record observations about housing type, density, age, space around the buildings, proximity of trees, accessibility of recreational facilities, playgrounds, and gardens, and even signs of vandalism and graffiti. While it is labor-intensive to collect data in this manner, a range of variables can be studied, including many that have great a priori appeal in characterizing the quality of places.

Some studies have assessed exposure by surveying respondents about their perceptions of these conditions. For example, people can be asked to rate the “walkability” of their neighborhoods (Leyden, 2003), the safety of allowing their children to walk to school (Dellinger and Staunton, 2002), or other perceived features of the built environment. In one investigation conducted as part of the Alameda County Study in California, participants were asked to rate the seriousness of six potential neighborhood problems: crime, nighttime lighting, traffic, excessive noise, trash and litter, and access to public transportation (Balfour and Kaplan, 2002). Other areas of concern may relate to the availability of stores that sell tobacco, firearms, and liquor to youth. Such measures carry the problems of many questionnaires, such as variable responses among participants and response bias. Moreover, there may be an element of self-fulfilling prophecy: respondents who rate their neighborhoods as more walkable are likely to walk more, but this may reflect factors other than the physical design of the neighborhood.

To the extent that environmental factors affect people’s behavior, that behavior might be considered an early biological effect (see Figure 5-1). An important example is travel behavior—the number of trips per household each day, the mode of travel used, and the distance per trip. A leading source of such information is the National Personal Transportation Survey, renamed in 2001 the National Household Travel Survey.⁴ Other features of traffic safety and injuries related to the physical constructions of vehicles and roads are also available. Additional data are gathered in academic and governmental travel studies such as SMARTRAQ.⁵ Such information was traditionally collected through surveys, using either retrospective recall or diaries; new techniques, such as personal digital assistants with global positioning system capability, have improved the completeness and accuracy of travel data.

Of note, studies may be conducted that assess the association between an exposure and a biological effect in the sense discussed here. For example, one might hypothesize that certain neighborhood features, such as low density or automobile dependence, are negatively associated with children walking or biking to school. While not extending all the way to a health outcome such as obesity, this association would be an important part of understanding the health implications of the built environment.