4 ms·

This is unintuitive and quite interesting. Whats your background?

cinntaile 5y ago
> urban planning background He just said it. The argument that I usually hear against more lanes is induced demand, but when I looked at some of the data this problem was very overblown. Going from 2 -> 3 or 3 -> 4. This is quite different from increasing 5 lanes to 6 though, then I can imagine it becomes chaotic and other problems such as the ones mentioned start to dominate.
- kqr 5y ago
  Well, the fixed parameter is people's tolerance for driving time. Everything else (including number of users) will scale up practically indefinitely. So if you give people more capacity, they will use it until they're back to their trip time tolerance, which is roughly at the same level of congestion. Is that not the case?
  - bobthepanda 5y ago
    It is, in the case where roads are free to use. Price is an extremely effective means of restoring flow to a congested system. Usually, how this works out is that a toll is placed on, say, an existing bridge to finance a parallel or upgraded replacement, and then magically traffic tends to disappear to the point where the new span is no longer needed. Of course, tolling untolled roads is extremely unpopular.
    - kqr 5y ago
      Right, of course! Price is a fantastic general way to reduce demand for limited resources so it matches supply. (It's also a great way to ensure the rich get richer, but that doesn't detract from your point.)
    - throwaway0a5e 5y ago
      >Price is an extremely effective means of restoring flow to a congested system. That's like saying famine is a great way of dealing with overpopulation. You're fundamentally taking a small subgroup (the trips that are justified despite the increased cost) and saying "look how good things are for them" while completely ignoring everything not in that subgroup. By that same (asinine) logic the SF housing market is great for homeowners. At best toll roads function as fuzzy rationing (and that isn't great either) where the masses only take the most critical trips.
      - bobthepanda 5y ago
        Roads don’t come for free, and user fees are a very widely used method of paying for them. Some of the trips brought on by induced demand is just shuffling around of economic activity that would’ve happened elsewhere. ——— You may not like that tolls can re-establish speeds on congested roads, but every solution has drawbacks, including building more free roads.
  - bluGill 5y ago
    No, because the limit isn't driving time, it is things they wany to do. Induced demand is people who can finally do something better. The only time you are right is when you are so far behind that people who used to stay home pick a second best thing because what they really want still isn't in their time tolerance. Note that building roads is in general not the rigjt answer, build good transit so they don't drive is much cheaper in the long run.
  - bobthepanda 5y ago
    As an addendum: Induced demand happens in all types of transportation, but cars are so space inefficient relative to other modes that it does not take a whole lot to get back to where you started, whereas the same cannot really be said of say a subway.
- jltsiren 5y ago
  There is also another mechanism. You can model traffic as something resembling a network flow problem. You have many individual drivers choosing routes, often using a greedy algorithm without full awareness of traffic conditions. If you add more capacity somewhere, some drivers will start making different choices. That could make existing bottlenecks worse, reducing overall throughput. On the other hand, if you have a good central traffic planner that gives everyone a route, adding more capacity will never reduce throughput. Some drivers may just get routes that are suboptimal from their perspective. This is the expected outcome of any system where people make individual choices.
- mrfusion 5y ago
  I wish we could put the induced demand idea to bed. There are a finite number of people so eventually more roads will help. There are a certain class of ideas that make people feel superior in conversation so they just stick around no matter how true.
  - throwaway0a5e 5y ago
    I don't take issue with the concept of induced demand but when people trot it out like it's a bad thing and is a reason not to build something it makes me want to buy a bulldozer and a welder and pay them a house call. Trips are a good thing. They're economic activity. They're people going to visit friends/family. Each trip is a case where the benefit outweighed the cost of the trip. The really infuriating part is that 99.999% (yes, 5 9s) of people who complain about induced demand on roads talk about it like it's a good thing when the context is literally any other form of infrastructure. You can induce demand for parks and subways by carpet bombing a city with them too or making the existing ones more attractive. The concept isn't just for public infrastructure. All sorts of business can induce a lot of demand if you plop it down in the right spot where a lot of potential customers were letting the need go unfilled because the opportunity cost of the next existing best option was too high.
    - ForHackernews 5y ago
      > Trips are a good thing. They're economic activity. Spoken like a true capitalist. Not all "economic activity" is good. Sitting in traffic driving to an out-of-town shopping centre to buy groceries because there are no local shops in walking distance may be economic activity, but I'd struggle to call these boring, wasteful trips I'm forced to make a good thing. More like an irritating waste of time and gas.
      - sokoloff 5y ago
        I have a neighborhood grocer in easy walking distance. The best way I could describe it is “imagine a tiny Whole Foods, except more expensive”. So, we drive to the neighboring, more suburban town to grocery shop.
- throwaway5486nv 5y ago
  I meant educational degree. What educational degree covers traffic analysis as part of the curriculum. I think civil engineering comes close but i am not sure.
rzzzt 5y ago
If you'd like to see similarly counterintuitive behavior in computer-land, check out Bélády's anomaly (increasing the number of pages in memory results in more page faults, not less for the FIFO page replacement strategy): https://en.wikipedia.org/wiki/B%C3%A9l%C3%A1dy%27s_anomaly https://en.wikipedia.org/wiki/B%C3%A9l%C3%A1dy%27s_anomaly

> urban planning background He just said it. The argument that I usually hear against more lanes is induced demand, but when I looked at some of the data this problem was very overblown. Going from 2 -> 3 or 3 -> 4. This is quite different from increasing 5 lanes to 6 though, then I can imagine it becomes chaotic and other problems such as the ones mentioned start to dominate.

kqr 5y ago
Well, the fixed parameter is people's tolerance for driving time. Everything else (including number of users) will scale up practically indefinitely. So if you give people more capacity, they will use it until they're back to their trip time tolerance, which is roughly at the same level of congestion. Is that not the case?
- bobthepanda 5y ago
  It is, in the case where roads are free to use. Price is an extremely effective means of restoring flow to a congested system. Usually, how this works out is that a toll is placed on, say, an existing bridge to finance a parallel or upgraded replacement, and then magically traffic tends to disappear to the point where the new span is no longer needed. Of course, tolling untolled roads is extremely unpopular.
  - kqr 5y ago
    Right, of course! Price is a fantastic general way to reduce demand for limited resources so it matches supply. (It's also a great way to ensure the rich get richer, but that doesn't detract from your point.)
  - throwaway0a5e 5y ago
    >Price is an extremely effective means of restoring flow to a congested system. That's like saying famine is a great way of dealing with overpopulation. You're fundamentally taking a small subgroup (the trips that are justified despite the increased cost) and saying "look how good things are for them" while completely ignoring everything not in that subgroup. By that same (asinine) logic the SF housing market is great for homeowners. At best toll roads function as fuzzy rationing (and that isn't great either) where the masses only take the most critical trips.
    - bobthepanda 5y ago
      Roads don’t come for free, and user fees are a very widely used method of paying for them. Some of the trips brought on by induced demand is just shuffling around of economic activity that would’ve happened elsewhere. ——— You may not like that tolls can re-establish speeds on congested roads, but every solution has drawbacks, including building more free roads.
- bluGill 5y ago
  No, because the limit isn't driving time, it is things they wany to do. Induced demand is people who can finally do something better. The only time you are right is when you are so far behind that people who used to stay home pick a second best thing because what they really want still isn't in their time tolerance. Note that building roads is in general not the rigjt answer, build good transit so they don't drive is much cheaper in the long run.
- bobthepanda 5y ago
  As an addendum: Induced demand happens in all types of transportation, but cars are so space inefficient relative to other modes that it does not take a whole lot to get back to where you started, whereas the same cannot really be said of say a subway.
jltsiren 5y ago
There is also another mechanism. You can model traffic as something resembling a network flow problem. You have many individual drivers choosing routes, often using a greedy algorithm without full awareness of traffic conditions. If you add more capacity somewhere, some drivers will start making different choices. That could make existing bottlenecks worse, reducing overall throughput. On the other hand, if you have a good central traffic planner that gives everyone a route, adding more capacity will never reduce throughput. Some drivers may just get routes that are suboptimal from their perspective. This is the expected outcome of any system where people make individual choices.
mrfusion 5y ago
I wish we could put the induced demand idea to bed. There are a finite number of people so eventually more roads will help. There are a certain class of ideas that make people feel superior in conversation so they just stick around no matter how true.
- throwaway0a5e 5y ago
  I don't take issue with the concept of induced demand but when people trot it out like it's a bad thing and is a reason not to build something it makes me want to buy a bulldozer and a welder and pay them a house call. Trips are a good thing. They're economic activity. They're people going to visit friends/family. Each trip is a case where the benefit outweighed the cost of the trip. The really infuriating part is that 99.999% (yes, 5 9s) of people who complain about induced demand on roads talk about it like it's a good thing when the context is literally any other form of infrastructure. You can induce demand for parks and subways by carpet bombing a city with them too or making the existing ones more attractive. The concept isn't just for public infrastructure. All sorts of business can induce a lot of demand if you plop it down in the right spot where a lot of potential customers were letting the need go unfilled because the opportunity cost of the next existing best option was too high.
  - ForHackernews 5y ago
    > Trips are a good thing. They're economic activity. Spoken like a true capitalist. Not all "economic activity" is good. Sitting in traffic driving to an out-of-town shopping centre to buy groceries because there are no local shops in walking distance may be economic activity, but I'd struggle to call these boring, wasteful trips I'm forced to make a good thing. More like an irritating waste of time and gas.
    - sokoloff 5y ago
      I have a neighborhood grocer in easy walking distance. The best way I could describe it is “imagine a tiny Whole Foods, except more expensive”. So, we drive to the neighboring, more suburban town to grocery shop.
throwaway5486nv 5y ago
I meant educational degree. What educational degree covers traffic analysis as part of the curriculum. I think civil engineering comes close but i am not sure.

rzzzt 5y ago

If you'd like to see similarly counterintuitive behavior in computer-land, check out Bélády's anomaly (increasing the number of pages in memory results in more page faults, not less for the FIFO page replacement strategy): https://en.wikipedia.org/wiki/B%C3%A9l%C3%A1dy%27s_anomaly https://en.wikipedia.org/wiki/B%C3%A9l%C3%A1dy%27s_anomaly