think they'd really notice noise above everything else. Traffic noise in cities is a massive "silent" irritant. Even outside of cities, it's amazing how far you need to be from a road to hear unadulterated wilderness. Off-road, dirt bikes, quads, water craft and such can really impact enjoyment of outdoor recreational places.<p>I don't think the psychological impacts of this are fully understood, but it is understood that they aren't trivial. The whole world will get so much more peaceful with combustion engines out of the picture.<p>Air travel is the one area where reducing noise would carry an obvious economic benefit to airlines. Unfortunately it's the one of hardest ones to solve.