The Case for Moderate Growth in Vehicle Miles of Travel: A Critical Juncture in U.S. Travel Behavior Trends

Polzin, Steven Edward · 2006 · ROSA P / University of South Florida. Center for Urban Transportation Research

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Summary

This report, prepared for the U.S. Department of Transportation by Steven E. Polzin of the University of South Florida’s Center for Urban Transportation Research, addresses the hypothesis that the United States has reached a critical juncture in national mobility trends. The central research question concerns whether underlying socio-demographic conditions and travel behavior shifts will result in more moderate rates of annual vehicle miles of travel (VMT) growth in the future. The study was motivated by the need to inform transportation planning and policymaking amidst changing trends in congestion, demographics, and economic conditions. It specifically investigates whether slowing VMT growth will translate to reduced congestion, noting that non-linear relationships between volume and speed may complicate this outcome. The methodology involves a comprehensive review of empirical trend data from 1977 to 2001, utilizing sources such as the National Household Travel Survey (NHTS) and Federal Highway Administration (FHWA) databases. The analysis categorizes VMT drivers into indirect factors (socio-economic conditions, land use, and transportation system conditions) and direct drivers (trip rates, trip length, and mode choice). Key socio-economic variables examined include population age profiles, household size, labor force participation, auto availability, licensure rates, and real income. The report identifies several historic trend reversals or stabilizations, such as stabilizing average household sizes, female labor force participation, and zero-vehicle household shares, alongside the aging of the baby boom generation through its peak travel years. Additionally, the study notes that average travel speeds are declining due to congestion, which may further dampen VMT growth if travelers are unwilling to increase time spent traveling. To forecast future demand, the report develops two scenario models for VMT growth through 2025. Formula 1 calculates VMT based on population, person trips per person, person miles per trip, and vehicle miles per person mile. Assuming a 22% population growth, a 16% increase in trip rates, an 8% increase in trip length, and a 5% increase in vehicle miles per person mile, this scenario projects a total VMT growth of 60% (approximately 2% per year). Formula 2 calculates VMT based on population, travel time budgets, and VMT per person hour of travel. Assuming a 35% increase in travel time budgets and an 8% decline in VMT per person hour (reflecting slower speeds and mode shifts), this scenario projects a 51% total VMT growth (approximately 1.74% per year). Both scenarios represent significantly lower growth rates than the 151% increase observed between 1977 and 2001. The significance of these findings lies in the implication that while VMT growth may moderate, congestion challenges will persist. The report concludes that because much of the roadway system is already at or near capacity, even modest increases in demand can lead to disproportionate declines in travel speed. Furthermore, the study highlights that land use impacts on travel behavior and the continued growth in per capita travel time budgets remain poorly understood, representing weak links in forecasting accuracy. The report suggests that transportation planners must account for these stabilizing demographic trends and the potential for unanticipated phenomena, such as fuel price increases, which could further dampen demand. Ultimately, the findings urge a reevaluation of long-range travel demand forecasts to better align infrastructure planning with likely moderate growth trajectories.

Key finding

Two forecasting scenarios project that U.S. VMT growth will moderate to between 1.74 and 2 percent annually by 2025, yet slower VMT growth does not necessarily lead to reduced congestion rates.

Methodology

modeling

Provenance

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