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The minimum amount of time in the oncoming lane is 0

Sure. But now you have to spend more money on adding lanes for the same traffic capacity because the average speed drops.

Road capacity usually increases as speed goes down.

Cars per mile, yes. Cars per second no. If you follow the recommended 3 second spacing then it's a car per 3 seconds per lane.

So for a fixed distance, the slower you go, the longer you are on the road, the more road capacity you consume.

Traffic flow is more stable at lower speeds, so you get less congestion.