– related to latent demand
and generated demand
– is the phenomenon that after supply increases, and there is sufficient demand, price declines and more of a good is consumed. This is entirely consistent with the economic theory of supply and demand
; however, this idea has become important in the debate over the expansion of transportation
systems, and is often used as an argument against increasing roadway traffic capacity as a cure for congestion. This phenomenon, more correctly called "induced traffic" or consumption of road capacity, may be a contributing factor to urban sprawl
. City planner Jeff Speck
has called induced demand "the great intellectual black hole in city planning, the one professional certainty that everyone thoughtful seems to acknowledge, yet almost no one is willing to act upon."
Effect in transportation systems
Induced demand is often used as a catch-all term for a variety of interconnected effects that cause new roads to quickly fill up to capacity. In rapidly growing areas where roads were not designed for the current population, there may be a great deal of 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 clogging them up again.
But these individuals were presumably already living nearby; how did they get around before the expansion? They may have taken alternative modes of transport, traveled at off 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).
The technical distinction between the two terms, which are often used interchangeably, is that latent demand is travel that cannot be realized because of constraints. It is thus "pent-up". Induced demand is demand that has been realized, or "generated", by improvements made to transportation infrastructure. Thus, induced demand generates the traffic that had been "pent-up" as latent demand.
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 materializes as actual usage.
The effect was recognized as early as 1930, when an executive of a St. Louis, Missouri
electric railway company told a Transportation Survey Commission that widening streets simply produces more traffic, and heavier congestion.
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' biographer, Robert Caro
, in The Power Broker
During the last two or three years before [the entrance of the United States into World War II], a few planners had...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 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 – which would generate more traffic and become congested in their turn in an ever-widening spiral that contained the most awesome implications for the future of New York and of all urban areas.
The same effect had been seen earlier with the new parkways that Moses had built on Long Island
in the 1930s and 40s, where
...every time a new parkway was built, it quickly became jammed with traffic, but the load on the old parkways was not significantly relieved.
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.
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 newly create 200 miles (300 kilometers) of roads, including additional bridges, such as the Throgs Neck Bridge
and the Verrazano Narrows Bridge
.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?
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.
Leeming went on to give an example of the observed effect following the opening of the 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."
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.
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.
A 2004 meta-analysis, which took in dozens of previously published studies, confirmed this. It found that:
...on average, a 10 percent increase in lane miles induces an immediate 4 percent increase in vehicle miles traveled, which climbs to 10 percent – the entire new capacity – in a few years.
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."
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.
Price of road travel
When supply shifts from S1 to S2, the price (explained below) drops from P1 to P2, and quantity consumed increases from Q1 to Q2
When road capacity is increased, initially there is more road space per vehicle traveling than there was before, so congestion is reduced, and therefore the time spent traveling 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. This can be explained using the simple supply and demand
theory, illustrated below.
Elasticity of transport demand
A review of transport research suggests that the elasticity of traffic demand with respect to travel time is around −0.5 in the short term and −1.0 in the long term.
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 retime their travel (change in timing). For example, people might travel to work earlier than they would otherwise like, in order to avoid peak period congestion – 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.
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 traveled. 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
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.
In the UK, the idea of induced traffic was used as a 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.
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."
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.
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."
And 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”.
Induced demand other than in traffic
are another example of induced demand. As more items of a certain product are available in the vending machine, consumers are more likely to acquire them.
Of course this example works if demand exists for the items and prices are at a level that ration items freely.
Reduced demand (the inverse effect)
Just as increasing road capacity reduces the cost of travel and thus increases demand, the reverse is also observed – decreasing
road capacity increases
the cost of travel, so demand is reduced. This observation, for which there is much empirical evidence, has been called disappearing traffic
also traffic evaporation
or traffic suppression
, or, more generally, dissuaded demand
. So the closure of a road or reduction in its capacity (e.g. reducing the number of available lanes) will result in the adjustment of traveler behavior to compensate – for example, people might stop making particular trips, condense multiple trips into one, re-time their trips to a less congested time, or switch to public transport
, walking, bicycling or smaller motor vehicles less affected by road diets
, such as motorcycles, depending upon the values of those trips or of the schedule delay
In 1994, the UK advisory committee SACTRA carried out a major review of the effect of increasing road capacity, and reported that the evidence suggested such increases often resulted in substantial increases in the volume of traffic.
Following this, London Transport
and the Department of the Environment, Transport and the Regions
commissioned a study to see if the reverse also occurred, namely that when road capacity was reduced, there would be a reduction in traffic. This follow-up study was carried out by Sally Cairns, Carmen Hass-Klau and Phil Goodwin, with an Annex by Ryuichi Kitamura, Toshiyuki Yamamoto and Satoshi Fujii, and published as a book in 1998.
A third study was carried out by Sally Cairns, Steve Atkins and Phil Goodwin, and published in the journal Municipal Engineer
The 1998 study referred to about 150 sources of evidence, of which the most important were about 60 case studies in the UK, Germany, Austria, Switzerland, Italy, The Netherlands, Sweden, Norway, the US, Canada, Tasmania and Japan. They included major town centre traffic schemes to make pedestrian areas
closed to traffic, bus priority
measures (especially bus lanes), bridge and road closures for maintenance, and closures due to natural disasters, mostly earthquakes. The 2002 study added some extra case studies, including some involving cycle lanes. The Annex by Kitamura and his colleagues reported a detailed study of the effects of the Hanshin-Awaji earthquake
Taking the results as a whole, there was an average reduction of 41% of the traffic flows on the roads whose capacity had been reduced, of which rather less than half could be detected as reappearing on alternative routes.
Thus, on average, about 25% of the traffic disappeared. Analysis of surveys and traffic counts indicated that the disappearance was accounted for by between 15 and 20 different behavioural responses, including changing to other modes of transport
, changing to other destinations, a reduction in the frequency of trips, and car-sharing. There was a large variation around these average results, with the biggest effects seen in large-scale pedestrianisation in German town centres, and the smallest seen in small-scale temporary closures with good alternative routes, and small reductions in capacity in uncongested streets. In a few cases, there was actually an increase in the volume of traffic, notably in towns which had closed some town centre roads at the same time as opening a new by-pass.
Cairns et al. concluded that:
...the findings reinforce the overall conclusion of the original study—namely, that well-designed and well-implemented schemes to reallocate roadspace away from general traffic can help to improve conditions for pedestrians, cyclists or public transport users, without significantly increasing congestion or other related problems.
The European Union have produced a manual titled "Reclaiming city streets for people"
that presents case studies and methodologies for traffic evaporation in urban areas.
An early example of the reduced demand effect was described by Jane Jacobs
in her classic 1961 book The Death and Life of Great American Cities
. Jacobs and others convinced New York City to close the street that split Greenwich Village
's Washington Square Park
in two, and also not to widen the surrounding streets to service the extra capacity they were expected to carry because of the closing of the street. The city's traffic engineers expected the result to be chaos, but, in fact, the extra traffic never appeared, as drivers instead avoided the area entirely.
Two widely known examples of reduced demand occurred in San Francisco, California
and in Manhattan, New York City
, where, respectively, the Embarcadero Freeway
and the lower portion of the elevated West Side Highway
were torn down after sections of them collapsed. Concerns were expressed that the traffic which had used these highways would overwhelm local streets, but, in fact, the traffic, instead of being displaced, for the most part disappeared entirely.
A New York State Department of Transportation
study showed that 93% of the traffic which had used the West Side Highway was not displaced, but simply vanished.
The argument is also made to convert roads previously open to vehicular traffic into pedestrian areas
, with a positive impact on the environment and congestion, as in the example of the central area of Florence
. In New York City, after Mayor Michael Bloomberg
's plan for congestion pricing
in Manhattan was rejected by the New York State Assembly
, portions of Broadway
at Times Square
, Herald Square
and Madison Square
were converted into pedestrian plazas, and traffic lanes in other areas taken out of service in favor of protected bike lanes, reducing the convenience of using Broadway as a through-route. As a result, traffic on Broadway was reduced, and the speed of traffic in the area lessened. Another measure instituted was the replacement of through-lanes on some of Manhattan's north–south avenues with dedicated left-turn lanes and protected bike lanes, reducing the avenues' carrying capacity. The Bloomberg administration was able to put these changes into effect as they did not require approval from the state legislature.
Despite the success of the Broadway pedestrian plazas in Manhattan, in general, pedestrian malls, in which all traffic is removed from shopping streets, have not been successful, leading to the conclusion that only particular areas – such as in college towns and resorts, which already have sufficient population density or pedestrian traffic – can successfully pursue this path.
Of the approximately 200 pedestrian malls created from the 1970s on, only about 30 remained as of 2012, and many of these were in poorer areas of their cities. The exceptions, including the Third Street Promenade
in Santa Monica, California
and 16th Street in Denver, Colorado
, are indicators that conversion of shopping streets to pedestrian malls can be successful, although the proper preconditions must exist. Some of the failed pedestrian malls have improved by allowing limited automobile traffic to return.
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- ^ a b Speck 2012, p. 80
- ^ Mokhtarian, Patricia L. (ndg) "Understanding the Concept of Latent Demand in Traffic" State of California Department of Transportation
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- ^ Salzman, Randy (December 19, 2010) "Build More Highways, Get More Traffic" The Daily Progress, quoted in Speck 2012, p. 82
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- ^ "Reclaiming city streets for people: Chaos or quality of life?" European Commission
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- ^ a b Speck 2012, pp. 97–99
Hart, Stanley I. and Spivak, Alvin L (1993). The Elephant in the Bedroom: Automobile Dependence and Denial; Impacts on the Economy and Environment
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- Giles Duranton, Matthew A. Turner (2010), The Fundamental Law of Road Congestion: Evidence from US cities, University of Toronto
- UK Department for Transport guidance on modelling induced demand
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- Todd Litman (2001), “Generated Traffic; Implications for Transport Planning,” ITE Journal, Vol. 71, No. 4, Institute of Transportation Engineers (www.ite.org), April, 2001, pp. 38–47.
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