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Avoiding Logical Fallacies in Planning

Our profession relies on logical analysis of accurate data. There are an amazing number of ways to go wrong.

Todd Litman | March 21, 2012, 11am PDT
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Our profession relies on logical analysis of accurate data. There are an amazing number of ways to go wrong.

One of my favorite Wikipedia chapters is Logical Fallacies, which systematically describes numerous ways that people can misunderstand, misrepresent and misinterpret analysis. This can be useful and amusing. During your next city council meeting or a public hearing you can use this list to mentally categorize illogical arguments: "The man with the red jacket just used a conjunction fallacy, and then the woman with the blue hat tried an inconsistent comparison followed by a straw man argument." This list can also be useful for critiquing your own arguments to insure that they can withstand informed scrutiny.

A related issue is the Dunning-Kruger effect, which refers to the tendency of people who are unaware of how little they know about a subject to be overly confident of their abilities and judgment. Research indicates that ignorant people often rate their knowledge and ability higher than it actually is, suffering from illusory superiority, while more knowledgeable people underrate their own abilities, suffering from illusory inferiority. 

A recent Calgary Herald editorial demonstrated this when it criticized a current regional travel survey, arguing that sufficient information is already available for transport planning. This shows severe ignorance about the many layers of complexity involved in transport planning decisions. The editorial overlooked the possibility that transportation means anything other than driving, that transport planning could have objectives other than congestion reduction, or that planning decisions may have undesirable unintended consequences. For example, roadway expansion often shifts congestion problems to new locations, expanding roadways to increase automobile traffic speeds can degrade walking and cycling access and induce additional vehicle travel and land use sprawl. It would be irresponsible to make such decisions without considering these impacts.

The Calgary Herald editorial is just one of many examples indicting that people, including many decision-makers, fail to appreciate the complexity of planning issues. We have recently seen a wave of policy makers choosing ignorance over knowledge, for example, when the U.S. federal government de-funded the National Household Travel Survey and the Canadian government ended the census long form, both well-established statistical series that provide invaluable support for countless planning decisions and research activities. We are left with gaps in information.

It is up to us, planners and our allies, to communicate the value of good data. We need to explain how such data are used, how they help solve problems and help decision-makers respond to people's needs, and the inefficiencies that can result if we lack such information.

For example, we could point out that travel survey data allows planners to determine the portion of traffic on a particular roadway that is traveling through the region, the portion of personal that originates from a particular part of the region and so might be diverted if public transit were improved on that corridor, and the portion of personal travel that originates nearby could be diverted to cycling if a bicycle path or route were developed. Survey data can help determine whether shopping trips are declining due to internet shopping, and the amount that Baby Boomers reduce their vehicle travel as they enter retirement age. We could also describe some of the interesting research questions that can be answered by travel surveys, such as the portion of residents that achieve physical fitness targets by walking and cycling, and how neighborhood design factors such as the quality of sidewalks and the proximity of stores, schools and parks affects how residents travel.

We need to put the value of this information into perspective. A few hundred thousand dollars for a better travel survey can help improve transport planning decisions that involve hundreds of millions of dollars in direct expenditures, which affect many billions of dollars in costs to residents and businesses. For example, a better understanding of the travel demands on an urban corridors (when, where, how and why people want to travel, and what factors affect those decisions) can help optimize investments in sidewalks, bicycle facilities, roads and public transit services: with better planning we can help consumers save time and money, be safer and healthier, and meet our accessibility needs if for any reason we cannot rely on automobile travel.

The demands on planners continually increase, so it is tragic when we are deprived of the most basic requirement for our job: logical analysis based on good information. This is an issue that is so sad and frustrating that it is funny. Let me know if you have any good examples of illogical thinking in your planning work.


For More Information

1000 Friends of Oregon (1999), "The Debate Over Density: Do Four-Plexes Cause Cannibalism" Landmark, 1000 Friends of Oregon (, Winter 1999.

Susan Beck (2004), The Good, The Bad & The Ugly: or, Why It's a Good Idea to Evaluate Web Sources, New Mexico State University Library  (

Dunning–Kruger effect (

David Huron (2000), Sixty Methodological Potholes, Ohio State University (; see appendix below.

Todd Litman (2005), Evaluating Research Quality, VTPI (; at . 

Wikipedia, Information Literacy ( discusses how to identify, locate, evaluate and effectively use that information for evaluating an issue. 

Wikipedia, Logical Fallacies (


Ad captandum   Ad hominem    Anangeon    Anecdotal evidence

Appeal to probability   Appeal to ridicule   Argument from beauty

Argument from setting a precedent   Argumentum ad baculum    Argumentum e contrario


Begging the question


Category mistake    Conditional probability    Confirmation bias   Motivated reasoning

Confusion of the inverse   Conjunction fallacy   Correlative-based fallacies


Deductive fallacy   Definist fallacy   Denying the correlative

Descriptive fallacy   Double counting (fallacy)


Ecological fallacy   Etymological fallacy


Fallacies of definition    Fallacy of distribution   Fallacy of four terms  

Fallacy of quoting out of context   False attribution   False dilemma   False premise


Greedy reductionism


Halo effect   Hasty generalization   Historian's fallacy

Historical fallacy    Homunculus argument


Idola fori   Idola theatri   Idola specus   Idola tribus   If-by-whiskey

Incomplete comparison   Inconsistent comparison   Inconsistent triad

Infinite regress   Intensional fallacy


Judgmental language


List of incomplete proofs   Ludic fallacy


Masked man fallacy   Mathematical fallacy  Meaningless statement   Moving the goalposts


Nirvana fallacy   No true Scotsman   Non sequitur (logic)          


One-sided argument


Package-deal fallacy     Parade of horribles   Pathetic fallacy   Poisoning the well

Politician's syllogism   Post disputation argument   Presentism (literary and historical analysis)

Pro hominem   Proof by assertion   Prosecutor's fallacy   Proving too much   Psychologist's fallacy


Regression fallacy   Reification (fallacy)   Relativist fallacy   Retrospective determinism


Spurious relationship   Straw man    Suggestive question   Sunk costs


Third-cause fallacy   Three men make a tiger    Trivial objections   Truthiness


Van Gogh fallacy


Wisdom of repugnance 


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