Induced Traffic

Compiled by Darin Burleigh

Those of us who whine about road improvements maintain that expanding roads to accomodate a projected increase in traffic is a self-fulfilling prophecy: if you build it, they will come. This is referred to in the literature as 'induced traffic'. "Hey, look at all the new lanes on this road! I'm gonna drive on that one !" is how we envision the automobile-dependent society thinking to their collective self. Is there any truth to this viscious rumor?

The following is the result of a brief survey of the literature. Many thanks go to Tim & Scott from the bikies listserv, for pointing me in the right direction; and to Allison Dobbins & Nils Eddy from Ped-net, whose comments are included below (w/o permission).
For the reports I've been able to find, I added my laymen's interpretation or just quoted from the reports. As they say, your mileage may vary. In particular, much of these studies concern big freeways in California, while my immediate concern is a proposal for a much smaller urban area (Madison, WI). You can also find useful information here

Assessment of Induced Travel
A report prepared for the Transportation Planning Board (TPB) of MWCOG. Their conclusions are

The report is linked from a web site for Second Crossing.

Do New Roads Cause Congestion?
Hank Dittmar, STTP Executive Director
Progress, March 1998
A pretty good overview of the subject.

Widening may not make I-25 speedier Denver Post - Ricky Young.

"Minority Statement "
Michael Replogle,
Appendix E in Expanding Metropolitan Highways: Implications for Air Quality and Energy Use.
Special Report 245. Transportation Research Board. 1995.

"Review of Empirical Studies of Induced Traffic,"
Harry Cohen
Appendix B Expanding Metropolitan Highways: Implications for Air Quality and Energy Use. Special Report 245. Transportation Research Board. 1995.

Impacts of Highway Facility Improvements On Travel and Regional Development - Wisconsin TransLinks 21,
Charles H. Thompson, Secretray of Transportaion, Wisconsin
January 1994

Relationships between highway capacity and incuded vehicle travel (128 kb, PDF format)
Robert B. Noland
US EPA, Office of Policy, Paper no. 991069 (Nov 16, 1998)
Accepted for presentation at the 78th Annual Meeting of the Transportation Research Board, Jan. 1999
Accepted for presentation at the Annual Meeting of the Western Regional Science Association, Ojai, CA, Feb. 1999

Determining Generated Traffic External Costs
Todd Litman
available from the Victoria Transport Policy Institute Now available for free on the 'Net! (formerly, they charged for these reports).

"Transportation improvements can encourage more and longer trips, changes in travel patterns, and land use changes that require special consideration when assessing benefits and costs. Current transportation planning often fails to do this, resulting in incorrect conclusions. This paper summarizes current technical information on "generated" (or "induced") travel, describes how increased travel and related impacts should be assessed, and provides analysis tools for doing this."


An Analysis of the Relationship Between Highway Expansion and Congestion in Metropolitan Areas
Lessons from the 15-Year Texas Transportation Institute Study
This report by the Surface Transportation Policy Project (STPP) is an analysis of a study from the Texas Transportation Initiative (TTI). From their summary:

"By analyzing TTI's data for 70 metro areas over 15 years, STPP determined that metro areas that invested heavily in road capacity expansion fared no better in easing congestion than metro areas that did not. Trends in congestion show that areas that exhibited greater growth in lane capacity spent roughly $22 billion more on road construction than those that didn't, yet ended up with slightly higher congestion costs per person, wasted fuel, and travel delay. The STPP study shows that on average the cost to relieve the congestion reported by TTI just by building roads could be thousands of dollars per family per year. The metro area with the highest estimated road building cost was Nashville, Tennessee with a price tag of $3,243 per family per year, followed by Austin, Orlando, and Indianapolis."
This report received press coverage in the Washington Post, National Public Radio, San Jose Mercury News, and ContraCosta newspapers.

A good place to start is a recent issue of a publication put out by the Transportation Research Board and the National Research Council.
Highway Capacity Expansion and Induced Travel
Evidence and implications

Transportation Research Circular, No. 481, Feb. 1998
In addition to the four articles below, it also includes an Introduction and Summary of Discussion.

Accounting For Induced Travel In Evaluation Of Urban Highway Expansion
By Patrick DeCorla-Souza and Harry Cohen . This article describes a methodolgy, and includes a spreadsheet model. for estimating the effects of induced travel.

"Road Supply and Traffic in California Urban Areas"
Mark Hansen and Yuanlin Huang
Transportation Research, Part A, Vol. 31A, No. 3. (May, 1997)
60%-90% of increased road capacity is filled with new traffic within 5 years [NE].

"Do new highways generate traffic? "
Mark Hansen
Access : research at the University of Transportation Center no. 7 (Fall 1995), p. 16-22

"Air Quality Impacts of Urban Highway Capacity Expansion: Traffic Generation and Land Use Changes"
Mark Hansen, et al.
Research Report UCB-ITS-RR-93-5
Institute of Transport Studies, University of California at Berkeley, 1993.
Total vehicle travel increased 1% for every 2%-3% increase in highway lane miles [NE].

"The effects of new high-occupancy vehicle lanes on travel and emissions "
Robert A. Johnson and Raju Ceerla
Transportation research. Part A, v. 30A, no. 1 (Jan. 1996), p. 35-50.
As the title implies, the focus of this report is the effect on emissions, in particular, compliance with US EPA and the California Clear Air Act. They argue that current traffic models are deficient in a number of areas for predicting emissions. I found the following quotes enlightening:

"While the inducement of additional trips by new roadway capacity is difficult to accurately project, in general (Kitamura, 1991)) it is accepted that greater accessibiltity by auto increases auto ownership and auto tripmaking. Also, the construction of new facilities that extend into developing areas is likely to increase the share of new house starts that are single family and that, in turn, generally increases trips per household... The effects of higher travel speeds on the number of trips and on trip lengths can be projected with commonly used travel demand models."

and here is a meta-quote:
Sherret (1979) , in an interpretation of the BART reports, noted that "induced travel is a common phenonmenon... whenever an automobile route is heavily used..." (p. 14, cited in Sierra Club, 1990 p.8)

This tidbit is not referenced:
"Caltrans and official engineering bodies (Institute of Transportation Engineers, American Society of Civil Engineers, and Transportation Research Board) agree that we cannot build our way out of urban congestion anymore in the US."

And here they discuss other relevant reports:
"They [TRB, 1976] found that new highway capapcity attracts new auto travellers and was expensive and recommended transit development as moderate in cost, effective, and longlasting(p.4) The authors state that road congestions is self-limiting, especially in large urban areas. "
"An American Society of Civil Engineers Committee (1990) found that it is not practical to size freeways to handle peak-hour volumes (p.536)."
"An institute of Transportation Engineers survery (ITE, 1985) found that the most effective means of reducing traffic congestion were land use planning, transit, and vanpooling."
They found that the ranking of preferred projects changed significantly when generated traffic feedback is included. Specifically, capacity expansion options provide less congestion reduction benefit and increase air emissions, while demand management and no-build options offer greater benefits. [NE]

"The effects of added transportation capacity on travel: a review of theoretical and empirical results"
Ryuichi Kitamura

"Ryuicha Kitamura suggests that the addition of transportation capacity may have some significant long-range impacts on household automobile ownership, residence, and job location choices. Improved access to/from fringe areas provided by new or expanded service may promote the geographic expansion of an urban area, and eventually result in new levels and patterns of travel in an urban area (1991). He believes this development-inducing impact to be the most important impact, while the changes discussed earlier to be secondary because they may be small compared to the primary growth impact. He points out the difficulty in attempting to disaggregate the effects of transportation supply, land use, accessibility, and travel demand, which together form a dynamic and interrelated system."

1991 Conference on the Effects of Added Transportation Capacity, Bethesda, MD
USDOT, DOT-T-94-12 [published 1994?]
This report is also available as Institute for Transportation Studies report CD-ITS-RP-91-4, available from ITS. It is also in
Proceedings from Conference "Effects of Added Transportation Capacity", December 1991; Texas Transportation Institute, College Station, Texas.
Some report (possibly this one), by someone named Kitamura (possibly R. Kitamura) is included in the full report by Goodwin et. al.

"Evidence on the effects of road capacity reduction on traffic levels "
Phil B. Goodwin, Carmen Hass-Klau and Sally Cairns
Traffic engineering and control, v. 39, no. 6 (June 1998), p. 348-354
This is a summary of much larger study (see
below), whi ch was published in conjunction with a parallel study by MVA Limited. They studied over 100 locations where traffic capacity was reduced (e.g. to allow new construction, disaster, etc.) and evaluated its effect on the total traffic. The average was a 25% reduction in traffic. This is essentially the corallary to the 'induced traffic' effect, vis. if you unbuild it, they will go away. They list a number of caveats, notably that the data is purely empirical and no controlled studies were done. Also, the question remains, where did that 25% of traffic go? Presumably, car-pooling, other forms of transportation, reduced trips to avoid congestion, etc. They conclude:

"The balance of evidence is that measures which reduce or reallocate road capacity, when well-designed and favoured by strong reasons of policy, need not automatically be rejected for fear that they will inevitably cause unacceptable congestion.... Hence, the research results tend to support the view that an integrated transport policy should take account of the interaction between transport and other activities, as well as the interaction between different elements of the transport system itself."

An article on this report appeared in New Scientist. .

Traffic Impact of Highway Capacity Reductions: Assessment of the Evidence
Sally Cairns,Carmen Hass-Klau and Phil B. Goodwin
1998 - Landor Publishing, London

"Modelling the traffic impacts of highway capacity reductions"
Denvil Coombe, John Bates and Martin Dale
Traffic engineering and control, v. 39, no. 7/8 (July/Aug.1998), p. 430-433
This is a discussion of a much larger report, see below

Traffic Impact of Highway Capacity Reductions: Report on Modelling.
MVA Ltd.
1998 - Landor Publishing, London

"BART's First Five Years: Transportation and Travel Impacts", interpretive summary of the final report
Alistair Sherret
USDOT, Washington, DC

Sierra Club v. Metropolitan Transportation Commission
US District Court for the Northern District of California
No. C-89-2044 TEH and C-89-2064 TEH
Exhibit B to the declaration of Dr. P. R. Stopher in support of Sierra Club's objections to MTC's Proposed Conformity Assessment.

Peak-Period Traffic Congestion: Options for Current Programs.
R. Remak and s. Rosenbloom/Transportation Research Board (TRB)
NCHRP report No. 169, Washington DC

Gridlock - poll of engineers perceptions.
J. Transportation Engng v. 116, p. 532-549 (1990)
American Society of Civil Engineers (ASCE) Committee on Energy and Environmental Aspects of Transportation

"Automobile travel reduction options for urban areas"
Institute of Transportation Engineers Technical Committee 6A-26
ITE Journal p. 44-48 (1985)

1990 National Personal Transportation Survey: Summary of Travel Trends
US DOT, Federal Highway Administration

References cited in EPA study:
Coombe, Denvil, 1996, Induced traffic: what do transportation models tell us?, Transportation, 23: 83-101.
Goodwin, Phil B., 1996, Empirical evidence on induced traffic, a review and synthesis, Transportation, 23: 35-54.
Goodwin, Phil B., 1992, A review of new demand elasticities with special reference to short and long run effects of price changes, Journal of Transport Economics and Policy, 26: 155-169.
Hills, Peter J., 1996, What is induced traffic?, Transportation, 23: 5-16.
Mackie, Peter J., 1996, Induced traffic and economic appraisal, Transportation, 23: 103- 119.
SACTRA, 1994, Trunk roads and the generation of traffic, Department of Transport, Standing Advisory Committee on Trunk Road Assessment, London.
Transportation Research Board, 1995, Expanding metropolitan highways: implications for air quality and energy use, Special report 245, National Research Council, National Academy Press, Washington, DC.

"Md.'s Lesson: Widen the Roads, Drivers Will Come" By Alan Sipress, Washington Post Staff Writer, Monday, January 4, 1999; Page B1

Some things you are expected to pay for:

Transportation and Growth: Myth and Fact
Available from the Urban Land Institute