In this study, walking-only and any cycling for transit were associated with lower BMI, smaller waist circumference, and higher fitness, while only any cycling was associated with lower lifetime CVD risk classification. Parks and public transit stops were the most common amenities accessed using active transit, and walking-only was more common than any cycling. Transit modes varied by type of resource and individual characteristics, particularly gender and relationship, child, and employment status. These findings have implications for promotion of physical activity through community design and active transit research.
Characteristics and Destinations of Active Transit Users
Active transit was common in the CARDIA sample, with 42%–44% reporting walking-only and 11%–19% reporting any biking to neighborhood amenities. Active transit prevalence was higher than other studies (21%–28% walking10,11,30
and 6%–10% cycling30
for transit) which use more restrictive definitions incorporating trip duration or frequency not ascertained in the CARDIA study. Limiting examination of sociodemographic correlates of active transit to those who used each type of amenity helped to isolate associations related to mode choice by minimizing confounding due to differential availability and use of each amenity.
The finding of higher active transit prevalence in men and whites is consistent with higher total physical activity levels in these groups.31–34
Gender differences may stem from greater safety concerns in women,34
but relationships were similar after controlling for self-reported crime and lack of safety. Racial differences may reflect disparities in physical or social environments,35,36
suggesting that such disparities may need to be addressed for active transit to become a viable option. However, gender and race differences in walking and cycling appear even after controlling for built environment characteristics in prior research.37
Active transit was more common in those without a live-in partner or children, suggesting the importance of time constraints and lifestyle in mode choice decisions. Similarly, active transit was more common in individuals with less than full-time employment, who may have more time and less access to a car. Car transit may be more appealing for those with greater time constraints, or errands may be conducted en route
between work and home. Financial resources and car ownership may influence transit mode choices, but associations with employment emerged even after controlling for education and income. Interestingly, active transit to most neighborhood amenities was positively related to education but negatively related to income, although most of these associations were not significant. While counterintuitive, similar patterns have been observed elsewhere,9,11
perhaps reflecting complex roles of factors such as resources (e.g., access to a car) and social norms (e.g., environmentalism).
Differences in Sociodemographic Correlates of Active Transit by Type of Neighborhood Amenity
Active transit correlates were consistent across neighborhood amenities, with a few exceptions. Walking-only for transit was generally more common in whites except to grocery stores and fast food restaurants. Given that these two amenities were least likely to be accessed using active transit, these results may be driven by necessity (e.g., lack of access to a car) rather than mode choice preference. Likewise, active transit was less common in those with above-median income except for transit to parks, suggesting that walking only or any cycling to parks may reflect leisure rather than utilitarian activity.
The majority (72%) of public transit users walked to a public transit stop, perhaps reflecting environmental factors such as limited parking that influence public transit use, or dedication to alternative transportation modes. Such factors may more strongly influence transit mode decisions than sociodemographic characteristics, several of which were correlated with active transit to amenities other than public transit stops. Public transit promotion may be valuable because active transit to public transit stations is common regardless of many individual characteristics, and public transit has been shown to be related to increased physical activity.3
Active Transit Associations with CVD Risk Factors
Associations between walking-only and any cycling for transit with favorable BMI, waist circumference, fitness levels, and lifetime CVD risk classification are consistent with a growing literature showing that active commuting and walking are associated with lower CVD risk4,38
and more favorable CVD risk profiles.5
These associations could reflect health benefits of active transit, selection of active transit by healthier people, or both, but longitudinal studies are needed to distinguish among these mechanisms.
Both Active, but not Equivalent: Walking-Only and any Cycling
In several cases, male gender, white race, and higher education were more strongly associated with any cycling than walking-only for transit. That is, incorporation of any cycling into trips to neighborhood amenities may be more strongly influenced to social norms and influences related to these characteristics. Indeed, almost half of the current sample reported walking-only for transit, which is consistent with evidence of walking as accessible to the general population, as opposed to the small proportion reporting cycling. Walking requires minimal equipment, skill, and transportation infrastructure (e.g., bike lanes) in the community. Thus, relative to walking, promotion of cycling may require distinct or enhanced interventions to the physical and social environments.
Any cycling is also more strongly related to CVD risk factors than walking-only for transit. Compared to walking-only, transit incorporating cycling may be more beneficial to health, perhaps because cycling can be performed at more vigorous intensities, more strongly influenced by health status, or more strongly confounded by unmeasured attributes of cyclists. This finding is consistent with stronger relationships between any cycling (as opposed to walking-only) and high physical activity other than walking and cycling, which further suggest that relationships between health and active transit measures that include cycling may be more vulnerable to confounding by other physical activity or fitness levels. These findings suggest potential limitations of combined walking/cycling measures because they represent heterogeneity in physical activity intensity and potential health benefits that may vary cross-culturally39
and across demographic subgroups.
Strengths and Limitations
The CARDIA data are observational and cross-sectional and study results do not imply causality. The analysis was limited by self-reported physical activity and other lifestyle factors, and cannot completely control for misreporting, which may include over-reporting of active transit and walking; resulting bias may be exacerbated by exclusion of generally less healthy individuals due to missing data, who may less likely to over-report healthy behaviors. Additionally, examination of self-reported amenities in self-defined neighborhoods has limitations,40
but many concerns were avoided by examining transit modes among those reporting use of each amenity. Measures of walking and cycling for transportation do not reflect frequency or duration of activity. Due to the relatively small number of cyclists in the sample, this study examined “any cycling,” a heterogeneous measure which may include walking and therefore does not provide a clear comparison with walking-only. Further, car ownership may be an important determinant of active transportation not examined in this study.9,12
More can be learned from reports of barriers and facilitators to active transit, but these variables were not collected in the CARDIA study.
Finally, classification of respondents reporting blood pressure– or cholesterol-lowering medications into the “elevated” risk category was based on clinical guidelines for initiation for medical treatment but likely resulted in some misclassification.
Conversely, study strengths include use of detailed data on active transit, measures of a variety of types of physical activity from an instrument with known reliability and validity, and extensive CVD risk biomarker data. Further, this study controlled for leisure physical activity, which is uncommon in most studies relating walking to CVD risk factors.41