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Comment by dumbmatter | Hacker News Reader

Adding 10 percent more road capacity leads to 3-6 percent more vehicle miles in the near term and 6-10 percent more over many years.

So... sounds like less traffic. Wonder why the title says "more traffic".

Some of the cars on a new highway lane have simply relocated from a slower alternative route. But many are entirely new. They reflect leisure trips that often go unmade in bad traffic, or drivers who once used transit or carpooled, or shifting development patterns, and so on.

Those all sound like good things. The point of roads is to use them. So people use them to have a shorter commute, or live a bit further away from work to lower rent, etc etc etc.

What is the actual problem? Did anyone actually expect that building more roads would lead to no increase in road usage?

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The problem is you get into an endless cycle of adding more and more road capacity and still ending up with the same amount of congestion. If you invest that money in public transport or cycle infrastructure you end up lowering congestion and speeding up everyone's movement including those that still want to drive.

I still don't see why "Adding 10 percent more road capacity leads to 3-6 percent more vehicle miles in the near term and 6-10 percent more over many years" is being interpreted as "ending up with the same amount of congestion".

If public transit or bike lanes would provide a better benefit for the same cost, then I would support them. But that's a separate and complicated discussion. Instead, this article seems designed to mislead people into thinking that expanding roads has literally no benefit, when obviously it does even if congestion remained the same (which it doesn't).

Maybe I'm too cynical...

Well first congestion isn't linear to vehicle miles. In traffic movement small things have a massive knock on effect. On my daily route to work I see the effect that adding just a few more cars to a road does. Adding 5% more cars doesn't increase congestion by 5% it actually brings traffic to a standstill where it was moving at an effective pace before.

Adding extra capacity can even make the problem worse because of what happens when increased capacity meets lower capacity. Human driver are terrible at going from two lanes down to two. You could say that this is bad design but in cities especially there are constraints that you can't really design around very easily without destroying massive parts of the city. This was done in the past to allow highways to penetrate right into cities but those highways just slowed to a crawl.

As for cost effectiveness bike lanes are a lot cheaper than extra road lanes and very much cheaper than elevated highways. Bus rapid transit is also extremely cheap. That's before you start looking at other cost benefits such as increased health for more exercise and less pollution.

>Adding 5% more cars doesn't increase congestion by 5% it actually brings traffic to a standstill where it was moving at an effective pace before.

That seems to argue in favor of the expansion. The 10% road increase may only lead to a 4% decrease in car density, but that could be enough to bring traffic out of standstill.

In terms of resources used to gain ratio that's terrible. Methods that reduce traffic density are much better than ones that increase it. And that's before you think about what adding extra capacity does to a city. I've lived in quite a few cities in my time and the wider the roads in the city the worse the traffic, pollution and enjoyability of the city.

Yes, when you build useful things, people generally use them. Consider: "The parks are too crowded. But if we build parks, this mostly just lowers the density of park-goers in the short term. In the long term, people move to live near the parks and use the parks more, so that the density of park-goers rises again, and we loose most of the benefit!"

There's lots of constructive arguments to be had about the relative benefits of investment in roads vs. rail vs. whatever, and tons of good criticism to level against subsidies and un-taxed externalities (for all those options). But arguments complaining about the "endless cycle" of building useful things that subsequently get used are silly. This lowers the quality of the conversation about public transit, making it more difficult to convince others of the relative merits.

The use of roads is to get people and goods from A to B. If there's a way of doing that that doesn't involve endlessly building roads then do that. Roads are inherently inefficient for moving people around because use about 2 tons of car that takes up a great deal of space to move one person. They're slightly more efficient for goods but even then there are much more efficient ways to move goods most places. Endless cycle of building an inefficient solution to fix a problem caused by inefficient design is a perfectly valid argument. And yes the inefficiency was implied by the contrast to public transport investment.

"Did anyone actually expect that building more roads would lead to no increase in road usage?"

I think that's exactly it.

Perhaps all road building initiatives should be publicised as short term congestion relief and long term preference of cars over other forms of transport.

Because there are alternatives that also increase people's mobility, as 'hammock' wrote below: "the alternative to falling into the induced demand trap by building more roads, is to improve public transportation infrastructure. Such improvement has been shown, perhaps counterintuitively, to have the biggest effect on road traffic."

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