A possible long-term solution is smart vehicles that are able to travel autonomously with very small gaps between each vehicle. Two lanes would almost certainly be enough with a third for joining & leaving the "train" and for the rare breakdowns. It may be the one place high speed autonomous vehicles make sense.
Talking about small gaps.......I was surprised to learn that the maxium traffic capacity is achieved when traffic is travelling at around 15 mph, ie. traffic travelling closer together.
Forget about Smart Motorways......impose a speed restriction of 15 mph, sorted, haha !!!