In a recent review of the state-of-the-art, two planning researchers conclude that traffic forecasts often fail to accurately predict that demand for new transportation infrastructure.

Traffic forecasts play an important role in the planning and approval of new road and rail projects. They are important for determining both the expected economic and environmental project impacts, and therefore often end up being a highly debated item in the planning process.
In a new article published in Transport Reviews, two researchers from Aalborg University in Denmark show that decision makers should probably take the forecasts with a pinch of salt. After reviewing the largest studies that compare forecasts with actual demand, they conclude that "it is clear that demand forecast inaccuracy is problematic for all project types." The studies included in the review examined projects on all six continents from the 1960s to 2012.
The review confirms earlier results from individual studies that have shown road traffic to be underestimated on average and rail patronage to be overestimated on average. According to the researchers, both tendencies are problematic as planners tend to overestimate the congestion relief from new roads as well as the attraction value of new rail projects. However, even if forecasts were unbiased on average, the article argues that decision makers should be careful with relying too heavily on forecasts. Traffic on two projects could easily be 30% below the forecast for one and 30% above the forecast for another. In such a case there would be no bias, but the general lack of precision makes it problematic to assess individual projects.
The researchers argue that better monitoring is a first step in improving the usefulness of traffic forecasts. "It ought to be a key priority for funding institutions to implement better monitoring of project impacts, develop standardised archival procedures and make the data publically available, in order to facilitate more detailed studies of why travel demand forecasts are still often highly inaccurate."
FULL STORY: Ex-Post Evaluations of Demand Forecast Accuracy: A Literature Review

Planetizen Federal Action Tracker
A weekly monitor of how Trump’s orders and actions are impacting planners and planning in America.

Chicago’s Ghost Rails
Just beneath the surface of the modern city lie the remnants of its expansive early 20th-century streetcar system.

San Antonio and Austin are Fusing Into one Massive Megaregion
The region spanning the two central Texas cities is growing fast, posing challenges for local infrastructure and water supplies.

Since Zion's Shuttles Went Electric “The Smog is Gone”
Visitors to Zion National Park can enjoy the canyon via the nation’s first fully electric park shuttle system.

Trump Distributing DOT Safety Funds at 1/10 Rate of Biden
Funds for Safe Streets and other transportation safety and equity programs are being held up by administrative reviews and conflicts with the Trump administration’s priorities.

German Cities Subsidize Taxis for Women Amid Wave of Violence
Free or low-cost taxi rides can help women navigate cities more safely, but critics say the programs don't address the root causes of violence against women.
Urban Design for Planners 1: Software Tools
This six-course series explores essential urban design concepts using open source software and equips planners with the tools they need to participate fully in the urban design process.
Planning for Universal Design
Learn the tools for implementing Universal Design in planning regulations.
planning NEXT
Appalachian Highlands Housing Partners
Mpact (founded as Rail~Volution)
City of Camden Redevelopment Agency
City of Astoria
City of Portland
City of Laramie