> How does it increase safety? Someone else will just catch up to him on that hill.

Instead of having an ever growing queue of cars that try to pass on the hill, they pass before hand on the flats. So there's not mass chaos when 100 cars who have been trying at 10 mph under the speed limit for an hour all try to pass on the uphill, while the truck is slowing from 60-40.

> If safety is your concern, then it's even easier and safer to predict conditions without passing until a passing lane opens up, or a turn out is reached for him.

Sure, some do that, it's the drivers choice. Personally if passing is allowed, I like to pass slower vehicles. My car accelerates quickly, so I like to pass quickly and spend the minimum amount of time in the oncoming traffic lane.

Just seems natural to let the cars get ahead whenever they can, after all the cars usually have a different speed limit than the trucks. Thus the carefully engineered lane markers, truck specific speed signs, and often passing lanes on the hills.

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.