By Robert B. Noland
Induced travel occurs when new roads or lanes are built with the goal of reducing traffic congestion. What this means in practice is that new travel fills the new roads or lanes such that the goal of congestion reduction is not met. While many local factors can affect the magnitude of induced travel, the basic response is a simple matter of economic supply and demand. Increasing the supply of a road, or its capacity to move more vehicles, reduces the cost or amount of time needed to travel. This leads to an increase in demand to use that road, which can undermine the goal of congestion reduction. While there may be benefits to increasing travel, this is typically not the stated goal of most projects.
While this basic theoretical relationship has been well-known for decades and supported by empirical research, many methods to forecast growth in travel do not adequately account for induced travel. Thus, we have developed the New Jersey Induced Travel Calculator based on a similar model being used in California (Volker & Handy, 2022). The calculator provides a range of estimates of induced travel associated with increases in the lane-miles or capacity of a road. The calculator is designed to estimate induced travel for all road types in New Jersey and for all 21 counties, using empirical estimates developed and published in the literature.
The calculator also provides a way to check the quality of existing forecasts of increased travel. Forecasts of travel should be included in environmental assessments of specific projects, and for large projects an environmental impact report is usually required by the National Environmental Policy Act. These documents are intended to inform decisionmakers and the public of any adverse environmental impacts associated with a project and to investigate possible ways to mitigate those impacts. Often, these assessments do not consider the impacts of induced travel in their forecasts of traffic growth.
Induced travel is important to consider as any increase in vehicle travel will also increase greenhouse gas emissions, possibly undermining the state’s goal of reducing greenhouse gas emissions by 80% below 2006 levels by 2050, with an interim target of a 50% reduction by 2030 (New Jersey’s Priority Climate Action Plan, March 2024). Not accounting for induced travel can also make the benefits of a project seem better; that is, the congestion reduction will appear to be better than it actually is. The calculator includes an estimate of annual greenhouse gas emissions associated with a capacity expansion.
We see the induced travel calculator as a tool to assist transportation planners and the public to better assess the impacts of projects. The output from the calculator provides a range of estimates allowing planners and travel demand forecasters to better assess whether their forecasts are within expected ranges.
The calculator can be accessed here: Home | New Jersey Induced Travel Calculator (travel-calculator-henna.vercel.app)
References (see “About” section on the calculator website for a full list of references informing the development of the empirical estimates):
New Jersey’s Priority Climate Action Plan, March 2004, https://dep.nj.gov/wp-content/uploads/climatechange/nj_pcap_final-1.pdf
Volker, J., & Handy, S. L. (2022). Updating the Induced Travel Calculator. https://escholarship.org/content/qt1hh9b9mf/qt1hh9b9mf.pdf