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The situation is complicated by induced demand [0], the phenomenon in which providing more roadway capacity tends to raise demand for it by reducing the cost of

by pash 11y ago

The situation is complicated by induced demand [0], the phenomenon in which providing more roadway capacity tends to raise demand for it by reducing the cost of travel. What typically happens when a new roadway opens or a congested route is widened is that travel times first fall, as expected; but soon the reduced travel times attract drivers who previously had used other routes, or who had commuted at off-peak times, or who had used public transit. Thus the improved roadway ends up with significantly more traffic than the old roadway, and the price (in travel time) of the increased capacity is bid back up near its previous level. Added roadway capacity can lead to a welfare improvements, but it mostly comes in the form of fewer people shifting their schedules to beat rush-hour traffic or deigning to take public transit, and not through reductions in travel times. In actual fact, as we've seen throughout the era of American suburbanization, vast new roadways have probably substantially increased average travel times [1] by markedly changing patterns of land use (i.e., by giving rise to the modern low-density, car-dependent suburb), albeit while also engendering some welfare gains (e.g., increased suburban living space). If you want to prevent induced demand from gobbling up expected improvements in travel times when you build new roads, you pretty much either have to (a) decrease demand by deliberately increasing the financial cost of travel (e.g., through tolls or congestion charges), or (b) over-build roadway capacity in the extreme, to the extent that just about everyone in the region can fit on the roads at once, alone in his or her own car (as is the situation in cities like mine). 0. https://en.m.wikipedia.org/wiki/Induced_demand https://en.m.wikipedia.org/wiki/Induced_demand 1. I mean in comparison to a counterfactual alternative history in which the urban-suburban commuter freeway systems typical of American cities went unbuilt. Commute times in European metropolises, which mostly lack the extensive commuter roadways and concomitant suburban sprawl of their American counterparts, are broadly lower than in American metros of similar sizes, for example.

sandworm101 11y ago
That is true if the new road does is indeed the source of the new traffic. But it all falls apart if that future increase in traffic will be coming regardless of whether the road is built. that's where the paradox really breaks down. You cannot solve the problem of an external source of new traffic, be it a new stadium at the edge of town or new machine added to the edge of a network, by not building in any new capacity. Well, you could if you simply chose not to serve that new node. Not running any new roads to a new airport is a great way to get that airport to close, just as not serving that new customer on your network is a great way to make that customer go elsewhere. The other flipside to the pardox is that it really is applicable to any network. Sometimes the best way to improve the efficiency of mass transit is to cull bus/train routes. But try selling that in a modern city.
- derefr 11y ago
  To put the original paradox another way, though: if you run a road out of town far enough, somebody will build an airport at the end. The airport is indeed an "external source of new traffic", but it was built because the traffic system had enough unused capacity to absorb the projected load.
  - sandworm101 11y ago
    I;m not convinced that the people building stadiums and airports go around looking for cities with extra traffic capacity. They are responding to forces well outside local traffic issues. The fact that they will generate increased traffic is a factor during planning and construction, but it is certainly not in any way a driving force behind the decision to build.
cLeEOGPw 11y ago
You kind of imply that building a new road is useless if travel time stays the same. But increased traffic bandwidth IS improvement, even if individual drivers don't feel it. Also same thing applies to this paradox. Even if individual travel time is reduced for some, I doubt that overall traffic throughput is reduced. At worst case scenario it should stay the same.
- chipsy 11y ago
  That only appears to be the best option if you specify all trips to be made via private automobile. Road bandwidth is more efficient on bike or bus, but people will default to a car if the road is well suited and there is parking at the destination. It's not that the road is useless, it's that it perpetuates demand for driving and so can't solve congestion except by brute force saturation. It's one of the big shifts in planning thought to get away from a simple engineering problem of moving more cars faster and try to integrate different modes and give land use a more careful impact assessment.
  - cLeEOGPw 11y ago
    That assumes that people will change from a car to other options. At least where I live, that will not happen. They tried to do exactly what you said - instead of increasing size of roads, they broadened roads but made the new lanes only for public transport. The expected happened - everyone who used cars before, were using them after, and only thing that improved is people who ride bus reach destination a little faster. Same would be with bikes and other things - nothing will have impact, car throughput is basically only thing that matters in traffic congestion reduction.
    - chipsy 11y ago
      If you expand capacity in one mode but hold the other ones steady, you're still running into Braess' paradox. It doesn't matter that there is more bus capacity if people were tolerating the existing car congestion before. People will not switch until the roads are jam-packed. The paradox still applies when we consider the Tokyo rail network. There's lots of rail in Tokyo - it transports most of the commuters. Major lines are quite literally packed during rush hour. [0] Adding more rail will make the system even more popular, so the problem won't get better. OTOH consider congestion charges. When interviewed[1], individuals do not believe they were significantly affected by the introduction of a charge, even though overall traffic levels go down substantially, by one fifth in the example used of Stockholm. What we are changing when we change capacities is not speed or comfort, but a preference for what kind of congestion we get and how cheaply it will be filled. If our only goal is congestion reduction we should not be looking at the roads at all. The San Francisco area has plenty of car traffic, but more recently, within the past decade or so, the public transit has become extremely popular. This is not because the transit has gotten substantially better by quality or speed metrics. It is because the population is making more trips and longer ones, and the public transit systems were the last resort for capacity. Demand for new trips is in turn caused by available housing being located distantly from workplaces. If newcomers were able to live where they worked, congestion would drop significantly. [0] https://www.youtube.com/watch?v=pRBLnth4oSg https://www.youtube.com/watch?v=pRBLnth4oSg [1] https://www.youtube.com/watch?v=wC33HAq--x8 https://www.youtube.com/watch?v=wC33HAq--x8
mjevans 11y ago
The entire problem is that people look at the effect (Full roadways) and fail to see the related causes (the rent is too ed high (because we aren't building enough valid working and living (not just homes) spaces in the city)).

by pash 11y ago

sandworm101 11y ago

That is true if the new road does is indeed the source of the new traffic. But it all falls apart if that future increase in traffic will be coming regardless of whether the road is built. that's where the paradox really breaks down. You cannot solve the problem of an external source of new traffic, be it a new stadium at the edge of town or new machine added to the edge of a network, by not building in any new capacity. Well, you could if you simply chose not to serve that new node. Not running any new roads to a new airport is a great way to get that airport to close, just as not serving that new customer on your network is a great way to make that customer go elsewhere. The other flipside to the pardox is that it really is applicable to any network. Sometimes the best way to improve the efficiency of mass transit is to cull bus/train routes. But try selling that in a modern city.

derefr 11y ago
To put the original paradox another way, though: if you run a road out of town far enough, somebody will build an airport at the end. The airport is indeed an "external source of new traffic", but it was built because the traffic system had enough unused capacity to absorb the projected load.
- sandworm101 11y ago
  I;m not convinced that the people building stadiums and airports go around looking for cities with extra traffic capacity. They are responding to forces well outside local traffic issues. The fact that they will generate increased traffic is a factor during planning and construction, but it is certainly not in any way a driving force behind the decision to build.

cLeEOGPw 11y ago

You kind of imply that building a new road is useless if travel time stays the same. But increased traffic bandwidth IS improvement, even if individual drivers don't feel it. Also same thing applies to this paradox. Even if individual travel time is reduced for some, I doubt that overall traffic throughput is reduced. At worst case scenario it should stay the same.

chipsy 11y ago
That only appears to be the best option if you specify all trips to be made via private automobile. Road bandwidth is more efficient on bike or bus, but people will default to a car if the road is well suited and there is parking at the destination. It's not that the road is useless, it's that it perpetuates demand for driving and so can't solve congestion except by brute force saturation. It's one of the big shifts in planning thought to get away from a simple engineering problem of moving more cars faster and try to integrate different modes and give land use a more careful impact assessment.
- cLeEOGPw 11y ago
  That assumes that people will change from a car to other options. At least where I live, that will not happen. They tried to do exactly what you said - instead of increasing size of roads, they broadened roads but made the new lanes only for public transport. The expected happened - everyone who used cars before, were using them after, and only thing that improved is people who ride bus reach destination a little faster. Same would be with bikes and other things - nothing will have impact, car throughput is basically only thing that matters in traffic congestion reduction.
  - chipsy 11y ago
    If you expand capacity in one mode but hold the other ones steady, you're still running into Braess' paradox. It doesn't matter that there is more bus capacity if people were tolerating the existing car congestion before. People will not switch until the roads are jam-packed. The paradox still applies when we consider the Tokyo rail network. There's lots of rail in Tokyo - it transports most of the commuters. Major lines are quite literally packed during rush hour. [0] Adding more rail will make the system even more popular, so the problem won't get better. OTOH consider congestion charges. When interviewed[1], individuals do not believe they were significantly affected by the introduction of a charge, even though overall traffic levels go down substantially, by one fifth in the example used of Stockholm. What we are changing when we change capacities is not speed or comfort, but a preference for what kind of congestion we get and how cheaply it will be filled. If our only goal is congestion reduction we should not be looking at the roads at all. The San Francisco area has plenty of car traffic, but more recently, within the past decade or so, the public transit has become extremely popular. This is not because the transit has gotten substantially better by quality or speed metrics. It is because the population is making more trips and longer ones, and the public transit systems were the last resort for capacity. Demand for new trips is in turn caused by available housing being located distantly from workplaces. If newcomers were able to live where they worked, congestion would drop significantly. [0] https://www.youtube.com/watch?v=pRBLnth4oSg https://www.youtube.com/watch?v=pRBLnth4oSg [1] https://www.youtube.com/watch?v=wC33HAq--x8 https://www.youtube.com/watch?v=wC33HAq--x8

mjevans 11y ago

The entire problem is that people look at the effect (Full roadways) and fail to see the related causes (the rent is too ed high (because we aren't building enough valid working and living (not just homes) spaces in the city)).