Induced travel refers to the increase in travel associated with increases in road capacity. This relationship occurs when new capacity, intended to reduce congestion, leads to changes in the relative travel time on the road network (reducing the cost of travel). This leads to various changes in when, where, and how people travel to take advantage of the new capacity. They also may change where they live and work.  It has been found that prior levels of congestion return fairly quickly. While transportation planners are well aware of this impact, most travel demand models do not properly measure the increase in vehicle-miles of travel from induced travel.

The Induced Travel Calculator can provide an estimate of the increased travel associated with increases in road capacity. The Calculator is based on “elasticities of travel” as estimated by many studies over the last 30 years. Users need to select the area (county or metro area, depending on the calculator) and the lane-miles for the type of road proposed for expansion. The output provided is a range of likely estimates of increased vehicle-miles of travel.

One can also estimate whether existing forecasts of increased vehicle-miles of travel for a project are realistic by entering that forecast amount into the “back-calculator.” The output provided is an “implied elasticity” which can be compared to those values reported in the literature.

While many road projects may be justified to account for increased mobility demands, planners need to recognize that justifying increased capacity on the basis of congestion reduction is flawed. New road capacity also leads to increased greenhouse gas emissions and other adverse environmental impacts. The Calculator provides an estimate of the annual increase in carbon emissions associated with increased road capacity.

Calculators currently available and produced by VTC include the following:

The New Jersey Induced Travel Calculator

as well as:

The Maryland Induced Travel Calculator
The Minnesota Induced Travel Calculator
The Pennsylvania Induced Travel Calculator

Full documentation is available at each site.  All code is available at: www.github.com/rbnoland

More calculators are currently under development. If your state is in need of one, please contact Bob Noland (rnoland@rutgers.edu).

Calculators developed elsewhere include:

The California Induced Travel Calculator – National Center for Sustainable Transportation, UC Davis
The Colorado Induced Travel Calculator – Rocky Mountain Institute
The SHIFT Calculator – State Highway Induced Frequency of Travel – Rocky Mountain Institute