Wednesday, July 21, 2004

Induced Demand

Ahhh, "induced demand." Another perennial favorite of the transport willfully discordant. My take on the entire subject;

IIRC the range of non-consensus was from 3% to 10% of measured VMT on new capacity could be called "induced" but that this number went lower when it was realised that the predictions weren't good enough to account for 3%-5% accuracy in the model. Calling the component that came from unknown or modelling errors "induced" is not any more accurate than calling it anything else. Subtracting out the 3-5% unknown that frequently gets added on to "induced traffic" generation takes that 3-10% number lower. How much lower? Who knows, that is exactly the point.

"Induced traffic" isn't as important as many posters seem to believe; not because they don't see more traffic but because they can't identify which traffic is induced anymore than someone could predict fluid flow by looking at a single molecule of water. Modeling or measurement don't work that way, they breakdown when the phenomena being observed approaches the error level of the measurement.

Induced demand is not vehicle trips that realign to reduce congestion on surface streets, induced demand is what is left over after all the known sources of traffic are counted. Generally a few percent and not coincidentally about the margin of error for such studies. You've fallen for the transit advocacy lie based upon the discredited work of Hanson, Huang and others. The study most commonly used is the infamous, "Mark Hansen and Yuanlin Huang, "Road Supply and Traffic in California Urban Areas," Transportation Research A, Vol. 31, No. 3, 1997, pp. 205-218:

Every 1% increase in new lane-miles generated a 0.9% increase in traffic
in less than five years, which led Hansen and Huang to conclude that
"With so much induced demand, adding road capacity does little to reduce

Was this after controlling for secular growth, population growth and latent demand, or is the total growth? It's even less controlled than that. Let's look at the 3 largest regions of California (1996):

population Lane Miles DVMT
per 1000 per
persons person

Los Angeles 12.2 mill 2.1 21.6
San Franscisco/Oakland 3.9 mill 2.3 20.8
San Diego 2.6 mill 2.3 21.7

Ave of regions >500k pop 123.6 mill 3.3 21.4

See the problem? California urban areas have 50% fewer lane miles per person than the AVERAGE urban area. Can you say latent demand? Notice also that there is little difference in DVMT. Not as car crazy as most think eh?

A 1% increase of road capacity in any of these areas will be overwhelmed by a 5 year 15% increase in population and 20% increase in driving population and commensurate increase in the physical size of the urban areas and ...

There is so much latent demand in California's urban areas that any measure of induced demand is impossible. Indeed, Hansen and Huang finding only 90% after 5 years can easily be interpreted as evidence that increasing road capacity has a negative impact on induced demand!

When freeways get congested, trips go back to other arterials and neighborhood streets. When freeway capacity is expanded, much comes back. The result is a marked improvement in safety and speed. People forget that in an environment of existing latent demand limited access interstate class freeways are capable of the most capacity and that this impacts positively every component of a roads network. This means that sometimes the way to fix neighborhood congestion is to build a freeway someplace else.

Most likely you were exposed to the poisonous opinions of the Surface Transportation Policy Project. The STPP is an overtly agenda based organization engaged in a desperate search of supporting data and relentless media campaign. When the well regarded (but flawed) Texas Transportation Institute (TTI) failed to produce the answers they needed to promote their anti-highway agenda the STPP recalculated the TTI numbers. The most famous case being the TTI congestion index.

The STPP misuses the TTI data. The TTI calculated "congestion" measures are actually more properly called roadway utilization ratios. STPP knows this but it isn't in their interests to acknowledge the formula. This ranks places like LA with a very high road utilization because of density and regionalism as being very TTI Congested when in fact, LA's commute times are typical.

As mentioned above, the TTI is not perfect. When all you have is a hammer, everything starts looking like a nail. TTI is the only organization out there trying to quantify "congestion." So regardless of the quality of the baseline and cross regional comparisons; it is the only game in town.

An example of where the formula breaks down. A repeat of the snowstorm of '78 would today account for anywhere from a 6% to 20% reduction in Boston's congestion index.

The TTI -data- are fine. It's the -calculations- they perform that are highly suspect. For one, a place with bad weather is by that factor alone less "congested." Now maybe you think having only one lane open pushing 800 cars per hour in a Metro Boston snow emergency is uncongested like the TTI formula implies but I think the experience would prove otherwise.

They do a good job of bringing together FHWA and Census data in a uniform manner then sadly a few derived calculations, with the goal of a predetermined conclusion, are performed to justify their "solutions." TTIs "solutions" fly in the face of their own data because of this. Once you read and understand that their "congestion index" is a calculation and not a measurement then you can break out the actual data and perform more honest evaluations.

With respect to the myth of induced demand, you would have been better served with a more accurate assessment at:

Other references not necessarily still available included:

Without an acquaintance with the fundamental concepts and methodological issues associated with a full-cost framework, planners and policymakers will be unable to take the first steps towards a more comprehensive evaluation of alternative urban forms, and the policies and investments that cause them to occur. Some of the main concepts and issues explored in this report include the following:

- Costs are real economic resources used by a policy or project.
- Benefits are negative costs; costs are negative benefits.
- Benefits and costs must be defined in a way that is both comprehensive and mutually exclusive.
- Measuring all benefits and costs means considering some that do not have obvious market prices.
- A full-cost accounting framework must look at all impacts, both benefits and costs, that result from a defined change in the state of the world (in the case of this study, either a change in urban form or, more correctly, from a change in policy that attempts to change urban form).
- A full-cost accounting framework must consider all the people affected by the change. Many people may feel the change not just as residential consumers, but also in their capacities as employees of businesses and government.

"For policy makers faced with the controversial issue of induced travel, the critical issue is not whether highway capacity additions result in induced travel, but whether net societal benefits, after accounting for the external costs of induced travel, will exceed the public costs and social costs to be incurred in implementing the capacity addition"

The authors of this paper from the Federal Highway Administration web site clearly believe that induced traffic exists and the only question is to what degree. They argue it is minimal - but gets worse the more congestion exists. Several other studies using different assumption arrive at very different conclusions.

A personal favorite also from:

"A frequent statement advanced by transportation professionals
is that highway improvements, by inducing travel, create more
congestion they eliminate. Although few data exist to
support this statement, it has gained legitimacy by sheer

(A statement from 25 years ago. Still valid today.)

Induced demand is a real thing inasmuch there is a measurable increase within a transportation corridor associated with an increase in transportation capacity. Almost all of this is the existing unmet demand that justifies the expansion. Almost all of the rest is general growth preferentially going where there is capacity. The little bit left over is the part that truly is induced demand. 5% is fine and not worth refining further.

By now the myths of induced demand, global climate change, auto sprawl, conspiracies, transit benefits, subsidies should be part of everyone's worldview or at the very least all should be wary of parroting the latest Newsweek headline.

The future does not lie in "collective" anything. Balkanization and increasing fractionation are the rule.

There's only a very few controlling principles. One is transport choice. It affects several others, desire to be useful (work) and desire to live in a pleasant place and a desire to be as free as practical. There's a list for those issues as well.

1 comment:

Seb said...