摘要：Objectives. We isolated physical activity attributable to transit use to examine issues of substitution between types of physical activity and potential confounding of transit-related walking with other walking. Methods. Physical activity and transit use data were collected in 2008 to 2009 from 693 Travel Assessment and Community study participants from King County, Washington, equipped with an accelerometer, a portable Global Positioning System, and a 7-day travel log. Physical activity was classified into transit- and non–transit-related walking and nonwalking time. Analyses compared physical activity by type between transit users and nonusers, between less and more frequent transit users, and between transit and nontransit days for transit users. Results. Transit users had more daily overall physical activity and more total walking than did nontransit users but did not differ on either non–transit-related walking or nonwalking physical activity. Most frequent transit users had more walking time than least frequent transit users. Higher physical activity levels for transit users were observed only on transit days, with 14.6 minutes (12.4 minutes when adjusted for demographics) of daily physical activity directly linked with transit use. Conclusions. Because transit use was directly related to higher physical activity, future research should examine whether substantive increases in transit access and use lead to more physical activity and related health improvements. Physical inactivity is prevalent and a worldwide public health concern. 1 Increasing active transport is an appealing strategy to increase overall physical activity, although more clarity is needed about the amount of physical activity directly attributable to transportation choices. Users of public transit (e.g., bus, train) engage in more overall physical activity than do nonusers and more often meet daily physical activity recommendations (≥ 30 min/d on most days), likely from the active transport involved with accessing transit (e.g., walking to a bus stop). 2–5 Reported total walking time is also higher among transit users than among nonusers. 4,6 National travel diary data indicate that the average American transit user (approximately 2%–3% of adults) walks 19 minutes per day to and from transit, and approximately one third of these transit users attain recommended levels of physical activity based solely on the amount of walking related to their transit use. 2,3 Better public transit access and more use appear related to more overall physical activity and specifically walking. 7 However, many previous studies about the relation between transit use and physical activity fail to address the critical issues of possible confounding and potential substitution. Regarding confounding, examining only overall physical activity or total walking among transit users versus nonusers without disaggregating physical activity into constituent types and purposes of walking makes it difficult to determine how much physical activity is directly related to using public transit (i.e., walking or biking to and from public transit stops), separate from other types of utilitarian (e.g., walking to the store) or recreational physical activity. This is problematic because other factors could readily account for the relation between transit use and higher overall physical activity and walking. For example, built environment factors such as residential density and land use mix are related to transit use because transit access is higher in more dense and higher mix areas; however, these built environment variables are also related to walking to and from other nonresidential locations (e.g., stores, restaurants). 8 Thus, without more precision, it is not possible to rule out a spurious relation (e.g., through built environment or other shared variables) between transit access and use and physical activity. The issue of substitution is also critical to measuring the health effect of transit use on physical activity. If transit users decrease the time spent in other activities in lieu of the time spent in transit-related walking, attempts to increase transit use would not yield increases in physical activity but merely shift from one form to another form of physical activity. Studies that provide more precise estimates of walking to and from transit use have not examined whether such substitution occurs. A recent US time use study suggested that some substitution may be happening as individuals with longer commutes, which are often characteristic of public transit use, engage in less recreational physical activity than do those with shorter commutes. 9 Large-scale randomized trials manipulating transit use are not feasible, but one approach to a better understanding of the relation between transit use and physical activity is to obtain an objective overall and type-specific assessment of physical activity among transit users and nonusers and, for the former, to examine walking behavior directly linked to transit use. For the current study, we hypothesized that transit users would be more physically active than nonusers and that increased activity would be directly linked to transit use (i.e., no or little confounding). Thus, we expected that physical activity would be higher among more frequent transit users than among less frequent users and that more physical activity would occur only on days when transit was used as a result of the transit-related physical activity and not changes in other types of physical activity (i.e., no or little substitution occurs).
