10 ms·

Do you happen to have any examples of where induced demand has not happened? My understanding is that it is pretty much guaranteed, especially in larger cities.

Do you happen to have any examples of where induced demand has not happened? My understanding is that it is pretty much guaranteed, especially in larger cities. I think New York's bridges and tunnels are the canonical examples of this. I say pretty much because I assume there must be cases where it didn't happen, but I am unaware of them.

tgb 10y ago
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.
- mnw21cam 10y ago
  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.
- skybrian 10y ago
  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.
- audleman 10y ago
  > 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.
- jdmichal 10y ago
  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.
Retric 10y ago
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.
- bryanlarsen 10y ago
  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.
  - Retric 10y ago
    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.
dx034 10y ago
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.
- emodendroket 10y ago
  How many people commute by plane? It's not a good comparison.
  - jdmichal 10y ago
    Well, considering that time to get through security equates to a large number of commutes, and you haven't even gone anywhere yet...
jessriedel 10y ago
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-%20Induced%20Travel%20Studies.pdf 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.
- jandrese 10y ago
  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-use-of-495-express-lanes-in-virginia-f/ http://www.washingtontimes.com/news/2013/nov/17/not-so-fast-...
maxsilver 10y ago
> 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) 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).
- audleman 10y ago
  > 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-uspirg/424700/ http://www.citylab.com/commute/2016/01/highway-boondoggle-us...
  - true_religion 10y ago
    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.

tgb 10y ago

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.

mnw21cam 10y ago
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.
skybrian 10y ago
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.
audleman 10y ago
> 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.
jdmichal 10y ago
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.

Retric 10y ago

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.

bryanlarsen 10y ago
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.
- Retric 10y ago
  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.

dx034 10y ago

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.

emodendroket 10y ago
How many people commute by plane? It's not a good comparison.
- jdmichal 10y ago
  Well, considering that time to get through security equates to a large number of commutes, and you haven't even gone anywhere yet...

jessriedel 10y ago

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-%20Induced%20Travel%20Studies.pdf 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.

jandrese 10y ago
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-use-of-495-express-lanes-in-virginia-f/ http://www.washingtontimes.com/news/2013/nov/17/not-so-fast-...

maxsilver 10y ago

> 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) 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).

audleman 10y ago
> 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-uspirg/424700/ http://www.citylab.com/commute/2016/01/highway-boondoggle-us...
- true_religion 10y ago
  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.