Induced Demand: An Axiom of Biology

Figuring out the relationship between this tweet and this article is left as an exercise for the reader.

Induced demand is the observed fact that if you make something easier to do, people will do it more.  For example, if you create new capacity for cars in a place where travel demand is high, the result is more cars.  If you build more capacity to “fix congestion”, you end up back near the same level of congestion you had before.

After decades of observing this pattern, most people are still reluctant to face what this means.  Part of the problem is that we’re presenting induced demand as an observed discovery, which allows us to quarrel over data, research methods etc.

But induced demand isn’t just an observed fact.  It’s also an axiom of biology.  We are as sure about it as we are of the facts of math.  This means we don’t really need to be doing this experiment over and over, just as we don’t need to keep measuring circles to be sure of the value of pi.

In this context an axiom is a statement that can be taken as true because it is part of the definition of a concept you are using, or follows logically from that definition.  The value of pi is axiomatic because it follows from the definition of a circle, in standard Euclidean space that describes our everyday world.

Now consider the concept of an organism.  It implies that:

  • The thing consumes some resource from its environment, in order to have enough energy.
  • It will expend energy getting this resource.
  • Therefore, it must run a positive balance sheet: The energy it spends getting the resource must be less than the energy the resource will provide.

We humans are organisms, so we do what they do.  In particular:

  • To have enough energy, we seek resources.  Money is a symbolic resource in this sense.  We also have other needs like shelter and security that also require resources.
  • We need to get these things in way that minimizes energy costs.  This is what I mean by easy.

We are mobile creatures, rather than barnacles or oysters, so this pursuit of resources often requires traveling. This means that time is also part of this basic calculation that determines survival.  The time spent pursuing a resource can’t be spent pursuing other resources, or doing other necessary things like sleeping.  So we must minimize our costs in both energy (which includes money) and time, while gaining as much energy (money) as possible.

From this it follows that people will tend to travel in ways that maximize their access to resources while minimizing energy cost (time and money) and danger.  People don’t always do this exactly, but the underlying biological imperative is unavoidable.

This means that:

  • If driving suddenly becomes easier (lower time and energy cost) than taking transit, more people will shift to driving, increasing congestion.  This is why road widening in high-demand places tend to lead to more traffic.
  • If a road widening makes it possible for developers to save money (i.e. energy) by building in more distant places where land is cheaper, they will do that.
  • This process changes the shape of the urban area so that people travel longer distances (due to sprawl) at slower speeds (due to congestion).
  • Therefore the average organism will need to expend more time and energy to reach the same resources it reached before.  (Your job flees from downtown to a distant business park where taxes are lower.  Your grocery store closes because a WalMart opened two miles away where you can’t walk to it, or even walk from the nearest point that a bus could get to.)  The organism will also be exposed to more danger as a result.
  • On average, organisms in this system end up in a weakened state, with a worse balance sheet of energy expended vs energy gained.

The organisms in this story are all trying to harvest more energy than they spend in the act of harvesting.   Even unimaginable aliens on distant planets would do this in the same situation.  So it’s axiomatic that, in the absence of other pressures, road widening in a high-demand area will induce more traffic and more sprawl.

So although road-building departments keep doing the induced demand experiment many times every year, and getting the same results, you don’t need to do more experiments, just as you don’t need to keep measuring circles to be sure of the value of pi.  You can add complexity by taking this into the human sciences and trying to model subtleties of human behavior, but all the resulting insights will be marginal compared to the axiomatic fact that above all, we’re organisms, so we’ll do what organisms do.

16 Responses to Induced Demand: An Axiom of Biology

  1. Anti-tax January 12, 2022 at 9:29 pm #

    You’re trying to use an analogy as some kind of proof, while the side that you oppose has actually researched the various presented arguments. I.e. some people ignore population growth when determining demand; and/or some people think that if roads aren’t built then that will just force people to use transit; etc.

    • asdf2 January 16, 2022 at 10:07 pm #

      The induced demand argument is still valid, even in places with zero transit.

      That’s because it’s not just about driving vs. transit, it’s also about driving vs. not making the trip at all. Or, driving during peak hours vs. driving during off-peak hours. Or, making separate trips to shop at different stores in the same neighborhood vs. combining them. Or, driving long distances to a place that you have long gone to out of habit vs. taking the time to look for an alternative that is equally as good, but closer to home. Or, family members in the same household choosing to carpool vs. driving separately, just in case one person wants to leave 10 minutes before the other.

      Even in the most car-oriented cities you can imagine, these tradeoffs still apply. Not every trip is all that important to begin with; not every trip has to be made at a specific time; just because you’re used to driving 15 miles to see a dentist and the dentist does a good job does not mean that isn’t an equally good dentist out there with half the commute distance. etc.

      When highways are widened indefinitely, people are encouraged to ignore these tradeoffs altogether, and the result is a lot of extra driving for the tiniest of reasons that often don’t provide enough cumulative public benefit to justify the cost of the widening project. It’s the classic tragedy of the commons argument. Anytime a good or service is offered for free, in unlimited quantities, people will consume more of it, for the tiniest of reasons. When the cumulative benefits to the users are less than the cost to provide the free service, it’s a net loss for society. This is true, whether the item in question is potato chips or access to freeways.

      When you add transit to the mix, the transit is simply just another option people can use to avoid getting stuck in traffic, alongside avoiding unnecessary trips, rescheduling trips, etc. Whether the best solution is to actually ride the transit vs. reschedule the trip, cancel the trip, or just put up with driving in traffic, is up to the individual to decide. Nobody is “forced” to do anything.

      • Anti-tax January 17, 2022 at 5:39 pm #

        Did you read the article? It addresses all those points.

        What I would say about your response specifically, is that it shouldn’t be up to you to determine which of other people’s trips are important to them.

        And no one is advocating indefinite highway widening, we know that there is not the funding or resources for such. Usually when there’s no more possible widening then what happens is that one or two “express” toll lanes are built, which use varying toll prices to regulate demand.

        • Sailor Boy January 27, 2022 at 7:57 pm #

          “And no one is advocating indefinite highway widening, we know that there is not the funding or resources for such”

          You obviously don’t talk to a very wide range of people!

  2. Kyle Marshall January 13, 2022 at 4:13 pm #

    Instead of congestion, why not focus on growth and other principles? Is the opposite of growth self-evidently absurd? Do you, or anyone/thing you love, truly want to focus on degeneration/entropy? Even if we want to conserve, or maintain, our current state, don’t we have to at least grow at times to balance the natural degeneration/entropy? Growth can be up, out, in, etc., all useful at different times, and growth of any sort requires energy. Therefore, maintaining also requires energy too. As I’m writing this response, the root of frustrations in maintaining aspects of my life, like my bike for example, seems self-evident to me now: a lack of focus on growth. It’s easy to steal money from a child, but why would anyone feel guilty in doing so, if it were revealed to them that that is what they are doing? Even so, every person on Earth is limited by death and we will all have to answer to the limitless/unknown eventually, yet we can still have children and hope for the future resides in them.

    I don’t believe direction needs to be reduced down to power, but likely there are exceptions. Power can be money or energy or politics or whatever else to which power is equated. Having nearly acted as though I believed in nihilism and allowing responsibility to nearly crush me, I can appreciate that everything is pointless because maybe there are many points and focusing on the wrong one not only leads to instability, but also blinds you to the many points that are needed. Recently, I have found the secular work of Stephen Covey to be useful and he argues to center on principles. I didn’t really understand what that means, so I have read his more religious book, Divine Center, and I took away that Christians argue to center on Jesus/God/Holy Ghost and all other points in balance will be revealed afterward. In the same way that the sun gives us energy, so does the Son, as Christians may say. That may seem spooky or you can be cynical about it, like I have been most of my life. That cynicism and enemy-centeredness hasn’t worked for me, especially during the COVID-19 pandemic, so I have been picking up my pieces. However, there is something magical in the words, opportunity and adventure, that has been a bridge to principles for me. As an example, my wife and I originally set our wedding date for the summer of 2020 after getting engaged the year before. Of course we had to delay the ceremony due to COVID; however, we wanted to see opportunity in the limitations. We asked ourselves what we could do, now that our ceremony must be delayed, and what we may have never prioritized otherwise. So, we chose to try and make our wedding photos as beautiful as possible by taking them during the fall while the leaves were changing colors. Since that was many months after the summer, I also decided to work out and lose 20 pounds too. Furthermore, we spent more time researching, personalizing, and writing our vows. We did all of this where we first met, a place not ideal for our ceremony, yet this place is now more special and nostalgic for us to revisit.

    In regards to growth, why not get more people up to speed? Why not consider a transit technology that leverages principles of highway design? What if a train could always be moving and dock with another set of trains that are traveling from station to station? Passengers could shuffle between the two moving vehicles when they have docked so as to not slow down the speed of passengers traveling through stations. Perhaps this, in addition to frequency and access, is enough to get passengers up to speed. I’m not sure where this will go, and it’s not my idea, but hopefully we can go on an adventure.

    • RossB January 14, 2022 at 10:17 am #

      There is no analogy. The author is simply describing human nature. As human beings, we consider the options to achieve a goal, and pick the ones that appeal to us. Being delayed by traffic is not appealing, nor is waiting endlessly for a bus. They are in competition with each other. That is the point. Building more roads will get more people on them, as will building transit. The big difference is that transit scales so much better.

      • Kyle Marshall January 14, 2022 at 9:07 pm #

        I appreciate the emphasis about scaling. It’s not something that comes to my attention often, but it is important to consider and ponder, even for growth. I hope for win-win solutions, but it’s quite possible that we are limited to win-lose at many times, fortunately or unfortunately. Also, I really hope I did not ignore the point(s) being made in this blog or your response; however, this is my interpretation and attempt at responding and it’s quite more revealing how I view the world than as is the world in actuality.

  3. RossB January 14, 2022 at 10:22 am #

    As I mentioned in my previous response, the big difference between adding more transit and adding more lanes is that transit scales so much better. Adding more lanes is often a zero sum game. Add lanes on one roadway, and it adds traffic on another. Those riders might be better off, but other drivers are worse off.

    In contrast, transit works the opposite way. Add transit on one section, and more people ride it on another. At first glance, this appears to be the same problem, but it isn’t. If a bus or train is crowded, you improve it. You either run it more often (reducing the wait) or you run it more directly (reducing travel time for those riders). Either way it becomes *more* affordable to do that, because of fare revenue. Transit scales, more roads does not.

    • Johnny January 14, 2022 at 6:00 pm #

      Except if the train line already runs at maximum capacity, then you have to build longer platforms or build a new train line. This costs extra money, so in this case the cost per passenger actually goes up when ridership rise.

      Or, which is what usually happens in practice, you remove seats and try to squeeze in more standing passengers in the same amount of trains. Then the riders are worse off when ridership rise.

      • asdf2 January 16, 2022 at 10:19 pm #

        That’s true in extreme cases, but transit can accommodate many more riders per hour than a lane of freeway before those issues start to arise.

        When overcrowding does start to become an issue, the same induced demand issues that apply to driving also become relevant to transit. You can manage it by raising fares and/or charging higher fares during peak hours, just like you manage freeway congestion by charging higher tolls during peak hours. The result is that people move discretionary trips off peak, replace short trips with walking or cycling, replace long trips with substitutes closer to home that are cheaper to get to, etc. However, there are very few cities in the US where transit ridership ever gets high enough for such passenger congestion to become an issue (although, it can be fairly common in other parts of the world).

      • RossB January 17, 2022 at 8:46 am #

        “Except if the train line already runs at maximum capacity, then you have to build longer platforms or build a new train line. This costs extra money, so in this case the cost per passenger actually goes up when ridership rise.”

        Not necessarily. Longer platforms may be relatively cheap, and more than made up for by the additional riders that they allow. Same with running the trains more often.

        A new train line is more expensive, but it also adds value. Again, it may pay for itself, but even if it doesn’t, you are providing something of additional value. A large percentage of freeway expansion does nothing more than add lanes to the same roadway. When there is no traffic, it adds no value. This sort of thing is rare when it comes to transit. Even if a new line helps with a capacity problem it almost always makes a new connection.

  4. Adam Short January 14, 2022 at 3:26 pm #

    As you can see especially by the first comment, you might be making a common technocratic assumption that the problem is a lack of understanding on the part of policymakers.

    In fact highway expansion is sold to the public as a means to alleviate congestion because that is a popular message. This does not imply that the people who are selling the policy in this way actually believe that this is the purpose, or the likely result of the policy.

    As Lorna Finlayson put it, the political is political. Conservatives want highway expansion and car-centric policies for political reasons, and are willing to make whatever arguments that are likely to garner public support. Since conservatives monolithically oppose public transit, and non-conservatives do not monolithically oppose highway expansion, highway expansion almost always wins.

    This is a political problem, not a problem of understanding of a technical principle. Conservatives understand induced demand very well. If you read the linked Reason article above you will see that quite well – the disingenuous arguments made therein could not be crafted by someone who did not understand the principles they were trying to obfuscate.

    • Sean Gillis January 21, 2022 at 8:13 am #

      This post by Adam Short. Truth.

  5. Rob January 23, 2022 at 11:39 pm #

    I’m disappointed this is presented this way. Jared, you’re describing the effect of latent demand, which explains the dynamics you’re describing well. When there is more demand than can be accommodated, people switch to second and third-best solutions — changing their route or more, their home or work location, or not making the trip at all. Urban areas always have latent demand because of the geometry problem you’ve described many times. When new capacity is added in a place with latent demand, people switch from their second and third best options to a better one and there’s a new equilibrium but congestion stays constant.

    The notion of induced demand is different, and drives to different policies and solutions. Induced demand is the ideological notion that capacity itself causes increases demand to drive. It’s part of a theology that believes people only drive because of marketing and pervasive cultural messages, not because it actually can add value.

    If you believe there is latent demand, you can still propose solutions that use road capacity thoughtfully. Sometimes you can reduce congestion at a bottleneck without reducing through traffic capacity. Sometimes you can price or manage capacity to offset latent demand shifts to the new capacity. Transportation models factor in latent demand, but the don’t factor in added demand that materializes out of nowhere due to the persuasive power of highway capacity. If you’re *not* in an urban area with latent demand, you can actually add capacity and relieve congestion sustainably. If you believe in induced demand, none of those solutions are sensible.

    I don’t understand why it’s necessary to introduce an article of faith to explain what latent demand already explains very well.

    • Henry January 25, 2022 at 12:55 pm #

      I think this is splitting hairs. People *do* propose road pricing to solve induced demand. However, very few people talk about it, because the politics of toll roads make them political suicide in pretty much every place they exist. Even in places where only tolls apply to new roads or new lanes, there are movements to bring them back under public control and remove the tolls.

      Induced demand is a type of latent demand, with the main difference being that most systems have some kind of check to determine when latent demand is too marginal to be worth spending money on. The (American) public strongly demands that highways be free, there is pretty much always a real estate and construction lobby willing to fight for ever more marginal extensions, and neither running out of maintenance money nor running out of dedicated fund money seems to stop this either.

      The first Interstates shaved the time to drive across the country from months to days. We have long moved away from that, and at some point spending hundreds of millions to shave off intersection timings is diminishing returns.

    • Jarrett January 27, 2022 at 11:40 am #


      I guess I don’t hear “induced demand” as having the ideological connotation that you do.

      In your terms, by induced demand I mean latent demand released by capacity expansion. But “latent” is also misleading because it’s not like there’s an objective quantity of demand that is waiting to be revealed, like pressure in a boiling teapot. Because people adapt to the transportation network, capacity constraints cause people to reorganize their lives so that they no longer produce demand that can’t be met. Capacity expansion pushes the same mechanism in the other direction, which is what I mean by induced demand. And that’s even before we include the new demand created by land use changes triggered by the new capacity. Semantics I think?