> My understanding is that it is pretty much guaranteed, especially in larger cities.

Induced demand can only happen when there is a non-trivial amount of people who have and use some other option. (NYC, for example, where there is a large subway network. A new freeway might convince some subway-users to switch, inducing demand)

For a city with no alternatives (like no meaningful public transit), it's impossible to induce traffic demand, because demand is already at 100%. Traffic still grows as population grows, but a person can only physically drive one car at a time, it's currently impossible to induce any higher demand than that.

> Do you happen to have any examples of where induced demand has not happened

https://en.wikipedia.org/wiki/M-6_(Michigan_highway)

"A year after the freeway opened, traffic volumes along parallel roads like 44th, 56th and 68th streets dropped 40–50%"

That's pretty typical for proper freeway construction/expansion. It's just that we stopped doing that 20-30 years ago, but population growth hasn't stopped in the meantime, so we're way behind on our urban transit infrastructure (all of it, freeways, rails, and public transit).

> For a city with no alternatives (like no meaningful public transit), it's impossible to induce traffic demand, because demand is already at 100%. Traffic still grows as population grows, but a person can only physically drive one car at a time, it's currently impossible to induce any higher demand than that.

This is not true. An example is Houston building a 26 lane highway only to lead to ~40 increased commute times [1]. Initially commute times decreased, but then the population felt they could move farther out into the suburbs, get a more affordable house, and still have the same commute time. Once enough people did this traffic times got worse.

So, demand increased.

[1] http://www.citylab.com/commute/2016/01/highway-boondoggle-us...

It'd seem in that case it's not that demand increased but that people moved further way from their final destination so their commute time was increased.

It may very well be that the same people are still driving or wanting to drive, but simply by living further away the commute times have gone up.

If people travel further then demand has increased. It is strange how people think about congestion in terms of number of people rather than miles driven. A single car that hits several choke points on a 50 mile commute is causing far more congestion than a vehicle travelling 5 miles. Yet the users are treated exactly the same and never get any social pressure to reduce their demand.

There's no such thing as demand being at 100%, certainly not in this context. Demand is a curve. If the cost of driving decreases--i.e. less congestion, easer traveling--people will drive further and more often. It won't happen instantaneously, of course, but demand will adjust even if the population stays the same.

That doesn't mean the new road will fill up completely. There may be other constraining factors. But if a route was highly congested before expansion, you can bet demand will go up after expansion.

  > "A year after the freeway opened, traffic volumes along parallel roads like 44th, 56th and 68th streets dropped 40–50%"

That doesn't tell you anything about the total volume of traffic. And in any event it's precisely what you'd expect to see. If a line at the checkout counter has 10 people in it and new counter opens up, expect about 5 people to immediately move over. But _also_ expect a few more people to join one or the other line now that they'll clear quicker.

The total number miles driven on roads will almost always go up when you build more roads, just like increasing the supply of apples will lead to more apples being eaten. (The rare exceptions would be if there are a fixed number of people taking a long circuitous route from A to B, and a shortcut road is built; then the fixed number of drivers would get there more directly, reducing the total number of miles driven.) What normally happens with apples is that the price of apples goes down when there is new supply, roughly analogous to a decrease in congestion (i.e., the "price", in their time, that drivers pay to use a road).

When people talk about induced demand as an argument for building roads, they are claiming that induced demand will mean that congestion on the existing roads will increase (or at least not decrease). Here you have to take into account that most places where you build roads are growing for exogenous reason, so it doesn't make sense to compare post-construction congestion to pre-construction congestion. You have to compare post-construction congestion to the counterfactual where no construction was done. When you do that, you find (my emphasis)

> for every 100 percent increase in capacity there’d be an eighty percent increase in travel, reflecting increased travel speeds and land use shifts along improved corridors. However, only around half the increases in speed and growth in building permits was due to the added capacity. Factors like employment and income growth accounted for the other half. Accordingly, the traffic gains that one can attribute to the added capacity is actually around half of eighty percent, or forty percent. This is substantially less than reported by past induced-demand studies.

http://www.uctc.net/access/22/Access%2022%20-%2004%20-%20Ind...

The answer to all of this is well known: dynamically price road usage to keep roads at max capacity without noticeable congestion. Then each new road built is pure benefit.

We have one of those dynamic price roads around here. It starts out overpriced and goes up from there. It's basically the road for the 1%. Usage has been a quite a bit lower than the estimates so the taxpayers have been on the hook for lost profits.

http://www.washingtontimes.com/news/2013/nov/17/not-so-fast-...

I heard once that there was a government-managed construction project not featuring dynamic pricing, and that it cost much more than expected so the taxpayers were on the hook for the cost.

Maybe induced demand has always happened, in the sense of demand increasing when the product is more available. But it's plenty common for the amount that demand increases to be less than the amount of extra capacity available. Furthermore, is induced demand that bad? If you double the capacity and it again gets filled to the point of gridlock, well then at least twice as many people are now able to be in gridlock, which they apparently prefer to the alternatives.

The example of when induced demand is bad is when it causes increased pollution. In many parts of the world, there is a deliberate movement towards avoiding building too many roads, in order to decrease road use and therefore pollution levels.

However, a car sitting in traffic will produce much more pollution per mile than one travelling at 70mph on a clear road, so the thinking behind this may be flawed.

It's possible to change the incentives.

If the roads are free they fill up until gridlock makes them more painful than the alternatives. If you charge money then the expense becomes part of the cost/benefit calculation. It favors car pooling, rich people (who don't care about the price) and other people who urgently need to get somewhere during rush hour (who can pay the price that one time).

You can also just do car pool lanes, which has some similar benefits.

To be fair, for traffic, it's not necessarily the number of cars that's increasing. If everyone doubles the number of miles they drive, that will have a roughly equivalent effect. Car-miles is really the metric to watch here.

> Furthermore, is induced demand that bad?

It's good in cases where you can eventually meet that demand. Say induced demand for broadband leading to more fiber optic cables being laid down.

However there isn't enough space in cities to ever meet demand. What needs to happen is a shift away from the single-car culture in America and towards mass transit.

City's tend to have a lot of latent demand. There are some highway segments outside of city's that get minimal usage after they where built but which then grew over time instead of the sudden surge due to latent demand ex: 81 in southern VA which now sees heavy use, but was fairly empty right after construction.

However, in the city case new roads with high latent demand you may see a shorted rush hour which is a very useful side effect.

It seems to me that an urban car tunnel would be the most likely thing to induce demand: it's useless for local traffic, and is so expensive that it's only going to built in places with lots of latent demand.

Build one and I agree with you, build 10 and that's no longer true. So, the question becomes can you make it 10 times cheaper.

Remember, congestion lowers throughput and surface traffic scales really poorly.

Airports. Especially in Europe, many small cities built their own airports, hoping they'd get passenger traffic. Many of those are now closed or have to be sold off for a nominal amount, as flights concentrated on major airports instead of spreading out.

How many people commute by plane? It's not a good comparison.

Well, considering that time to get through security equates to a large number of commutes, and you haven't even gone anywhere yet...