I’ll cherrypick from the intro & conclusion, but do read the whole thing if you have time (and do some google searching if the concept interests you):
>Traffic engineers often compare traffic to an uncompressible fluid, assuming that a certain volume must flow through the road system, but it is more appropriate to compare urban traffic
to a gas that expands to fill available space (Jacobsen 1997). Traffic congestion tends to maintain equilibrium: traffic volumes increase to the point that congestion delays discourage additional peak-period vehicle trips. Expanding congested roads attracts latent demand, trips from other routes, times and modes, and encourages longer and more frequent travel. This is called generated traffic, referring to additional peak-period vehicle traffic on a particular road. This consists in part of induced travel, which refers to absolute increases in vehicle miles travel (VMT) compared with what would otherwise occur (Hills 1996; Schneider 2018)
.
>Generated traffic reflects the economic “law of demand,” which states that a good’s consumption increases as its price declines. Roadway improvements that reduce the user costs
of driving (i.e., the price) encourage more vehicle travel. In the short-run generated traffic represents a shift along the demand curve; reduced congestion reduces travel time and vehicle operating costs. Over the long run it represents an outward shift in the demand curve as transport systems and land use patterns become more automobile dependent, so people must drive more to maintain a given level of accessibility to goods, services and activities (Lee 1999).
.
>This is not to suggest that increasing road capacity provides no benefits, but generated traffic affects the nature of these benefits. It means that road capacity expansion benefits consist more of increased peak-period mobility and less of reduced traffic congestion. Accurate transport planning and project appraisal must consider these three impacts:
>1. Generated traffic reduces the predicted congestion reduction benefits of road capacity expansion (a
type of rebound effect).
>2. Induced travel increases many costs, including user expenses, downstream congestion, crashes, parking costs, pollution, and other environmental impacts. Many of these costs are external and therefore inefficient and unfair.
>3. The additional travel that is generated provides relatively modest user benefits, since it consists of marginal value trips (travel that consumers are most willing to forego).
>Ignoring these factors distorts planning decisions. Experts conclude, “…the economic value of a scheme can be overestimated by the omission of even a small amount of induced traffic. We consider this matter of profound importance to the value-for-money assessment of the road programme” (SACTRA 1994). “…quite small absolute changes in traffic volumes have a significant impact on the benefit measures. Of course, the proportional effect on scheme Net Present Value will be greater still” (Mackie, 1996) and “The induced travel effects of changes in land use and trip distribution may be critical to accurate evaluation of transit and highway alternatives” (Johnston, et al. 2001)
Conclusion:
>Ignoring generated traffic results in self-fulfilling predict and provide planning: Planners extrapolate traffic growth rates to predict that congestion will reach gridlock unless capacity expands. Adding capacity generates traffic, which leads to renewed congestion with higher traffic volumes, and more automobile oriented transport and land use patterns. This cycle continues until road capacity expansion costs become unacceptable. The amount of traffic generated depends on specific conditions. Expanding highly congested roads with considerable latent demand tends to generate significant amounts of traffic, providing only temporary congestion reductions. Generated traffic does not mean that roadway expansion provides no benefits and should never be implemented. However, ignoring generated traffic results in inaccurate forecasts of impacts and benefits. Road projects considered cost effective by conventional analysis may actually provide little long-term benefit to motorists and make society overall worse off due to induced travel external costs. Other strategies may be better overall. Another implication is that highway capacity expansion projects should incorporate strategies to avoid increasing external costs, such as more stringent vehicle emission regulations to avoid increasing pollution and land use regulations to limit sprawl.
Both show complete ignorance of well-understood and uncontroversial concept of Induced Demand. Both **assume** economic benefits from a reduction in congestion, even though real world evidence is fairly conclusive that congestion won’t decrease.
Before complaining about Friends of the Irish Environment, environmentalists in general, or serial objectors – read all the links in this post and ask yourself why exactly professional planners in Galway County Council, the Department of Transport & An Bord Pleanala don’t know elementary half-century old transport planning concepts?
It’s not going to fix the traffic issues in the city.
Hopefully it succeeds.
All that open space to the south of the city, and they want to displace 500 people and demolish 40+ homes?
Scandalous! I wonder how many of those houses are after being purchased by TDs in the last few years?
.
.
.
[Only ttp in case anyone missed it.]
Fucking Greens
What other city is so utterly disconnected and split into two halves with so few crossings
They want to see Galway wither and die while building more infrastructure in their leafy D4 suburbs
6 comments
Nothing ever gets done in this country
I had just posted this on another thread about Dublin congestion, but it’s as relevant here: https://www.vtpi.org/gentraf.pdf
I’ll cherrypick from the intro & conclusion, but do read the whole thing if you have time (and do some google searching if the concept interests you):
>Traffic engineers often compare traffic to an uncompressible fluid, assuming that a certain volume must flow through the road system, but it is more appropriate to compare urban traffic
to a gas that expands to fill available space (Jacobsen 1997). Traffic congestion tends to maintain equilibrium: traffic volumes increase to the point that congestion delays discourage additional peak-period vehicle trips. Expanding congested roads attracts latent demand, trips from other routes, times and modes, and encourages longer and more frequent travel. This is called generated traffic, referring to additional peak-period vehicle traffic on a particular road. This consists in part of induced travel, which refers to absolute increases in vehicle miles travel (VMT) compared with what would otherwise occur (Hills 1996; Schneider 2018)
.
>Generated traffic reflects the economic “law of demand,” which states that a good’s consumption increases as its price declines. Roadway improvements that reduce the user costs
of driving (i.e., the price) encourage more vehicle travel. In the short-run generated traffic represents a shift along the demand curve; reduced congestion reduces travel time and vehicle operating costs. Over the long run it represents an outward shift in the demand curve as transport systems and land use patterns become more automobile dependent, so people must drive more to maintain a given level of accessibility to goods, services and activities (Lee 1999).
.
>This is not to suggest that increasing road capacity provides no benefits, but generated traffic affects the nature of these benefits. It means that road capacity expansion benefits consist more of increased peak-period mobility and less of reduced traffic congestion. Accurate transport planning and project appraisal must consider these three impacts:
>1. Generated traffic reduces the predicted congestion reduction benefits of road capacity expansion (a
type of rebound effect).
>2. Induced travel increases many costs, including user expenses, downstream congestion, crashes, parking costs, pollution, and other environmental impacts. Many of these costs are external and therefore inefficient and unfair.
>3. The additional travel that is generated provides relatively modest user benefits, since it consists of marginal value trips (travel that consumers are most willing to forego).
>Ignoring these factors distorts planning decisions. Experts conclude, “…the economic value of a scheme can be overestimated by the omission of even a small amount of induced traffic. We consider this matter of profound importance to the value-for-money assessment of the road programme” (SACTRA 1994). “…quite small absolute changes in traffic volumes have a significant impact on the benefit measures. Of course, the proportional effect on scheme Net Present Value will be greater still” (Mackie, 1996) and “The induced travel effects of changes in land use and trip distribution may be critical to accurate evaluation of transit and highway alternatives” (Johnston, et al. 2001)
Conclusion:
>Ignoring generated traffic results in self-fulfilling predict and provide planning: Planners extrapolate traffic growth rates to predict that congestion will reach gridlock unless capacity expands. Adding capacity generates traffic, which leads to renewed congestion with higher traffic volumes, and more automobile oriented transport and land use patterns. This cycle continues until road capacity expansion costs become unacceptable. The amount of traffic generated depends on specific conditions. Expanding highly congested roads with considerable latent demand tends to generate significant amounts of traffic, providing only temporary congestion reductions. Generated traffic does not mean that roadway expansion provides no benefits and should never be implemented. However, ignoring generated traffic results in inaccurate forecasts of impacts and benefits. Road projects considered cost effective by conventional analysis may actually provide little long-term benefit to motorists and make society overall worse off due to induced travel external costs. Other strategies may be better overall. Another implication is that highway capacity expansion projects should incorporate strategies to avoid increasing external costs, such as more stringent vehicle emission regulations to avoid increasing pollution and land use regulations to limit sprawl.
Having read that, have a look at the initial preliminary appraisal for the need for a Galway ring road here: http://www.n6galwaycity.ie/wp-content/uploads/2016/Phase%201/Phase%201%20Preliminary%20Appraisal.pdf
And the Environmental Impact Assessment Report Need for the Proposed Road Development here: http://www.n6galwaycityringroad.ie/media/N6%20EIAR%20-%20Chapter%203%20Need%20for%20the%20Proposed%20Road%20Development.pdf
Both show complete ignorance of well-understood and uncontroversial concept of Induced Demand. Both **assume** economic benefits from a reduction in congestion, even though real world evidence is fairly conclusive that congestion won’t decrease.
Before complaining about Friends of the Irish Environment, environmentalists in general, or serial objectors – read all the links in this post and ask yourself why exactly professional planners in Galway County Council, the Department of Transport & An Bord Pleanala don’t know elementary half-century old transport planning concepts?
It’s not going to fix the traffic issues in the city.
Hopefully it succeeds.
All that open space to the south of the city, and they want to displace 500 people and demolish 40+ homes?
Scandalous! I wonder how many of those houses are after being purchased by TDs in the last few years?
.
.
.
[Only ttp in case anyone missed it.]
Fucking Greens
What other city is so utterly disconnected and split into two halves with so few crossings
They want to see Galway wither and die while building more infrastructure in their leafy D4 suburbs