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, including many road-building authorities, are still reluctant to face what this means. Part of the problem, surely, is that we’re presenting induced demand as an observed discovery, allowing us to perform quarrels over data, research methods etc.
But induced demand isn’t just an observed fact. It’s also an axiom of biology, so 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. We seek resources in the easiest possible way (i.e. with the lowest possible expenditure of energy.) So:
- If driving suddenly becomes easier than taking transit (due to a road widening, for example), more people will shift to driving, increasing congestion.
- 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 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.)
- 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 parable 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.