Traffic modeling is guided by a series of professional and administrative guidelines.  In the case of the proposed $7.5 Interstate Bridge Replacement Project, IBR and Metro modelers did not follow or violated these guidelines in many ways as they prepared their traffic demand modeling.  IBR modelers:

• Didn’t assess accuracy of their previous modeling
• Failed to calibrate their models to observed traffic levels
• Failed to accurately account for capacity constraints
• Ignored other models and more accurate data that contradicted their conclusions
• Failed to exhibit scientific integrity
• Failed to document their data and methods
• Failed to commission an independent review of their analysis

Each of these errors constitutes a violation of professional standards for traffic forecasting, and invalidates the claims made the the IBR Draft Supplemental Environmental Impact Statement.

The Interstate Bridge Replacement (IBR) project’s traffic modeling efforts represent a significant departure from professional standards and federal guidelines, raising serious questions about the reliability of its projections and the integrity of its environmental review process. Sponsoring agencies are generally given substantial deference in their technical analyses, but such deference is completely unwarranted when the agencies systematically disregard or violate professional standards.

A detailed examination of the project’s modeling work reveals systematic failures to follow established professional practices, federal directives, and state guidelines.

Ignoring errors in previous forecasts

Perhaps most troubling is the project team’s failure to examine the accuracy of their previous forecasting efforts. Federal Highway Administration (FHWA) guidelines explicitly direct agencies to:

“. . . critically review past efforts to be aware of the prior work and to improve on or complement that work” before undertaking new projections.

FHWA,  Interim Guidance On The Application Of Travel AndLand Use Forecasting In NEPA, 2010, page 6. 

The National Academy of Sciences specifically recommends that agencies periodically report forecast accuracy, noting that such reporting “reveals any bias in the traffic forecasts” and “provides the empirical information necessary to estimate the uncertainty surrounding their traffic forecasts.” (National Academies of Sciences, Engineering, and Medicine. 2020. Traffic Forecasting Accuracy Assessment Research. page S-10).

IBR officials utterly failed to look at their prior modeling efforts and acknowledge or learn anything from their earlier failures.  The evidence is stark: While the CRC modeling predicted 1.5 percent annual traffic growth in vehicle traffic on the I-5 bridges between 2005 and 2030, actual growth  was just 0.3 percent annually through 2019, and 0.1 percent per year from 2005 through 2023.

Despite this significant disparity, the latest iterations of Metro and IBR models inexplicably predict that traffic growth will more than double to 0.63-0.79 percent annually through 2045 in the “No-Build” scenario. This represents a fundamental failure to learn from past mistakes and adjust methodologies accordingly as demanded by FHWA and NAS standards.

Metro and IBR models simply failed to look at whether their previous models were accurate; instead choosing to hide these forecast misses from the public.  The SDEIS fails to report that the forecasts that were contained in the 2008 Environmental Impact Statement have proven to be wrong by a factor of four.  This failure is a fundamental violation of professional standards.

Failure to calibrate the model to actual conditions

The modeling effort suffers from serious calibration issues. FHWA guidance emphasizes that traffic models must be accurately calibrated to real world values for use in NEPA analyses

In the context of a NEPA study, it is important for the study team to focus any calibration and validation efforts that they undertake on the study area. Typically, a regional travel demand model will have been adequately calibrated and validated at least at a regional level prior to adoption. While it is important for the study team to critically review the documentation of this effort, it is suggested that more emphasis be placed on checks at the study area level. It is suggested that the study team scale their calibration and validation effort according to the scale of the analysis, such as its geographic scope.

Calibration A meaningful calibration effort would include: . . .

• Comparison of modeled traffic volumes with traffic counts both for individual roadway segments and at more aggregate levels such as throughout the study area

Federal Highway Administration, Interim Guidance On The Application Of Travel And Land Use Forecasting In NEPA, March 2010, page 10 (emphasis added)

Metro’s own validation report reveals that their model overestimates daily traffic volumes on I-5 by tens of thousands of vehicles per day, and over estimates PM peak hour northbound traffic at the I-5 Bridge by 18 percent. This overestimation leads to exaggerated predictions of congestion and potentially misrepresents both the project’s benefits and environmental impacts.  These calibration errors are not disclosed the the DSEIS.

Not constraining “No-Build” forecasts to reflect highway capacity

Both Metro and IBR models predict physically impossible traffic flows, violating FHWA guidance on realistic capacity constraints. The FHWA Traffic Analysis Toolbox specifically warns that “care must be taken to ensure that forecasts are a reasonable estimate of the actual amount of traffic that can arrive within the analytical period.”  Federal guidance makes it clear that this is critical to the accuracy of the modeling.

While the I-5 bridges’ northbound capacity is approximately 5,000 vehicles per hour, the models estimate peak hour flows of 5,740 to 6,290 vehicles – 16 to 25 percent above capacity. These predictions continue to rise in the No-Build scenario, reaching 6,375 (Metro) and 6,905 (IBR) vehicles by 2045.  The IBR Traffic Technical Report fails to acknowledge that its estimates of peak hour traffic flows exceed the physical capacity of the roadway, in plain violation of federal guidance.

Deficient documentation and lack of transparency

The project’s modeling process operates as a “black box,” contradicting established professional standards. As NCHRP Report #765 emphasizes:  “Providing transparency in methods, computations, and results is essential.”

IBR and Metro have effectively concealed many of the details and limitations of their traffic forecasts.  Professional standards impose an affirmative responsibility on modelers to fully disclose this information.  Metro and IBR generally refused to provide detailed information about their traffic modeling, except when compelled to do so under public records laws.  Metro does not publish on its website the calibration report for its travel demand model, which shows that the model systematically over-states I-5 traffic levels.  As noted above, Metro and IBR models failed to disclose the errors in their earlier forecasts.

Providing transparency in methods, computations, and results is essential. . . . The analyst should document the key assumptions that underlie a forecast and conduct validation tests, sensitivity tests, and scenario tests—making sure that the results of those tests are available to anyone who wants to know more about potential errors in the forecasts.

National Cooperative Highway Research Project Report, “Analytical Travel Forecasting Approaches for Project-Level Planning and Design,” NCHRP Report #765

Federal regulations (40 CFR § 1502.23) explicitly require “scientific integrity” in environmental documents, mandating that agencies “shall identify any methodologies used and shall make explicit reference to the scientific and other sources relied upon for conclusions in the statement.”

Ignoring and omitting better models and more accurate data

Perhaps most egregiously, the IBR project has completely disregarded more precise modeling work it already paid for. In 2013, the states spent over $1.5 million commissioning CDM Smith to prepare a detailed “Level 3” traffic model for the Columbia River Crossing. This model achieved less than 1 percent variance with actual traffic levels – far better than Metro’s current 18 percent overestimation. In 2023, spent nearly $800,000 to have traffic modeler Stantec estimate traffic levels for the I-5 bridges.   The Stantec model was calibrated with about 2.5 percent variance from actual traffic levels.  Yet the IBR team has made no mention of these more accurate modeling efforts in the Supplemental Draft Environmental Impact Statement.

Not obtaining independent review

The U.S. Department of Transportation’s guidance is unequivocal about the need for independent review: “A senior-level peer review, internal and/or external, is therefore necessary… An external peer review by an independent third party can greatly improve its credibility with potential investors, lenders, government officials with oversight and approval responsibilities, and others.” Yet neither Metro nor IBR commissioned such a review.

Not verifying value of time assumptions

The IBR will be a tolled facility; accurately forecasting traffic levels on tolled roads and bridges depends directly on accurately estimating the value that travelers attached to time savings on tolled routes.  Guidance from the Federal Highway Administration directs states to document the reasonableness and reliability of their value of time estimates. (Federal Highway Administration, Interim Guidance On The Application Of Travel And Land Use Forecasting In NEPA, March 2010,  page 13).

Both the Metro “Kate” travel demand model and the Stantec Level 2 traffic and revenue study purport to predict traveler reactions to varying toll levels, but rely on questionable and largely un-documented assumptions about travelers’ value of time. Stantec’s own report acknowledges the speculative nature of their work, stating that “in many instances, a broad range of alternative assumptions could be considered reasonable… and any changes in the assumptions used could result in material differences in estimated outcomes.” Stantec explicitly disclaims liability for their assumptions, noting that “by their very nature, assumptions regarding information or data are accepted as true or certain to happen without actual proof of same.”  Because these value of time assumptions are undocumented, it is likely that the travel demand model substantially mis-estimates actual traffic patterns, invalidating the project’s claims about environmental impacts.

Implications

These systematic failures in traffic modeling raise fundamental questions about the Interstate Bridge Replacement project’s planning process and environmental review. The consistent pattern of overestimating traffic growth, ignoring capacity constraints, and failing to document key assumptions suggests that the project’s justification rests on questionable analytical foundations.

The project’s refusal to acknowledge past forecasting errors, examine more accurate existing models, or subject their work to independent review indicates a troubling lack of scientific rigor. These shortcomings not only violate federal guidelines and professional standards but also potentially mislead decision-makers and the public about the project’s necessity and impacts.

As the project moves forward, these modeling deficiencies must be addressed. At minimum, the SDEIS should incorporate the more accurate Level 2 and Level 3 modeling (like that done by CDM Smith and Stantec), properly document assumptions and methods, and submit their work for independent peer review. Without these basic steps, the project’s traffic projections – and the broader environmental review process that depends on them – lack the scientific integrity required by federal law.