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Comment by HPsquared | Hacker News Reader

Any time there's a debate about building or enlarging a road, the idea of induced demand is brought up. More capacity leads to more demand (i.e. people bring able to travel more and/or further then they otherwise would have), causing traffic to increase over time until the new road is also congested.

A similar pattern would emerge for any infrastructure investment, really. Some regulating force is needed, or it will emerge naturally via congestion and queuing.

Edit: though once the tracks etc are built, public transport scales much better than roads so maybe it never becomes an issue.

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That is correct, but induced demand here is capped by each person only able to travel so much. Even if the person lives on the train he could only occupy one seat at one time. And I would imagine a very small subset of population would actually want to do that.

Right, but in his example, you are also capped by the ability to also only drive one car at a time. And yet highways are overflowing with cars during peak commute hours.

Because they don’t have capacity for all of the population and also because public transportation is so terrible people prefer (or have) to endure congested traffic over it.

Induced demand itself isn't a problem. The problem is that it counteracts congestion-relief efforts, which is often the (stated) motivation for things like road-widening.

Whilst road-widening can improve other things, like throughput (a congested three-lane road can shift more people than a congested two-lane road), once we start comparing such measures we find that public transit, etc. does even better.

The same argument doesn't apply to induced demand for transit (we can't do even better by switching to public transit, since that's what we're already doing!). At that point, maybe there aren't any easier options; and maybe we have to build more (bus) lanes or rails, or run more vehicles, etc.

> Even if the person lives on the train he could only occupy one seat at one time.

Living in NYC I've seen hobos lying down across 6 seats on the subway smelling so bad that 10+ seats or an entire half of a car is vacated because of them.

Has New York by any chance… considered offering showers to people who need them, without tying nonsense requirements to said service?

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