3 ms·

The idea that if 4 lanes are not enough we just have to add more lanes, ad infinitum, until we eventually hit the upper limit were traffic won't increase anymor

The idea that if 4 lanes are not enough we just have to add more lanes, ad infinitum, until we eventually hit the upper limit were traffic won't increase anymore, has, to my knowledge, no empirical foundation. Current knowledge says that "on average, a 10 percent increase in lane miles induces an immediate 4 percent increase in vehicle miles traveled, which climbs to 10 percent – the entire new capacity – in a few years." https://en.wikipedia.org/wiki/Induced_demand https://en.wikipedia.org/wiki/Induced_demand

TeMPOraL 6y ago
On the other hand, taking existing empirical data and extrapolating linearly from it seems like a bad idea in this case. There are obvious limits to how much latent demand is there. IIRC, we never did a proper experiment of suddenly e.g. quintupling the capacity of some major congested road. That said, I think the source of this paradox is ultimately not in the road one is thinking of expanding, but in all the connections between it and other roads. It buys you little to turn one lane into five if all the exits are still the same capacity.
- bildung 6y ago
  While there obviously has to be an upper limit somewhere, this isn't (only) about latent demand, but induced demand: People actually drive more because they have to. More lanes have more consequences than wider streets alone, they enable sprawl, funnel funding away from other transport options etc. That has consequences for the housing market, average distances between work and home etc. This paper explains this quite well: https://link.springer.com/article/10.1007/BF00166218 https://link.springer.com/article/10.1007/BF00166218 - I've heard it is available on sci-hub. Your second point is very important though, and IIRC is the main reason why so many city planners are so critical of the boring company idea of underground car tunnels: It tries to solve an aspect of traffic that isn't the problem.
  - AnthonyMouse 6y ago
    > People actually drive more because they have to. More lanes have more consequences than wider streets alone, they enable sprawl, funnel funding away from other transport options etc. That has consequences for the housing market, average distances between work and home etc. You're really just saying the same thing as I did -- congestion suppresses demand. Suppose you have an uncongested two lane road. Making that a four lane road is not going to induce anything, because the widened road has more capacity than the original one, but you weren't even using the full capacity of the original one. It doesn't encourage anyone to do anything they hadn't already been able to. The reason it "enables sprawl" is that if the original road was congested, people wouldn't want to travel it to commute, so buying a house in the suburbs and commuting via that road is suppressed. Congestion suppresses demand. If you reduce the congestion by widening the road, some of the demand comes back. And notice that the same thing happens if you alleviate congestion in some other way -- if you somehow make it more attractive to take the subway and then more people do, there is less traffic on the roads and it becomes more attractive to drive. But you don't actually want the congestion. Traffic congestion sucks. You want to get rid of it somehow. Part of the solution could be expanding the road, but you could have to expand the road a lot to satisfy all of the demand that would exist with uncongested roads, if you want to use that as the only solution. Which is why you shouldn't. There are other things that reduce congestion. Like building higher density housing, so that more people can live closer to where they work and have a shorter commute, which in turn makes mass transit more viable. Then that in combination with widening some roads relieves the congestion. The real problem here is the politics of it. Increasing the housing stock sufficiently to satisfy the demand would tend to reduce housing costs, which existing property owners don't want. So they fight high density construction with restrictive zoning rules and then the high cost and low supply of housing in the city pushes people into the suburbs, which increases the demand for roads.
AnthonyMouse 6y ago
> Current knowledge says that "on average, a 10 percent increase in lane miles induces an immediate 4 percent increase in vehicle miles traveled, which climbs to 10 percent – the entire new capacity – in a few years." There is an obvious confounder here, which is that anywhere they add lane capacity is going to be a place undergoing growth, which is why they felt the need to add lane capacity. But if the growth the continues, the new lane capacity once again becomes insufficient at some future point. I would expect to see the same pattern with electrical transmission capacity. Where they add more capacity, you then see an increase in consumption. Which is to be expected, because why else would they add the capacity? The difference with roads is that they don't catch fire if you want until after you're over-capacity before expanding them, which makes it more likely that the demand will consume all of the new capacity, since you're starting off from the point of already being underwater.

TeMPOraL 6y ago

On the other hand, taking existing empirical data and extrapolating linearly from it seems like a bad idea in this case. There are obvious limits to how much latent demand is there. IIRC, we never did a proper experiment of suddenly e.g. quintupling the capacity of some major congested road. That said, I think the source of this paradox is ultimately not in the road one is thinking of expanding, but in all the connections between it and other roads. It buys you little to turn one lane into five if all the exits are still the same capacity.

bildung 6y ago
While there obviously has to be an upper limit somewhere, this isn't (only) about latent demand, but induced demand: People actually drive more because they have to. More lanes have more consequences than wider streets alone, they enable sprawl, funnel funding away from other transport options etc. That has consequences for the housing market, average distances between work and home etc. This paper explains this quite well: https://link.springer.com/article/10.1007/BF00166218 https://link.springer.com/article/10.1007/BF00166218 - I've heard it is available on sci-hub. Your second point is very important though, and IIRC is the main reason why so many city planners are so critical of the boring company idea of underground car tunnels: It tries to solve an aspect of traffic that isn't the problem.
- AnthonyMouse 6y ago
  > People actually drive more because they have to. More lanes have more consequences than wider streets alone, they enable sprawl, funnel funding away from other transport options etc. That has consequences for the housing market, average distances between work and home etc. You're really just saying the same thing as I did -- congestion suppresses demand. Suppose you have an uncongested two lane road. Making that a four lane road is not going to induce anything, because the widened road has more capacity than the original one, but you weren't even using the full capacity of the original one. It doesn't encourage anyone to do anything they hadn't already been able to. The reason it "enables sprawl" is that if the original road was congested, people wouldn't want to travel it to commute, so buying a house in the suburbs and commuting via that road is suppressed. Congestion suppresses demand. If you reduce the congestion by widening the road, some of the demand comes back. And notice that the same thing happens if you alleviate congestion in some other way -- if you somehow make it more attractive to take the subway and then more people do, there is less traffic on the roads and it becomes more attractive to drive. But you don't actually want the congestion. Traffic congestion sucks. You want to get rid of it somehow. Part of the solution could be expanding the road, but you could have to expand the road a lot to satisfy all of the demand that would exist with uncongested roads, if you want to use that as the only solution. Which is why you shouldn't. There are other things that reduce congestion. Like building higher density housing, so that more people can live closer to where they work and have a shorter commute, which in turn makes mass transit more viable. Then that in combination with widening some roads relieves the congestion. The real problem here is the politics of it. Increasing the housing stock sufficiently to satisfy the demand would tend to reduce housing costs, which existing property owners don't want. So they fight high density construction with restrictive zoning rules and then the high cost and low supply of housing in the city pushes people into the suburbs, which increases the demand for roads.

AnthonyMouse 6y ago

> Current knowledge says that "on average, a 10 percent increase in lane miles induces an immediate 4 percent increase in vehicle miles traveled, which climbs to 10 percent – the entire new capacity – in a few years." There is an obvious confounder here, which is that anywhere they add lane capacity is going to be a place undergoing growth, which is why they felt the need to add lane capacity. But if the growth the continues, the new lane capacity once again becomes insufficient at some future point. I would expect to see the same pattern with electrical transmission capacity. Where they add more capacity, you then see an increase in consumption. Which is to be expected, because why else would they add the capacity? The difference with roads is that they don't catch fire if you want until after you're over-capacity before expanding them, which makes it more likely that the demand will consume all of the new capacity, since you're starting off from the point of already being underwater.