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I think this might explain it better than induced demand. I'm going to use a simple analogy.
Say you have a network of water pipes connected to each other in a grid pattern.
You want to make water flow faster so you replace one section of pipe with
a larger diameter pipe. Will the water flow faster? No. Because only one section of
the pipe has been widened. The pipes in the network that have not been widened create a bottleneck.
What about adding more pipes? Well if you take a drinking straw and made it longer does it
make the liquid flow any faster? No of course not.
Inappropriate analogies.
Lengthening a straw is not equivalent to "adding more pipes". Try adding a second straw or widening the straw and you will have greater flow.
Similarly, just because one doesn't widen all pipes in a network does not mean that widening one pipe alone will have zero effect on the flow. If that pipe was the bottleneck you just improved the flow when you replaced that pipe with a larger one (or multiple pipes).
Quote:
Originally Posted by cisco kid
Same idea with roads. Widening a major road does little or nothing to reduce traffic congestion because of the bottleneck created by the grid network of roads connected to it. Traffic basically flows as fast as the slowest road in the network. To really make traffic flow faster you'd have to widen or add more lanes to every major road in the entire city, but of course that is cost prohibitive. And even if you did that it wouldn't improve the flow because of induced demand.
This simply is not true particularly if the major road you widened was the bottleneck. Moreover you are assuming a closed system when the network isn't closed. Consult a textbook on linear programming/optimization.
Bad analogy. Road networks are not like water systems. Water systems generally have one or a few entry points, which flows outward to many end points. Road networks are much more complex, with many entry and exit points.
Water systems can and do have several entry points but that's that's irrelevant to the point.
Widening one pipe does little or nothing to improve water flow unless all pipes in the system are widened.
Generally what happens is places that are growing build roads. Since they don't build roads faster than they grow, the congestion doesn't get any better. If you're some wonky third world dictator who builds a super road to a capital that nobody lives in, that's slightly different and you end up with what impala pictured. Generally that doesn't happen, however. It's very rare for road capacity to outpace demand for road capacity.
It's not rare in certain midwestern and rust belt areas. Roads were built under the assumption of growth that never occurred.
As for induced demand, just because some vehicle growth is from population growth doesn't mean it's not a real feature, but perhaps hard to isolate. It's not that unreasonable to assume less congestion would encourage more people to take more trips (if one could create a highway making it much faster for Long Islanders to commute to New Jersey some would take New Jersey jobs and it would likely get congested going back to the same situation as before but with more intra-metro trips) . But there must be a practical limit of how much people would travel congestion or no congestion. And any induced trips from residential-induced sprawl or commuting to further jobs would time — people don't change jobs or especially move overnight.
It's not rare in certain midwestern and rust belt areas. Roads were built under the assumption of growth that never occurred.
As for induced demand, just because some vehicle growth is from population growth doesn't mean it's not a real feature, but perhaps hard to isolate. It's not that unreasonable to assume less congestion would encourage more people to take more trips (if one could create a highway making it much faster for Long Islanders to commute to New Jersey some would take New Jersey jobs and it would likely get congested going back to the same situation as before but with more intra-metro trips) . But there must be a practical limit of how much people would travel congestion or no congestion. And any induced trips from residential-induced sprawl or commuting to further jobs would time — people don't change jobs or especially move overnight.
Induced demand coming from population growth IS induced demand. Induced demand isn't limited to just the existing population drives more. The trend of ever increasing VMT per capita ended before the recession even began.
Lengthening a straw is not equivalent to "adding more pipes". Try adding a second straw or widening the straw and you will have greater flow.
Yes you will get more flow by adding more straws, but not necessarily faster flow unless you widened out all the other straws in the network. Which is fairly cheap and easy to do with straws and pipes, but to widen every arterial road in a city grid is cost prohibitive. Well actually replacing every water pipe in a house wouldn't be cheap and easy either.
There are numerous studies that show that widening roads or increasing capacity does not solve traffic problems but makes them worse. Andres Duany had a series of videos illustrating examples from across the country about 10 years ago. They can be found on YouTube. Here is a recent piece which sums it up nicely.
There are numerous studies that show that widening roads or increasing capacity does not solve traffic problems but makes them worse. Andres Duany had a series of videos illustrating examples from across the country about 10 years ago. They can be found on YouTube. Here is a recent piece which sums it up nicely.
Great article. I can attest that driving on the 405 is just as bad now if not worse than
it was 10 years ago despite years and billions spent on widening it.
I was just visiting recently, but sure am glad I don't have to commute there.
Freeway traffic especially between LA and OC is an absolute nightmare.
Water systems can and do have several entry points but that's that's irrelevant to the point.
Widening one pipe does little or nothing to improve water flow unless all pipes in the system are widened.
It's very relevant. Because roads have so many entry and exit points, widening one section can induce more demand on that section, even if other sections are unimproved.
Plumbing or electrical circuits are poor analogies for traffic planning. Unlike
It's not a static model - peoples' tolerances can change over time to be sure, but the tale of BART is (IMO) instructive. I lived in SF while BART was a big hole in the middle of Market Street, and I can remember clearly all the ballyhoo about how BART would make commuting on the Bay Bridge so much easier, as all those cars would be replaced by happy commuters traveling in sleek trains under the bay.
Well, not so much. All BART did was free capacity on the Bay Bridge (and on the Nimitz and on I-80 and other roads) so that many other people, with new homes in places like Concord or Walnut Creek, could fill it back up until they reached their own points of equilibrium. Result: full roads and more of them, increased population on the edges of the conurbation, more congestion than the sum of the original parts.
Sometimes more is not better, it's just more.
I remember an environmental group actually opposed BART, largely for those reasons. They surmised it would bring so much business in, attracting more traffic. And certainly encouraged building at the far edges of the lines, not just Concord or Walnut Creek, which still require driving for the majority of trips that are not commutes.
The theory of induced demand says that if you build more roads, you end up with more cars traveling on it. It's completely contrary to your explanation.
Generally what happens is places that are growing build roads. Since they don't build roads faster than they grow, the congestion doesn't get any better. If you're some wonky third world dictator who builds a super road to a capital that nobody lives in, that's slightly different and you end up with what impala pictured. Generally that doesn't happen, however. It's very rare for road capacity to outpace demand for road capacity.
True, but its also true that capacity will never keep up with the voracious, infinite demands of the
automobile. The more you build, the more demand it creates. You'll be forever widening and building roads
in perpetuity in a neverending, resource draining vicious cycle. Doesn't sound like a very sustainable nor
environmentally friendly solution.
Quote:
Originally Posted by darkeconomist
It's very relevant. Because roads have so many entry and exit points, widening one section can induce more demand on that section, even if other sections are unimproved.
Quote:
Originally Posted by darkeconomist
Bad analogy. Road networks are not like water systems. Water systems generally have one or a few entry points, which flows outward to many end points. Road networks are much more complex, with many entry and exit points.
Right, and that's what an analogy is for. It helps us to visualize very complex things in simpler terms
that we can better understand. It's not going to be perfect, but no analogy is.
Quote:
Originally Posted by darkeconomist
Congestion, while problematic, isn't itself a problem, it's a symptom or a characteristic. Congestion is the natural result of many people placing value on a resource that is so deeply underpriced that the only way to value it is in terms of time and hassle. It's "free", so we're faced with the tragedy of the commons. The pricing, or lack thereof, of that resource is the true problem. Price that resource based upon demand and the congestion subsides--it doesn't go poof in some magical way, it recedes--as people then have an accurate way to value their trips.
I'm all for congestion pricing. But you have to give people alternatives to driving along with that.
Good transit, walkable urban areas and neighborhoods, etc. Or they wont have a choice but to drive,
and will bite the bullet on the congestion charges.
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