What commuting statistics can’t tell us about transportation use

We’ve been digging into the recently released county-to-county commuting flows in some detail over the past few weeks. The 2009-2013 American Community Survey (ACS) provides rich insight into where workers live, where they work, and how they travel between home and work. At multiple points, readers and colleagues expressed surprise about patterns emerging in the data that contradicted their on-the-ground perceptions of transportation use. Upon reflection, it was clear that these contradictions were due, in part, to some of the limitations of the data.

Fewer than 1 in 5 trips are for commuting

First, the ACS provides us with a limited picture of the spectrum of transportation use. These data are focused only on commuting—the journey from one’s residence to one’s workplace. But commuting is only one of many reasons that individuals are traveling. In a brief titled The Role of Commuting in Overall Travel, researchers from the American Association of State Highway and Transportation Officials (AASHTO) Census Transportation Planning Products Program (CTPP) write:

Commuters’ vehicles share the roadways with vehicles carrying persons who are traveling for school, shopping, personal business, social and recreational activities, and other purposes. [T]hey also share the road with vehicles transporting freight and providing various services, from goods delivery to emergency response to service workers visiting customers. Visitors and tourists from other communities and countries also share the transportation facilities. Similarly, transit services transport work commuters but also travelers pursuing other activities.

Although commuting is a key aspect of transportation utilization, it does not represent the majority of transportation usage. Nationally, commuting accounts for only 16% of all trips made by individuals. For specific travel modes, commuting accounts for a higher proportion of all travel: commuting trips comprise 28% of all miles traveled by private vehicle and 39% of all passenger miles on public transit.

Use of multiple transportation modes is not captured in the ACS

Second, the ACS data capture the most common mode of transportation. Specifically, if workers use multiple modes of transportation to get to work, they are asked to identify the transportation mode that was used for the greatest distance traveled. For workers in the Triangle and other metros who drive to a Park-and-Ride before boarding public transit, this would lower the estimate of public transit commuters if they drive a greater distance than they travel by public transit.

When examining commuting statistics, keep in mind their strengths and weaknesses. The ACS data provide rich insight into where individuals live, where they work, and how (and when) they move in between their residence and workplace. But these data do not provide a full picture of transit use and demand. And they may underestimate the utilization of less common modes of transportation, such as public transit, walking, or bicycling.

About Rebecca Tippett

Rebecca Tippett is Director of Carolina Demography at UNC-Chapel Hill's Carolina Population Center.
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