Project at a Glance

Title: Effects of increased highway capacity on travel behavior

Principal Investigator / Author(s): Dowling, Richard G

Contractor: Dowling Associates

Contract Number: 92-325

Research Program Area: Economic Analysis

Topic Areas: Impacts, Mobile Sources & Fuels


Changes in highway capacity change travel behavior. Short term impacts may include changes in route choice, the time of day trips are made, mode choice, trip frequency, trip chaining (the way trips are linked together) and destination choice. Longer term impacts include changes in auto ownership, residential location, choice of workplace location, and land development patterns. These changes all occur against an ever shifting background of economic, demographic and transportation pricing changes affecting the population as a whole.

Although there has been considerable research and debate on this topic for many years, the complexity of the problem has frustrated any definitive answers - answers that are important because they affect our ability to predict impacts of new transportation improvements on the demand for facilities, congestion levels, vehicle miles of travel, and harmful vehicle emissions.

The purpose of this study has been to develop a better understanding of the effects of new high capacity on travel behavior, assess current travel forecasting model procedures against that enhanced understanding, recommend improvements to current understanding and identify areas for further research. This work gives the public a better understanding of the significance of this issue and transportation and air quality analysts a better sense of how to assess the induced travel impacts of new highway capacity.

A three-part approach has been used to accomplish this: the use of case studies of historic changes; the application of travel and economic theory to the problem; and a household survey of household travel behavior and activity patterns. The literature review and survey showed that changes in the time at which people make trips (trip scheduling changes) and the choice of the highway route driven are the two most important effects of increased capacity. A Dutch study found that these two effects account for over 90 percent of the observed increase in traffic volumes on a new freeway in a congested area. Changes in mode choice (transit versus auto), trip destination, and the frequency with which trips are made, turn out to be secondary effects of increased highway capacity. The household survey conducted as part of this study found that a five-minute time savings would cause survey respondents to make an extra stop or to change the destination for about 4 percent of the their trips.

The study concludes with recommendations for improvements to current Travel D modeling practices and for further research into the travel behavior effects of new capacity.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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