According to induced demand, the optimal highway would be one that is the smallest. Meaning 1 lane would be an optimal highway, but why is every country expanding their highways and by default they have more than 2-3 lanes.
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In addition to u/Beepboopbob1’s great answer about induced demand, let me just add that a single lane is never optimal because vehicles cannot pass each other, and so you will always be limited to the speed of whichever is the slowest vehicle ahead of you. E.g. where I live, trucks are limited to 80 km/h while regular cars can go 100 km/h (during the day, or 130 km/h at night). So with a single lane, everyone would be forced to go 80 because there’s always a truck somewhere ahead of you. Or, if a vehicle breaks down and blocks the lane, then that causes a complete stop of all traffic behind it. So you always want at least two lanes.
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