Induced demand: The traffic paradox that we all need to know about

Traffic is one of the most interesting modern phenomena. It ebbs and flows, acting like a part of a living organism, the lifeblood of a city, creating a pulse as it courses through its asphalt veins. As for any living body, keeping healthy should be the number one priority, and taming traffic to keep our cities healthy is no simple matter. Traffic doesn’t always react as you might expect, it’s unpredictable. As such, there are many paradoxes associated to it. Induced Demand is one that we should all know about because it can change how we think about driving, infrastructure and making mobility more sustainable.

More roads, less congestion? What is induced demand?

Build more roads, add more lanes and traffic jams are solved, right? On the surface, it makes sense. If the road can’t handle the capacity of traffic and there are jams every day, just increase the capacity of the road and watch the traffic dissipate. Unfortunately, it doesn’t work like this in practice.

While adding lanes to roads has been known to alleviate congestion for a short time, eventually traffic levels return. If you build more roads, more people end up taking to those roads, thanks to the increased capacity, and traffic jams and travel times return to pre-expansion levels.

This phenomenon is explained by an economic principle called induced demand, or induced traffic in this case. In simple terms, induced demand is the idea that creating more of something increases the consumption of that thing. And so, if you’re looking to reduce consumption of a good or service or utility, you won’t achieve that by making more of the good. It’s closely related to the Braess’ Paradox which states that adding roads to a network slows overall traffic flow.

There’s an ongoing debate surrounding induced demand, with varying opinions as to whether it’s good or bad. For example, while traffic jams and travel times may return after road widening works to pre-widening levels, road capacity (the total amount of traffic it can service) does increase, and more people can get from A to B — just not any quicker.

However, this then introduces the problem of where all those people will park when they get to their destination and if the destination can handle increased volumes of traffic. Not to mention the environmental impact of more traffic, unproductive land use, wider roads and cities that prioritize support for cars over all other forms of transit.

Whatever the case, there’s no denying the existence of induced demand — various studies have proven it.

There are examples from all over the globe that are cited regularly, such as the Katy Freeway in Texas that links suburban Houston with the city’s downtown area. The freeway has been expanded multiple times over the past 20 years, at a cost of nearly $3 billion and the road now has a massive 23 lanes.

It looks like this:

The Kay Freeway is a massive multi-lane highway that's been expanded multiple times to try and reduce congestion. Travel times have not improved since it was expanded, but it can handle more traffic at peak times. Outside of peak hours, it's a lot of wasted space.

Its capacity for traffic has certainly increased, which is considered an upside by some, but travel times have only gotten worse — leading many to see its expansion as a total failure of public spending, considering its goal was to alleviate congestion.

But why does this happen? The case of the Katy Freeway

One of the most widely accepted theories to explain why road widening doesn’t reduce congestion is Anthony Downs’ Triple Convergence theory. It states that induced traffic is the result of vehicles converging from different routes, transport modes and travel times because of the widened road’s perceived increase in capacity, convenience and reduced travel time.

In other words, drivers that would have otherwise taken a bus, train or bicycle, a different route, or even stayed at home, now drive on the wider road — because they can. And traffic that would have taken to the roads earlier or later in the day to avoid the original jams, now drives at the same time as everyone else.

The Triple Convergence Theory suggests that when roads are widened, drivers that would have traveled by another mode of transport, at another time or on a different route all converge to the newly widened road. This means that even though the road can handle more vehicles, travel time along it does not reduce. Also, more people drive than walk, cycle or take the bus or train.

There’s yet another issue to consider: what else has been impacted by the widening of roads and how does that impact traffic demand?

In the case of the Katy Freeway, hundreds of homes and businesses were demolished to make way for the road widening, displacing jobs, trade and people to further out the city, which ended up increasing the amount of road users. These changes end up creating additional demand as people need to live further from the city and now need to commute to their jobs. They cannot walk, due to the distance, and mass transit often isn’t an option as it’s unreliable or doesn’t exist. Instead, people have no option but to drive and take the freeway that they didn’t need to use before.

What’s more, with added road capacity and displaced residents, demand for housing increases, prompting a rise in out-of-town residential developments which further attract more people to the area, adding even more strain on the roads.

Texasfreeway.com and Houstonfreeways.com have published extensive documentation of the “right of way clearance” that took place when expanding the Katy Freeway. Whole suburbs, communities and businesses were destroyed and uprooted to make way for the freeway’s expansion. It should be noted that these “right of way clearances” are rarely equitable or fair. As this article from the Guardian explains, clearances typically impact underrepresented or minority groups more than others.

Despite clear evidence that road widening doesn’t achieve its primary goal of reducing traffic congestion and comes at a huge social cost, it continues to happen all around the world, negatively impacting people, communities, quality of life, the environment and the nature of our cities.

Massive multi-lane highways haven't always cut through the middle of our cities. Imagine all the homes, businesses and people that were affected when this interstate was built through the suburbs of Los Angeles.

Studies examining cases like the Katy Freeway, but across whole countries, find the phenomenon of induced traffic to be consistent, regardless of the location. So much so, that it’s referred to as the fundamental law of road congestion.

What is the fundamental law of road congestion?

While induced traffic states that widened roads will create more traffic through induced demand, the law of road congestion goes a step further to say that the travel speed on an expanded highway will eventually revert to its pre-expansion level.

The name of the law comes from the title of a 2011 academic paper by economists Gilles Duranton and Matthew A. Turner, which found a proportional link between the expansion of roads and an increase in traffic congestion across a number of U.S. cities. Repeating the study, researchers in Japan reached the same conclusion.

No matter how big we make our road networks, traffic, it seems, is an inevitability. Taming it requires smarter, more creative solutions that consider the whole transport network and city infrastructure.

Ralf-Peter Schäfer, TomTom’s VP Product Management of Traffic, is resolute in his belief that to beat congestion we must look beyond roads and build city-wide mobility systems that use creative blends of multi-modal mass transit, advanced data-driven traffic management tools and real-time mapping.

[Read our interview with Ralf-Peter here.]

Multi-modal, mass transit is proving to be the most effective and scalable way to move people around our cities.

City planners and engineers should also be using historic traffic data to understand how roads are used, to improve their flow and keep traffic moving rather than building more roads to simply increase capacity. In the pursuit of reducing congestion, we should focus on improving flow, not increasing capacity.

The theory of induced demand and the fundamental law of road congestion demonstrate just how unpredictable traffic is. With data, though, we can spot these anomalies. We can model how infrastructure changes might impact traffic and subsequently their viability and sustainability — before investing millions into their creation. We can zoom out and see how to improve whole cities of traffic, reducing emissions and travel times by using flow optimization strategies, like smart traffic signaling and optimized speed limits.

We can see that making roads wider does not improve congestion.