The Moderating Growth in Vehicle Miles Traveled

Posted on October 31, 2007 by | 65 Comments

There is precious little good news to report on the transportation front, but reader Danny Newton has brought to my attention a fascinating article published by the U.S. Department of Transportation with the ungainly title, “The Case for Moderate Growth in Vehicle Miles of Travel: A Critical Juncture in U.S. Travel Behavior Trends.” (I have linked to a PDF file here.)

While Americans will continue to drive more than in the past, they won’t rack up increased Vehicle Miles Driven at the same prodigious rate as in the past, argues the author, Steven E. Polzin with the Center for Urban Transportation Research at the University of South Florida. The strain on the nation’s transportation system may be less than widely anticipated. But there’s a downside, of course: There is so little slack left in the transportation system that any increases in demand will contribute to congestion.
The chart above (click on the chart to enlarge) summarizes what’s going on: (1) The population will grow at a slower rate between 2002 and 2025 than between 1977 and 2001; (2) the number of daily trips that people take will plateau; (3) the length of the trips will increase, but not as rapidly; and (4) the total Vehicle Miles Traveled will increase, but at only at 40 percent of the rate in the previous quarter century.

Here’s the cool part: Most of the change will be painless — the result of demographic shifts.

  • The population is getting older. People tend to do the most driving in the 25-to-50 age bracket. That’s when they’re commuting to work and schlepping their muchkins to activities all around town. The baby boom generation is aging out of that bracket and heading toward retirement, when people drive less.
  • Most women are driving now. The increase in VMT was pushed in recent decades by the increased demand for mobility by women. As women entered the workforce and moved to the suburbs, they needed automobiles to juggle their duties. Most of the women who want to drive now have cars.
  • Shrinking family sizes. Back to the care and feeding of the little monsters. Fewer children translates into less parental chauffeuring.
  • Mode shifts. Past increases in VMT were driven in part by shifts to cars from other modes: walking, biking, carpooling and mass transit. Those shifts appear to be bottoming out.
  • On the other hand… Travel demand is correlated with income and free time. Incomes are rising and people are enjoying more free time. (While Americans are spending more hours working and commuting, they are spending less time on family care and personal care.) With more money and time to burn, people engage in more leisure travel.

Concludes Polzin:

Collectively, this body of data provides a compelling case for anticipating that VMT growth is moderating. However … the apparent unrelenting growth in travel time budgets and growing trips lengths may offset some of the factors that would appear to dampen VMT growth pressures. …

The premise that the reserve capacity in our system has been nearly fully absorbed and travelers have made the easy adjustments in travel departure times and route choices to utilize the high performing roadway segments, suggests that subsequent increases in demand may result in proportionally more severe consequences in terms of congestion levels and declining speeds.

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