[Xorlev]

> (perhaps autonomous driving may allow a small %age increase but I am highly sceptical past that).

I'm imagining in the future that some roads will be reserved for automated use only except in emergencies. At that point, you greatly increase the trust in individual agents and planning can be communicated ("I plan to move 3 lanes to the right in 1.5 miles, FYI"). Average speed increases, "turbulence" reduces, which improves throughput and speed consistency though doesn't help with exit bottlenecks.

That said, you're right. The law of diminishing returns continues to apply even if it kicks the can down the road a bit (i.e. adding a 5th lane results in a 10% capacity gain without SDVs, 21% gain with SDVs -- numbers are completely made up).