[AnthonyMouse]

> Just to be clear, the "induced demand" that concerns me is not the latent demand of a few deferred trips being taken in the days/weeks/months/year after the road is widened, it's the long-term generated demand of people choosing to move further into the suburbs because they can commute more miles in the same number of minutes. The cumulative result is that property values on the periphery of the commuting zone increase and within a decade or so the highway traffic exceeds the optimal capacity again.

Property values increasing there actually offsets the problem by making it less desirable to live there.

The real trouble is that people build more houses there. But the reason people build more houses there, and suffer a 30 minute commute (which more congestion might have turned into a 60 minute commute), is that they can't afford to live in the place with a 15 minute commute. Typically because zoning prohibits building more housing there.

Now let's see what our choices are here.

We can do nothing at all. Well, now people are screwed. They still need somewhere to live, the place that now has a 60 minute commute is the only place housing can be built, so the housing still gets built there, but now the commute is longer. That's just horrible and helps no one.

Second, we could widen the road and that's it. The new housing still gets built in the suburbs but at least now people waste less time in their cars.

Third, we could loosen the zoning so higher density housing can be built closer to the city, but not widen the road. This is pretty good, because now the people who live in the new housing get the 15 minute commute. But the people who already live in the suburbs are still stuck with the 60 minute commute.

Fourth, we could loosen the zoning and widen the road. Then new housing gets built in the city instead of the suburbs, because people prefer a 15 minute commute to a 30 minute commute, but the people who already live in the suburbs still get a 30 minute commute instead of a 60 minute commute because of the wider road. And it stays that way because the new housing is getting built in the city instead of the suburbs. This is pretty obviously the one that we want.

[asoneth]

> but the people who already live in the suburbs still get a 30 minute commute instead of a 60 minute commute because of the wider road

That is true for several years after the highway widening. Which is good for politicians who operate one election cycle at a time.

But the population doesn't just sit back and enjoy shorter commutes after a highway widening project. The towns that used to be two hours outside the city are now only an hour drive away and commuters looking for a deal start to move there. The towns that used to be an hour away are now only a half-hour drive to the city and are now hotter as well. Some of that is due to a game of musical chairs in which people expand out of the city to the suburbs, some of that is new residents choosing to live further out than they would have if they arrived before the highway widening. All of this increases the number of vehicle miles traveled. Eventually (over the span of decades) this results in a new equilibrium in which the highway is nearly as congested as it was before the widening and everyone is once again living at about the time-limit of how far they are willing to commute.

It's not all bad of course because the new highway helps grow the metro area and it increases property values, especially in the suburbs that were on the periphery of the commuting zones. But to permanently eliminate congestion you'd either have to keep widening the road every decade or two to handle the growth (Houston, Dallas), allow the congestion itself to act as a natural limit (NJ), or institute tolls/congestion pricing (Singapore, London).

[AnthonyMouse]

> But the population doesn't just sit back and enjoy shorter commutes after a highway widening project.

Notice that this is true of anything that relieves the highway congestion in any way whatsoever. As soon as the road is clear, no matter why, the commute is shorter and it becomes more attractive to live further away in distance.

> Eventually (over the span of decades) this results in a new equilibrium in which the highway is nearly as congested as it was before the widening and everyone is once again living at about the time-limit of how far they are willing to commute.

The way out of this is to make some alternative to increasing sprawl more attractive than increasing sprawl. Relaxing zoning rules to allow higher density is an obvious one, because people would rather have a 15 minute commute than a 30 minute commute, so they'll prefer to build housing where it's a 15 minute commute unless that's prohibited by law. When it is, they have to build further away and you get more sprawl.

But also notice that you're assuming population growth.

If the population was stable, and you built a road which is sufficient now, it'll probably stay sufficient as long as the population remains stable, because who is building enough new housing to move the needle on traffic in a city that isn't growing?

If the city's population is growing, continued population growth will require you to expand highways and such over time in proportion to the population. What else would you expect? The only alternative is intentional scarcity.

[asoneth]

> Notice that this is true of anything that relieves the highway congestion in any way whatsoever.

Is that true if we charge an appropriate price for road use? Isn't that what we do to manage availability of every other scarce resource to ensure it is being used optimally?

If you increase the price of using the road at the busiest times then eventually you will arrive at a price that maintains the optimum flow rate. You may need to adjust the price occasionally to track shifts in inflation and population, it has equity issues, and (like most sustainable solutions) it is politically difficult. But I don't understand what would cause it to stop working. In the places it has been implemented (e.g. Singapore, London, Stockholm) it is generally unpopular at first and then extremely popular after a year or so. And administering it should become cheaper as technology improves which may make it feasible for smaller cities as well.

> If the population was stable, and you built a road which is sufficient now, it'll probably stay sufficient as long as the population remains stable, because who is building enough new housing to move the needle on traffic in a city that isn't growing?

Even in metro areas that have stable populations, heavily subsidizing transportation infrastructure (whether transit or highways) between the city and the suburbs often has the effect of slowly shifting the current metro population outward to those suburbs. By subsidizing suburban commuters you are making living in the suburbs more attractive than it would be otherwise. As you make it more attractive, more people who currently live in the city will rationally choose to move to the suburbs.

I'm not saying it's a bad idea to build more lanes, just that on it's own it is not a sustainable approach. As long as rent/land prices are largely set by the market and roadways are free or heavily subsidized then it's not surprising that people take advantage of that and you end up with a shortage of road space.

(I agree with your comments on zoning.)