Editing Induced demand (section)

== {{anchor|Transportation}} In transportation systems ==
===Definitions===
According to ''[[CityLab (web magazine)|CityLab]]'':

<blockquote>Induced demand is a catch-all term used for a variety of interconnected effects that cause new roads to quickly fill to capacity. In rapidly growing areas where roads were not designed for the current population, there may be significant latent demand for new road capacity, which causes a flood of new drivers to immediately take to the freeway once the new lanes are open, quickly congesting them again. {{parabr}}But these individuals were presumably already living nearby; how did they get around before the expansion? They may have taken alternative modes of transport, travelled at off-peak hours, or not made those trips at all. That’s why latent demand can be difficult to disentangle from generated demand—the new traffic that is a direct result of the new capacity. (Some researchers try to isolate generated demand as the sole effect of induced demand.)<ref name=benjamin /></blockquote>

The technical distinction between the two terms, which are often used interchangeably, is that latent demand is travel that cannot be realised because of constraints.  It is thus "pent-up".  Induced demand is demand that has been realised, or "generated", by improvements made to transportation infrastructure.  Thus, induced demand generates the traffic that had been "pent-up" as latent demand.<ref>[[Patricia Mokhtarian|Mokhtarian, Patricia L.]] (ndg) [http://www.dot.ca.gov/researchconn%/past_speakers%/DrMokhtarian/induced_demand_powerpoint.ppt "Understanding the Concept of Latent Demand in Traffic"] {{Webarchive|url=https://web.archive.org/web/20200611224559/https://www.dot.ca.gov/researchconn%/past_speakers%/DrMokhtarian/induced_demand_powerpoint.ppt |date=2020-06-11 }} State of California Department of Transportation</ref><ref>{{cite journal |last1=Clifton |first1=Kelly J. |last2=Moura |first2=Filipe |title=Conceptual Framework for Understanding Latent Demand: Accounting for Unrealized Activities and Travel |url=https://www.researchgate.net/publication/318693342 |journal=Transportation Research Record: Journal of the Transportation Research Board |date=January 2017 |volume=2668 |issue=1 |pages=78–83 |doi=10.3141/2668-08|s2cid=157228080 }}</ref><ref>Rodrigue, Jean-Paul (2016) [https://transportgeography.org/?page_id=186 "Transportation as a Derived Demand"] ''The Geography of Transport Systems''</ref><ref>{{cite book |last=Cervero |first=Robert |author-link=Robert Cervero |chapter-url=https://escholarship.org/content/qt5pj337gw/qt5pj337gw.pdf |chapter=Induced Demand: An Urban and Metropolitan Perspective |pages=55–73 |date=March 2001 |publication-date=2002 |title=Working Together to Address Induced Demand: Proceedings of a Forum |publisher=[[Eno Transportation Foundation]] |publication-place=Washington, DC |isbn=978-0971817548 }}</ref>

===History===
Latent demand has been recognised by road traffic professionals for many decades, and was initially referred to as "'''traffic generation'''". In the simplest terms, latent demand is demand that exists, but, for any number of reasons, most having to do with human psychology, is suppressed by the inability of the system to handle it.  Once additional capacity is added to the network, the demand that had been latent materialises as actual usage.<ref name=vanderbilt>Vanderbilt, Tom (2008) ''Traffic: Why We Drive the Way We Do (and What It Says About Us)'' New York; Knopf. pp.154-156. {{ISBN|978-0-307-26478-7}}</ref>

The effect was recognised as early as 1930, when an executive of a [[St. Louis, Missouri]], electric railway company told the Transportation Survey Commission that widening streets simply produces more traffic, and heavier congestion.<ref>''Report of the Transportation Survey Commission of the City of St. Louis'' (1930), p.109, cited in Fogelson, Robert M. (2001) ''Downtown: Its Rise and Fall, 1880–1950'' New Haven, Connecticut: Yale University Press. p.66. {{ISBN|0-300-09062-5}}</ref>  In New York, it was clearly seen in the highway-building program of [[Robert Moses]], the "master builder" of the [[New York City]] area. As described by Moses's biographer, [[Robert Caro]], in ''[[The Power Broker]]'': <blockquote>During the last two or three years before [the entrance of the United States into World War II], a few planners had&nbsp;... begun to understand that, without a balanced system [of transportation], roads would not only not alleviate transportation congestion but would aggravate it.  Watching Moses open the [[Triborough Bridge]] to ease congestion on the [[Queensboro Bridge|Queensborough Bridge]], open the [[Bronx-Whitestone Bridge]] to ease congestion on the Triborough Bridge and then watching traffic counts on all three bridges mount until all three were as congested as one had been before, planners could hardly avoid the conclusion that "traffic generation" was no longer a theory but a proven fact: the more highways were built to alleviate congestion, the more automobiles would pour into them and congest them and thus force the building of more highways &ndash; which would generate more traffic and become congested in their turn in an ever-widening spiral that contained far-reaching implications for the future of New York and of all urban areas.<ref>{{harvnb|Caro|1974|p=897}}</ref></blockquote>  The same effect had been seen earlier with the new [[parkways]] that Moses had built on [[Long Island, New York|Long Island]] in the 1930s and 40s, where <blockquote>... every time a new parkway was built, it quickly became jammed with traffic, but the load on the old parkways was not significantly relieved.<ref>{{harvnb|Caro|1974|p=515}}</ref></blockquote>  Similarly, the building of the [[Brooklyn–Battery Tunnel]] failed to ease congestion on the [[Queens-Midtown Tunnel]] and the three [[East River]] bridges, as Moses had expected it to.<ref>{{harvnb|Caro|1974|p=911}}</ref> By 1942, Moses could no longer ignore the reality that his roads were not alleviating congestion in the way he expected them to, but his answer to the problem was not to invest in mass transit, it was to build even more roads, in a vast program which would expand or create {{convert|200|mile|km|-2|abbr=off|sp=us}} of roads, including additional bridges, such as the [[Throgs Neck Bridge]] and the [[Verrazano Narrows Bridge]].<ref>{{harvnb|Caro|1974|pp=96–97}}</ref><ref name=suburban88>{{harvnb|Duany|Plater-Zyberk|Speck|2000|p=88}}</ref>

[[J. J. Leeming]], a British road-traffic engineer and [[county surveyor]] between 1924 and 1964, described the phenomenon in his 1969 book, ''Road Accidents: Prevent or Punish?'': <blockquote>Motorways and bypasses generate traffic, that is, produce extra traffic, partly by inducing people to travel who would not otherwise have done so by making the new route more convenient than the old, partly by people who go out of their direct route to enjoy the greater convenience of the new road, and partly by people who use the towns bypassed because they are more convenient for shopping and visits when through traffic has been removed.<ref>{{cite book|title=Road Accidents: Prevent or Punish?|author=Leeming, J. J.|year=1969|publisher=Cassell|isbn=0304932132}}</ref></blockquote> Leeming went on to give an example of the observed effect following the opening of the [[A1 road (Great Britain)#Doncaster bypass|Doncaster Bypass section of the A1(M)]] in 1961. By 1998, Donald Chen quoted the British Transport Minister as saying "The fact of the matter is that we cannot tackle our traffic problem by building more roads."<ref name=chen>Chen, Donald D. T. (March 1998) "If You Build It, They Will Come ... Why We Can't Build Ourselves Out of Congestion" ''Surface Transportation Policy Project Progress''; quoted in {{harvnb|Duany|Plater-Zyberk|Speck|2000|p=89}}</ref>

In [[Southern California]], a study by the [[Southern California Association of Governments]] in 1989 concluded that steps taken to alleviate [[traffic congestion]], such as adding lanes or turning freeways into double-decked roads, would have nothing but a cosmetic effect on the problem.<ref name=suburban88 /> Also, the [[University of California at Berkeley]] published a study of traffic in 30 California counties between 1973 and 1990 which showed that every 10 percent increase in roadway capacity, traffic increased by 9 percent within four years time.<ref name=chen /> A 2004 meta-analysis, which took in dozens of previously published studies, confirmed this. It found that: <blockquote>... on average, a 10 percent increase in lane miles induces an immediate 4 percent increase in vehicle miles travelled, which climbs to 10 percent &ndash; the entire new capacity &ndash; in a few years.<ref>Salzman, Randy (December 19, 2010) "Build More Highways, Get More Traffic" ''[[The Daily Progress]]'', quoted in {{harvnb|Speck|2012|p=82}}</ref></blockquote>  An aphorism among some traffic engineers is "Trying to cure traffic congestion by adding more capacity is like trying to cure obesity by loosening your belt."<ref>{{harvnb|Duany|Plater-Zyberk|Speck|2000|p=89}}</ref>

According to city planner Jeff Speck, the "seminal" text on induced demand is the 1993 book ''The Elephant in the Bedroom: Automobile Dependence and Denial'', written by Stanley I. Hart and Alvin L. Spivak.<ref name=speck />

=== Price of road travel ===
A journey on a road can be considered as having an associated cost or ''price'' (the [[generalised cost]], ''g'') which includes the [[Out-of-pocket expenses|out-of-pocket cost]] (e.g. fuel costs and [[road pricing|tolls]])<ref name="Didi">{{cite book |last1=Rodrigue |first1=Jean-Paul |title=The Geography of Transport Systems |date=2020 |publisher=New York: Routledge |url=https://transportgeography.org/contents/chapter3/provision-and-demand-of-transportation/elasticity-road-transport/}}</ref> and the [[opportunity cost]] of the time spent travelling, which is usually calculated as the product of travel time and the [[value of time|value of travellers' time]]. These cost determinants change often, and all have variable effects on demand for transport, which tends to be dependent on the reason(s) as well as method of travel.<ref name="Didi"/>

[[File:San Francisco-Embarcadero Freeway demolition.jpg|thumb|left|Part of the [[Embarcadero Freeway]] in San Francisco being torn down in 1991. The removal of the freeway illustrates the inverse of induced demand, "reduced demand".]]

When road capacity is increased, initially there is more road space per vehicle travelling than there was before, so congestion is reduced, and therefore the time spent travelling is reduced – reducing the generalised cost of every journey (by affecting the second "cost" mentioned in the previous paragraph). In fact, this is one of the key justifications for construction of new road capacity (the reduction in journey times).

A change in the cost (or price) of travel results in a change in the quantity consumed. Factors such as petrol prices, as well as fuel costs, are the most common variables that influence the quantity demanded for transport.<ref name="Didi"/> This can be explained using the simple [[supply and demand]] theory, illustrated in this figure.

==== Elasticity of transport demand ====

The economic concept of elasticity measures the change in quantity demanded relative to a change in another variable, most commonly price.<ref>{{cite news |last1=Hayes |first1=Adam |title=Elasticity |url=https://www.investopedia.com/terms/e/elasticity.asp |publisher=Investopedia |date=August 1, 2021}}</ref> For roads or highways, the supply relates to capacity and the quantity consumed refers to [[vehicle miles of travel|vehicle miles traveled]]. The size of the increase in quantity consumed depends on the [[elasticity of demand]].

The elasticity of demand for transport differs significantly depending on the reason people are choosing to travel initially. The clearest example of inelastic demand in this area is commuting, as studies indicate that most people are going to commute to work, regardless of fluctuations in variables such as petrol prices, as it is a required activity to generate income.<ref name="Didi"/> This exemplifies the fact that activities that yield a high economic benefit, in this case, financial gain in the form of income, tend to be inelastic. In contrast, travel for recreational or social reasons has a low tolerance for price increases, and as such the demand for recreational travel when prices spike sees a sharp decline.<ref name="Didi"/>

A review of transport research suggests that the elasticity of traffic demand with respect to travel time is around &minus;0.5 in the short term and &minus;1.0 in the long term.<ref>{{cite journal |journal=Transportation |year = 1996|volume=23 |pages=35–54|title=Empirical evidence on induced traffic: A review and synthesis|author=Goodwin, P. B.|doi=10.1007/BF00166218|s2cid = 154827067}}</ref>  This indicates that a 1.0% saving in travel time will generate an additional 0.5% increase in traffic within the first year. In the longer term, a 1.0% saving in travel time will result in a 1.0% increase in traffic volume.

=== Sources of induced traffic ===
In the short term, increased travel on new road space can come from one of two sources: diverted travel and induced traffic.  Diverted travel occurs when people divert their trip from another road (change in route) or reschedule their travel to avoid peak period congestion &ndash; but if road capacity is expanded, peak congestion is lower and they can travel at the time they prefer.

Induced traffic occurs when new automobile trips are generated.  This can occur when people choose to travel by car instead of public transport, or decide to travel when they otherwise would not have.<ref name="Litman">{{cite web |url=http://www.vtpi.org/gentraf.pdf |title=Generated Traffic and Induced Travel: Implications for Transport Planning |author=Litman, T. L. |year=2011}}</ref>

Shortening travel times can also encourage longer trips as reduced travel costs encourage people to choose farther destinations.  Although this may not increase the number of trips, it increases vehicle miles travelled.  In the long term, this effect alters [[land use]] patterns as people choose homes and workplace locations farther away than they would have without the expanded road capacity.  These development patterns encourage [[automobile dependency]] which contributes to the high long-term demand elasticities of [[road expansion]].<ref name="Litman"/>

=== Induced traffic and transport planning ===
Although planners take into account future traffic growth when planning new roads (this often being an apparently reasonable justification for new roads in itself – that traffic growth will mean more road capacity is required), this traffic growth is calculated from increases in car ownership and economic activity, and does not take into account traffic induced by the presence of the new road; that is, it is assumed that traffic will grow, regardless of whether a road is built or not.<ref name="Litman"/>

In the UK, the idea of induced traffic was used as grounds for protests against government policy of road construction in the 1970s, 1980s and early 1990s, until it became accepted as a given by the government as a result of their own Standing Advisory Committee on Trunk Road Assessment (SACTRA) study of 1994. However, despite the concept of induced traffic now being accepted, it is not always taken into consideration in planning.{{Citation needed|date=May 2025}}

=== Studies ===
[[File:The Vicious Cycle of Predict and Provide (cropped).png|thumb|The [[vicious cycle]] of predict and provide]]
A 1998 meta-analysis by the Surface Transportation Policy Project, which used data from the institute, stated that "Metro areas which invested heavily in road capacity expansion fared no better in easing congestion than metro areas that did not."<ref>{{harvnb|Speck|2012|p=83}}</ref>

On the other hand, a comparison of congestion data from 1982 to 2011 by the Texas A&M Transportation Institute suggested that additional roadways reduced the rate of congestion increase. When increases in road capacity were matched to the increase demand, growth in congestion was found to be lower.<ref>{{cite web |url=http://mobility.tamu.edu/ums/report/ |title=2012 Urban Mobility Report |author=David Schrank |author2=Bill Eisele |author3=Tim Lomax |date=December 2012 |publisher=Texas A&M Transportation Institute |access-date=May 14, 2013 |url-status=deviated |archive-date=2013-04-24 |archive-url=https://web.archive.org/web/20130424103215/http://d2dtl5nnlpfr0r.cloudfront.net/tti.tamu.edu/documents/mobility-report-2012.pdf}}</ref>

A study by [[Robert Cervero]], a professor of City and Regional Planning at the [[University of California, Berkeley]], found that "over a six-to eight-year period following freeway expansion, around twenty percent of added capacity is 'preserved,' and around eighty percent gets absorbed or depleted. Half of this absorption is due to external factors, like growing population and income. The other half is due to induced-demand effects, mostly higher speeds but also increased building activities. These represent California experiences from 1980 to 1994. Whether they hold true elsewhere is of course unknown."<ref>{{Cite web|url=http://americandreamcoalition-org.adcblog.org/highways/induced.pdf |access-date=April 5, 2017 |title=Are Induced-Travel Studies Inducing Bad Investments? |last=Cervero|first=Robert|date=Spring 2003|website=University of California Transportation Center |archive-url=https://web.archive.org/web/20190130110317/http://americandreamcoalition-org.adcblog.org/highways/induced.pdf|archive-date=2019-01-30|url-status=dead}}</ref>

And [[Patricia Mokhtarian|Mokhtarian]] et al. (2002) paired eighteen California state highway segments whose capacities had been improved in the early 1970s with control segments that matched the improved segments with regard to facility type, region, approximate size, and initial volumes & congestion levels. Taking annual data for average daily traffic (ADT) and design-hour-traffic-to-capacity (DTC) ratios during the 21 years 1976–1996, they found the growth rates between the two types of segments to be “statistically and practically indistinguishable, suggesting that the capacity expansions, in and of themselves, had a negligible effect on traffic growth”.<ref>{{Cite journal |vauthors=[[Patricia Mokhtarian|Mokhtarian PL]], Samaniego FJ, Shumway R, Willits NH |year=2002 |title=Revisiting the notion of induced traffic through a matched-pairs study |url=https://www.researchgate.net/publication/263187449|journal=Transportation|volume=29 |issue=2 |pages=193–220|doi=10.1023/A:1014221024304|s2cid=152745459}}</ref>

=== Policy implications ===
When evaluating induced demand traffic demand theoretically, consideration is mainly given to the actual amount of traffic that will arise from a certain scenario. In real world applications, policymakers must consider the benefits of new infrastructure with the potential negative impacts on the environment, public health, and social equity.

Carbon emissions have become a primary concern for policymakers in recent times and continues to be a consideration for infrastructure planning. An example of this is the [[Expansion of Heathrow Airport]], where hopes of additional runways would spur economic growth within the UK: increasing both the amount and frequency of direct flights.<ref>{{cite web|url=http://news.bbc.co.uk/2/shared/bsp/hi/pdfs/15_01_09decision_doc.pdf|title=Britain's Transport Infrastructure. Adding Capacity at Heathrow: Decisions Following Consultation|date=January 2009|website=bbc.co.uk|access-date=29 May 2023}}</ref> These expansion proposals posed climate concerns and prompted studies into its environmental viability. It was estimated by the government that such expansion plans would create 210.8 Mt (million tons) CO2 annually.<ref>{{Cite web |title=UK Government Web Archive |url=https://webarchive.nationalarchives.gov.uk/ukgwa/20100209050033/http://www.dft.gov.uk/pgr/aviation/heathrowconsultations/heathrowdecision/impactassessment/ia.pdf |access-date=2023-04-24 |website=webarchive.nationalarchives.gov.uk}}</ref> In addition, approximately 700 homes, a church, and eight listed buildings would have to be destroyed to make way for the project.<ref>{{Cite news |date=2006-02-21 |title=Village faces being wiped off map |language=en-GB |url=http://news.bbc.co.uk/2/hi/uk_news/4731948.stm |access-date=2023-04-24}}</ref> In 2020, the court of appeal ruled the expansion plans illegal due to the ministers’ lack of consideration towards the government’s commitments to climate change.<ref>{{Cite news |last1=Carrington |first1=Damian |date=2020-02-27 |title=Heathrow third runway ruled illegal over climate change |language=en-GB |work=The Guardian |url=https://www.theguardian.com/environment/2020/feb/27/heathrow-third-runway-ruled-illegal-over-climate-change |access-date=2023-04-24 |issn=0261-3077}}</ref>

In contrast to negative externalities, Bogotá, Colombia, has been recognized as a success story in managing induced demand for transportation by investing in new bike infrastructure. The city’s first bike path was established in 1974, with heavy investment in the late 1990s which eventuated in over 300 kilometers of bike lanes and dedicated bike paths.<ref>{{Cite web |last=City |first=Livable |date=2019-03-13 |title=The Beginning of Open Streets: Bogotá, Colombia Changes the Game |url=https://www.livablecity.org/the-beginning-of-open-streets-ciclovia-changes-the-game/ |access-date=2023-04-24 |website=Livable City |language=en-US}}</ref> This infrastructure has been credited with reducing traffic congestion through encouraging more people to bike as transport. Less traffic then directly leads to lesser emissions, improved air quality and healthier lifestyles for residents. In addition, the city has implemented additional policies such as a bike-sharing program, bike-friendly streets and education campaigns to promote biking as a healthy and sustainable mode of transportation.<ref>{{Cite web |last=Emblin |first=Richard |date=2022-09-30 |title=Bogotá joins world cities with bike-sharing mobility |url=https://thecitypaperbogota.com/bogota/bogota-joins-world-cities-with-bike-sharing-mobility/ |access-date=2023-04-24 |website=The City Paper Bogotá |language=en-US}}</ref>

=== Criticism ===

Critics of induced demand arguments generally accept their premise, but argue against their interpretation.  Steven Polzin, former director of the Center for Urban Transportation Research and former Senior Advisor at the US Department of Transportation, argues that most forms of induced demand are actually good things and that, due to changing transportation trends, past data cannot be applied to present circumstances.<ref>{{Cite web |last=Polzin |first=Steven |title=Examining the induced demand arguments used to discourage freeway expansion |url=https://reason.org/commentary/examining-the-induced-demand-arguments-used-to-discourage-freeway-expansion/ | date=2022-12-22 | access-date=2023-09-29 |website=[[Reason (magazine)|Reason]]|language=en-US}}</ref> Specifically, he argues:

# One type of induced demand is simply keeping up with population growth. This is a good thing.
# Another is traffic moving out of neighborhoods and onto newly expanded freeways. This is a very good thing.
# Another is people adjusting timing of trips to their desired timing, thus improving business efficiency and quality of life - both good things.
# Another is shifting transportation from non-auto transport to auto transport. Polzin does not argue that this is good, but rather that it's irrelevant (at least in a US context) as non-auto transport is such a small fraction of the total, and thus cannot meaningfully induce demand anymore (unlike in the past). By contrast, going in reverse would require unprecedented growth rates in public transport systems even just to keep up with population growth.
# Another is people taking trips to places that they wouldn't have gone before, such as shopping in new places or living further from work. Beyond arguments that this implies improved quality of life, while this appears to have been a major driver in induced demand in the past, it ignores trends. From 1980 to 2015 increases in road capacity in the US didn't even keep up with population growth, yet vehicle miles per capita doubled - a detachment between capacity growth and demand. But since the late 2000s, vehicle miles per capita have stagnated - and growing trends of [[telecommuting]] and [[e-commerce]] are likely to apply further downward pressure. I.e.: people don't drive further to shop or work if they're shopping or working from home either way.
# As personal road travel declines, commercial and service travel increases. This travel is not sensitive to road capacity and is not readily shifted to alternate modes of transportation.

Rather than limiting demand by reducing road capacity, Polzin argues for limiting demand via highway pricing, such as [[managed lane]]s, [[toll roads|toll highways]], [[congestion pricing]] or [[cordon pricing]], as this provides a revenue stream which can (among other things) subsidize public transportation.<ref>{{Cite web |last=Polzin |first=Steven |title=Examining the causes of induced demand and the future of highway expansion |url=https://reason.org/policy-brief/examining-the-causes-of-induced-demand-and-future-of-highway-expansion/ | date=2022-01-25 | access-date=2023-09-29 |website=Reason|language=en-US}}</ref>

Similar arguments have also been made by libertarian transportation policy analyst [[Randal O'Toole]],<ref>{{cite web|last=O'Toole|first=Randal |title=Debunking the Induced-Demand Myth |url=https://www.cato.org/blog/debunking-induced-demand-myth|date=2014-06-18|access-date=2023-11-18|website=[[Cato Institute]]|language=en-US}}</ref> economist William L. Anderson,<ref>{{cite web |last=Anderson |first=William L |url=https://www.pacificresearch.org/induced-demand-a-poor-excuse-not-to-build-highways/ |title='Induced demand' a poor excuse not to build highways |date=2023-05-04 |website=[[Pacific Research Institute]]}}</ref> transportation journalist and [[market urbanism|Market Urbanist]] director Scott Beyer,<ref>{{cite web|url=https://www.marketurbanist.com/blog/is-induced-demand-a-phony-theory|last=Beyer |first=Scott |date=August 18, 2018 |access-date=2023-11-18 |website=Market Urbanist |title=Is Induced Demand A Phony Theory?}}</ref> Professor of City and Regional planning Robert Cervero,<ref>{{cite web|url=https://www.accessmagazine.org/wp-content/uploads/sites/7/2016/07/Access-22-04-Induced-Travel-Studies1.pdf |title=Are Induced-Travel Studies Inducing Bad Investments? |website=Access |date=Spring 2003 |edition=22 |access-date=2023-11-18 |last=Cervero |first=Robert}}</ref> studies such as from WSP and Rand Europe,<ref>{{cite web |title=Department for Transport: Latest Evidence On Travel Demand: An Evidence Review |url=https://assets.publishing.service.gov.uk/media/5c0e5848e5274a0bf3cbe124/latest-evidence-on-induced-travel-demand-an-evidence-review.pdf |date=May 2018 |access-date=2023-11-18}}</ref> and numerous others.