Oliver Keating wrote...

I once toyed with the idea of a single-lane motorway construction for high
speed cars (150mph), with a signalling system to keep safe distances.

This would be better with more technological developments, but such a
"road
track" would be exceptionally expensive compared to the amount of traffic
it
could handle.

That sounds like the Frejus Tunnel (France - Italy, the most westerly of the
trans-Alpine road tunnels).

The return toll is about £25 (for about eight miles of road).