The original note, as written at the time
BELOW SURFACE HEAVY TRAFFIC. I think it would be beautiful to have a city where people can walk in peace without all the hooting, shuddering and blasting of rushing heavy traffic.
Ongoing development
A city where people can walk in peace, without the hooting, shuddering and blasting of rushing heavy traffic — because the heavy traffic is below the surface.
The engineering decision that matters is the gradient, and it is the one that kills most schemes. A tunnel that lorries can use needs a long, gentle approach, which means the portals have to be far apart and far from the places traffic wants to reach. Steep portals look cheap on paper and become unusable in the rain.
The second decision is ventilation, and it is the one that decides the cost. Heavy vehicles in a confined tube produce heat and particulates continuously, so the system needs either enormous airflow or a fleet of electric vehicles that produce neither. The electrification is the precondition, not the afterthought.
The third is that a tunnel is a route, not a destination. Every vehicle in it must still emerge somewhere, and wherever it emerges becomes a place where the street is destroyed again. So the portals must land in industrial ground, and there must be a plan for the last kilometre that is not a lorry on a residential road.
The prize is worth all of that. A street without heavy traffic recovers its frontage, its trees survive, its children can cross it, and the noise that makes a conversation impossible goes away.
Bury the through-movement, keep the access. That is the rule that keeps a city from building a motorway with a lid on it.