It'd be hard to quantify, but I'm sure some statistics person could compare transportation methods, that includes speed, distance, energy usage, population, capacity, and probably a few more, per capita.
You could isolate it to a country's top X biggest cities, and how traveling between them compares in all those metrics.
Spain and France especially seem to be doing a good job building high speed rail:
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It'd be hard to quantify, but I'm sure some statistics person could compare transportation methods, that includes speed, distance, energy usage, population, capacity, and probably a few more, per capita.
You could isolate it to a country's top X biggest cities, and how traveling between them compares in all those metrics.
It's just a pity that renfe's new high speed trains built by Talgo seem to be a shaky and noisy experience as highlighted here:
https://m.youtube.com/watch?v=dhEopBWNbTc