[vidarh]

The bigger problem with induced demand is that it's often poor ROI to add that lane where the demand is highest.

That is, imagine you have a big city. You can add capacity for 1m extra people to travel to the city centre, where there's lots of congestion. Or you find ways to induce demand around the other limits of town, even town current demand is low there.

Odds are you'll pick the first, because it's "obvious" and doesn't require much thinking to see it'd help. But we really ought to look at cost-benefit of the second option too, because repeatedly inducing demand in the centre keeps driving up the incremental cost of further improvements, along plenty of other undesirable second order effects.

[otherme123]

Adding lanes is like getting a bigger cache with the same throughput.

It's obvious at the supermarket: what goes faster, a single cashier processing four short lanes of 10 people with round robin, or two cashiers processing a single lane with 40 people?

Is the city center able to process 1m extra people? If not, it doesn't matter how many lanes you build.

[vidarh]

Well you often can make it able to "process" 1m extra people: You can build overpasses, and tunnels, and taller buildings. But the cost-per-extra-person will tend to go up accordingly, to the point where you could spend an extraordinary amount attracting people out of the centre.

E.g. London's "Crossrail" / Elizabeth line cost $24 billion. Granted, it also allows some people to go through London faster, but I can't help to wonder what that money could've done if applied to attract businesses out of the centre instead. E.g. upgrading links between towns on the outskirts, upgrading town centres, and generally try to make it more attractive for businesses to be located further out.

Given the extraordinary costs it takes to do large infrastructure projects in London, I'd be very surprised if you couldn't get a higher return on investment that way, or by investing similar sums elsewhere in the UK entirely.