Induced Demand

Friday December 25, 2020

If you talk to urbanists about projects to develop new highways, you’ll inevitably hear someone talk about induced demand. Here’s CityLab’s definition:

Induced demand is often used as a catch-all term for a variety of interconnected effects that cause new roads to quickly fill up to capacity. In rapidly growing areas where roads were not designed for the current population, there may be a great deal of latent demand for new road capacity, which causes a flood of new drivers to immediately take to the freeway once the new lanes are open, quickly clogging them up again.

But these individuals were presumably already living nearby; how did they get around before the expansion? They may have taken alternative modes of transport, traveled at off hours, or not made those trips at all. That’s why latent demand can be difficult to disentangle from generated demand—the new traffic that is a direct result of the new capacity.

This idea is commonly used to argue against building new roads as a way to fight congestion, since as soon as the new roads are built, they’ll fill right up again. This idea makes a nice just-so story, but it’s basically irrelevant to actually deciding what kind of transit to build — you can just as easily say that train or bus service shouldn’t be increased due to induced demand — run the trains faster, and more people will just fill them up again! But for some reason, people exclusively use induced demand to talk about why building highways is a bad idea, but never think to apply the same logic to increasing frequency of trains or other such projects.

This isn’t to say that building highways is good, actually — quite the opposite. But people who are using a newly-expanded highway that wasn’t practical for them to use before due to congestion are using transit in the same way that people using higher-frequency trains that weren’t before are using transit. The fundamental difference is that cars and trucks account for ⅕ of the United States’ carbon emissions, make a huge amount of noise and air pollution in cities, and are able to transport far fewer people per square foot used than trains or buses. That’s the reason that expanding freeways is usually bad, while increasing train or bus service is usually good. And hell, increasing train and bus service can reduce traffic congestion as well, since the hundreds of thousands of people in trains and buses are people who aren’t using the road, freeing up space for people who actually need to make trips that are inconvenient or impossible on other transit.

Talking about “induced demand” as a reason not to expand freeways sounds nice, but all it’s doing is ignoring the needs of the people who use roads, when peoples needs are the thing that should be centered when designing transit. If we’re going to try to get rid of freeways, we need to at least be honest about why.