Higher speeds do the opposite for congestion than what you'd think. If everyone and their mum wants to drive on the same stretch of highway at 5pm sharp, there's not enough space - at speed.

Obviously, everyone fits when standing still. The amount of asphalt is immense.

Reducing the speed limit from 120kph to 80 will allow 50% more cars to fit on the same stretch of road, thereby reducing stop and go, and not really impacting your average speed; you'd be stopping and going during rush hour, anyway. But now traffic flows, which is safer and easier to follow.

Why? Because - let's assume safe driving - every car has two seconds of safety distance to the car in front. Those two seconds remain two seconds, but that means the distances you need are twice as large with 120kph than with 60kph. Your car length doesn't even really matter, two seconds at 120kph are ~67m. So the road will always transport 1 car every two seconds per lane, no matter the speed.

You can either rage in a congestion, not knowing if farther up someone has caused a crash and completely blocked everything, or drive slower but steadily. The throughput of the road is the same.