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Why Expanding Roads Fails to Reduce Traffic Congestion

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Re: Why Expanding Roads Fails to Reduce Traffic Congestion

#11

Post by entropicdrifter »

foxyv wrote: post #9
dpark wrote: post #4

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. >…

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!

Re: Why Expanding Roads Fails to Reduce Traffic Congestion

#12

Post by scratcheee »

dpark wrote: post #4

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. >…

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.

Re: Why Expanding Roads Fails to Reduce Traffic Congestion

#13

Post by NomNew »

dpark wrote: post #8
NomNew wrote: post #7

Earlier quoted context omitted.

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 quest…

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.

Re: Why Expanding Roads Fails to Reduce Traffic Congestion

#14

Post by dpark »

NomNew wrote: post #13
dpark wrote: post #8

Earlier quoted context omitted.

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…

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).

Re: Why Expanding Roads Fails to Reduce Traffic Congestion

#15

Post by NomNew »

dpark wrote: post #14
NomNew wrote: post #13

Earlier quoted context omitted.

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…

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.

Re: Why Expanding Roads Fails to Reduce Traffic Congestion

#16

Post by NomNew »

dpark wrote: post #14
NomNew wrote: post #13

Earlier quoted context omitted.

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…

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).

And also - thanks for the comments and questions!
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