To manage traffic, researchers ask: “Whose miles are these anyway?”

By Chris McCahill

Several states and many local governments are growing more interested in measuring vehicle miles traveled (VMT) as a key indicator of transportation emissions levels and other unwanted impacts. But measuring VMT, especially at the local site level, can be more difficult than it seems. A new study in the Journal of the Planning Association points to data sources, shortcomings, and considerations for measuring VMT in the future. 

Estimating VMT across states or other large regions is straightforward. FHWA produces state-level estimates each month based on thousands of traffic counters. In cases like this, traffic volumes are multiplied by road lengths, after some assumptions are made for roads without traffic counts. 

At the local level, however, knowing where VMT is produced is more useful than the amount of VMT that occurs on the roads. To manage VMT growth, for example, it is important to understand the built environment and what other factors contribute to travel decisions at each end of someone’s trip, such as their home and workplace. 

To test different methods, a team of researchers looked at eight large office sites in the San Francisco Bay Area and the Salt Lake City region. They derived VMT estimates by multiplying trip generation rates and trip length estimates from various sources: 

  1. Rate: Trip generation rates from ITE (no trip length information is available) 
  2. Admin: Administrative data about employees from the U.S. Census Bureau 
  3. Device: Mobile device data from StreetLight Data 
  4. Model: Travel demand models developed using the open-source platform, ActivitySim 

By combining trip generation estimates from all four available sources and trip length estimates from three, they tested 12 methods overall. They found the VMT estimates to be wildly different—anywhere from twice as much to one-half of the overall average. 

Error of VMT estimates relative to the average across all methods. Source: Voulgaris, Macfarlane, & Kaylor (2024).

The authors note that the most common method, using published trip rates from ITE, is “deeply flawed.” They recommend the following:

“In the short term, we recommend that planning policies requiring the estimation of site-generated VMT explicitly specify methods to produce these estimates. In the long term, if VMT is to be used as a performance metric or as a project evaluation measure, we recommend the development of an industry-wide standard that achieves acceptable levels of consistency, cost effectiveness, closeness [accuracy], and conservatism.”

In the meantime, there are promising methods already in use. Cities like Madison, Wisconsin, and many across California are using point-based systems or calculators that approximate VMT. These are sufficient in a lot of common cases, since trip rates can depend just as much on the popularity of a restaurant or the remote work policies at an office, as they do on things that are easier to measure. Home-based VMT estimates—which are just as important as work- or shopping-based estimates—can be derived from data sources like the Bureau of Transportation Statistics.

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