Why Expanding Roads Fails to Reduce Traffic Congestion | Hacker News Reader

Why Expanding Roads Fails to Reduce Traffic Congestion

nominalnews.com

12 pointsNomNew15 comments

I have a lot of trouble believing the premise that road capacity is unrelated to congestion. Practically we know that road capacity is only increased when congestion reaches critical levels. We also know that there are some massive empty roads that no one magically came to fill (as shown on Top Gear). Anyone who has lived in an urban area knows that congestion increases over time, whether they add new roads or not.

> Overall, VKT in the US doubled from 1983 to 2003, from 7,700 VKT to 15,900 VKT for interstate highways. For major urban roads, a similar pattern occurred – VKT went up from 15,000 to 30,000. In the same time span, the number of lane kilometers of interstate highways remained basically constant, while on major urban roads, it went up from 3,800km to 6,500km.

I don’t know how someone can trot out this stat and then claim with a straight face that new lanes cause congestion. What this states is that car traffic doubled over a 20 year period, and it doubled whether lane capacity increased (cities) or not (interstates).

The study looked at what is the source of extra traffic. In most cases it was individuals living in the area doing more car trips.

Basically, an extra lane temporarily reduces congestion. The main cost of congestion to road users is time. Since now it's faster to travel, you're more likely to do an extra car trip. You continue taking extra car trips, until the cost in time is the same as pre-road expansion.

The question then is are the extra car-trips valuable.

My issue isn’t that they claim the increased congestion comes from additional personal trips. My issue is that they claim this is due to increased capacity which fundamentally seems untrue, as evidenced by the fact that traffic doubled both with and without increased road capacity.

In fairness, I have not read the 37 page paper yet. Maybe the paper makes a more compelling case than the summary article.

I'm not sure I see the nuance in the two issues you point to. The paper looks at how adding new lanes changed congestion on these roads. The results found no change in congestion, as shown by VKT growing the same amount as the increase in road capacity.

A driver, besides the cost of the car and fuel, faces a time cost. An extra lane will reduce the time-cost, assuming no new vehicles enter the road. But if time costs fall, it's in effect 'cheaper' to drive. So previous car trips that were not happening, because the time-cost was too high, are now occurring. The amount of extra car-trips is such that we are back to the same time-cost as before the road expansion. That's why VKT ends up growing one for one with road expansion.

I did not read the paper yet, but the article says that KVT doubled in 20 years for both highways (which did not add new lanes) and urban roadways (which did). So we see KVT double regardless of lane expansion.

To me this says that lane expansion is not the driver of KVT (at at least not the primary one).

Ah, I see. The key question the authors look at is what is the impact of adding a lane to VKT - this they found one-for-one growth. They control for many other factors that may have led to the growth over the observed time period (population being the big control).

The broad question is what would reduce congestion - currently, on the margin, adding a lane will not reduce it. I imagine that hypothetically adding many lanes may reduce congestion, although there may be city driven bottlenecks that don't make this feasible at all.

And also - thanks for the comments and questions!

Please I don't understand this.

It's actually a pretty complicated topic. CityNerd did a pretty good video on the topic here:

https://www.youtube.com/watch?v=za56H2BGamQ

Better yet, it's literally a complex system!

The implied part is “within common conditions” it’s a law limited to a specific regime, much like Newtonian physics. We know it’s not universally true but we can see it’s often true in common scenarios.

In the extreme case, as road coverage approaches 100%, the city stops containing buildings so the traffic will drop towards zero, so there’s actually a balance point somewhere, but roads are quite inefficient for high density cities, so probably the balance point would be less about fulfilling traffic demand and more about reducing the demand by demolishing most of the city.

All of the major freeway expansions I have experienced required at least two years to complete. During these periods, lanes in the construction zone were heavily constricted, resulting in all of the roads surrounding the expansion project being completely filled with stop-and-go traffic during peak hours until the projects were completed, which seems to suggest the number of lanes makes some kind of a difference in something, somehow.

The authors seem to suggest that demand for roads is infinite, as expanding roads merely increases the number of trips people choose to make, thus infinite expansion will result in infinite trips.

These analyses always appear to me as if they are without any understanding of how humans actually behave, resulting in nonsensical nonsense "laws".

> The authors seem to suggest that demand for roads is infinite, as expanding roads merely increases the number of trips people choose to make, thus infinite expansion will result in infinite trips.

Agree. They literally claim this with “increasing lane kilometers by 1% will increase VKT by 1.03%” but subtly acknowledge that this fundamentally doesn’t make sense with the hand-wavey “any feasible increase in roadways will have no impact on congestion”. Keyword feasible.

The real law doesn’t seem to be that congestion rises to meet capacity but that no one will ever fund enough road expansion to make peak congestion low.

One of the reasons why this keeps getting executed is that there is a latency between added capacity and induced demand. So you can 'solve' traffic temporary just around the next election.

Therefore, removing lanes does not make it worse? The title doesn't pass a simple smell test. This analysis looks to try to describe no reduction in overall transit time, not what the title says.

The demand fills capacity is not a good rule of thumb either, from an economist's perspective. Shame on you. Many multi-lane highways are rather empty, why? Many roads are basically never used, why?

Lanes (transit corridors) are a river of money (funnels). When you have populations that are exchanging goods, jevon's paradox fills lanes to increase capital velocity that scales beyond the average value of infrastructure. Infrastructure cost is balanced against a perceived value, which is always skewed toward the larger (poorer) part of the population.

In this case, it's not a paradox at all that capital self-generates demand for this space. This also explains why some corridors are emptied as capital flees a locale.

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