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Old 12-22-2014, 06:08 PM
 
4,023 posts, read 3,265,973 times
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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.

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.
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Old 12-22-2014, 07:48 PM
 
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What about the fact that in order to widen a roadway more Right of Way is required. Buying right of way means homes and businesses have to move, often further away from each other. That increases demand for travel.
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Old 12-22-2014, 08:52 PM
 
4,023 posts, read 3,265,973 times
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Quote:
Originally Posted by pvande55 View Post
What about the fact that in order to widen a roadway more Right of Way is required. Buying right of way means homes and businesses have to move, often further away from each other. That increases demand for travel.
True. But even for roads without any homes or businesses on them (freeways) it doesn't accomplish anything
unless maybe you widened the entire stretch of the freeway, which is not financially feasible.

State highway widening projects are a popular thing to do but really pointless. Adding lanes to a two or three
mile stretch of highway here and there does practically nothing to ease traffic congestion, because of the
bottlenecks of the portions of the highway that were not widened. On a 100 mile long state or interstate highway you would
have to widen all 100 miles of it before you might hopefully start to see any gains in traffic flow. But they can't afford to do that so
they'll widen a couple miles here and there, which is an exercise in futility and rather pointless waste of money.
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Old 12-23-2014, 09:21 AM
 
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You can't argue with the tried and true method of reducing traffic congestion over the past 100 years... build bigger and wider!


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Old 12-23-2014, 09:23 AM
 
Location: East of Seattle since 1992, originally from SF Bay Area
29,789 posts, read 54,440,540 times
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True, but widening can do a lot for the traffic flow if the exits are long enough to absorb much of the backup created by the people getting off onto the surface streets. For congested interchanges, often it's most effective to create a 2+ lane flyover that is long enough to absorb many of the cars rather than have them simply exit and merge to create gridlock.
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Old 12-23-2014, 10:17 AM
 
Location: Vallejo
14,062 posts, read 16,078,369 times
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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.
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Old 12-23-2014, 10:44 AM
 
2,553 posts, read 2,004,793 times
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Quote:
Originally Posted by cisco kid View Post
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.

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.
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.

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.

Here's what happens with induced demand. Imagine there's a candy everyone in the world loves. This candy is produced by the government, and is given away for free to whomever asks for it. But, the supply is limited. It's in your best interest to consume as much as you are able--we're nowhere near marginal value being a problem, either. Well, the gov sees that all the candy is being consumed, so the rate of production is increased. But now everyone just tries to consume more candy, with nothing left over. The change in supply only served to increase consumption.
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Old 12-23-2014, 01:53 PM
 
Location: Seattle
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Plumbing or electrical circuits are poor analogies for traffic planning. Unlike water molecules or electrons, drivers possess free will. This is a recurrent problem in planning and engineering - those pesky humans act like they haven't heard about fluid dynamics or Ohm's law. The water flowing through a pipe doesn't have the benefit of a radio program telling them there's a clogged T-joint up ahead, any more than an electron can delay its flow through the light bulb out of choice.

The best explanation I've heard for the phenomena of new roads getting filled up right away is an "equilibrium of annoyance" model. People will fill the available infrastructure - roads, buses, trains - until they reach the point where the nth minute spent commuting is too annoying and/or too expensive. At that point they'll settle for the status quo, or else do something individually to break the paradigm. Move closer to work, carpool, change working hours, whatever - in other words, use that free will.

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.
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Old 12-23-2014, 02:17 PM
 
1,478 posts, read 2,002,048 times
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How I would characterize it is if an extra lane increases capacity say 50%, but if another area down the road can only handle 30% more traffic (and 100% of vehicles continent to that point), then you only get a 30% improvement. That's in simple terms. In complicated terms, you need to consider crowding effects, which don't apply to something fluid like plumbing the same way. As the road exceeds capacity, speeds drop precipitously. 50 percent over might result in a near standstill where cars are going 0 to 15 mph. 30% over capacity might still allow cars to go 30mph consistently.

Even with a bottleneck further down the road in the original example, you're trading an 8mph zone for a 30 mph zone, which is a huge difference. On a city street (as opposed to expressway) it gets more complicated. There is a cascade effect at each light and route decisions may be very different. This depends upon the nature of the grid (standard urban grid vs. broken suburban grid where everyone takes primary roads).

Highway engineers are really pretty good at understanding the flow impact of a project absent changes in behavior/future development due to the capacity increase. This is where they miss the mark. If you add more lanes, you encourage greater residential and employment dispersion This is what consumes the capacity increase. You can make an entire road network 20 percent more efficient initially through substantial investment. Assuming no population growth, what ends up happening over several years is that people drive 20% farther, so the network hits the same usage on a VMT basis. It might even be worse than it was originally. As people disperse, transit is less conducive to meeting needs, so this may actually force more people into cars.

IMO, this is the big road expansion problem. Areas paint themselves into a corner: no better congestion than before, with more infrastructure to maintain, with alternatives less viable than the were pre-investment.
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Old 12-23-2014, 02:25 PM
 
Location: Vallejo
14,062 posts, read 16,078,369 times
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Quote:
Originally Posted by Chicago76 View Post
How I would characterize it is if an extra lane increases capacity say 50%, but if another area down the road can only handle 30% more traffic (and 100% of vehicles continent to that point), then you only get a 30% improvement. That's in simple terms. In complicated terms, you need to consider crowding effects, which don't apply to something fluid like plumbing the same way. As the road exceeds capacity, speeds drop precipitously. 50 percent over might result in a near standstill where cars are going 0 to 15 mph. 30% over capacity might still allow cars to go 30mph consistently.

Even with a bottleneck further down the road in the original example, you're trading an 8mph zone for a 30 mph zone, which is a huge difference. On a city street (as opposed to expressway) it gets more complicated. There is a cascade effect at each light and route decisions may be very different. This depends upon the nature of the grid (standard urban grid vs. broken suburban grid where everyone takes primary roads).

Highway engineers are really pretty good at understanding the flow impact of a project absent changes in behavior/future development due to the capacity increase. This is where they miss the mark. If you add more lanes, you encourage greater residential and employment dispersion This is what consumes the capacity increase. You can make an entire road network 20 percent more efficient initially through substantial investment. Assuming no population growth, what ends up happening over several years is that people drive 20% farther, so the network hits the same usage on a VMT basis. It might even be worse than it was originally. As people disperse, transit is less conducive to meeting needs, so this may actually force more people into cars.

IMO, this is the big road expansion problem. Areas paint themselves into a corner: no better congestion than before, with more infrastructure to maintain, with alternatives less viable than the were pre-investment.
Induced demand isn't that sophisticated. It just says 10% more road capacity = 10% more vehicle miles traveled. It's just looking at the very high level. It's not even looking at per capita and changes in population. It's also complex with lots of variables that are not static. For example, I can tell you the Bay Area hasn't had much road capacity added of late. Traffic, however, is getting much worse. Population is up (mostly in fill) and more people are working. If you just looked at the Bay area from 2009 to 2014, you'd conclude that induced demand is more like a 1:5 ratio rather than 1:1. That is a 1% increase in roads leads to a 5% increase in traffic. That's your correlation causation error. It wasn't the increase in roads that caused traffic. It's that there's a greater population and more people working.
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