One way to start neighborhood selection is to simply look at a map and consider the number of road intersections there are in different areas. To some extent you should be able to see differences in walkability with a visual comparison.

NQLS study neighborhood selection

Neighborhoods were selected based on walkability and income.

The walkability of the neighborhoods was established using an index based on:

The first measure included in the walkability index was net residential density; the ratio of residential units to the land area devoted to residential use per block group.

The second component, the retail floor area ratio, was the retail building square footage divided by retail land square footage and indicated the density of retail development. The rationale was that a low ratio indicated a retail development likely to have substantial parking, while a high ratio indicated smaller setbacks, and less surface parking; two factors thought to impact pedestrian access.

Intersection density measured the connectivity of the street network, represented by the ratio between the number of true intersections (3 or more legs) to the land area of the block group in acres. A higher density of intersections corresponds with a more direct path between destinations.

The fourth measure was the land use mix, or entropy score, indicating the degree to which a diversity of land use types were present in a block group. For this project, the mix measure considered five land use types: residential, retail (excluding region-serving or “big box” uses of 300,000 square feet or larger), entertainment (including restaurants), office, and institutional (including schools and community institutions).

Values were normalized between 0 and 1, with 0 being single use and 1 indicating a completely even distribution of floor area across the 5 uses. The four calculated values were normalized for each block group using a Z score.

Block groups were ranked and divided into deciles based on the normalized walkability index. The top four and bottom four deciles represented “high walkability” and “low walkability” areas.

Similarly, the median household income data for each block group were deciled and categorized into “high income” and “low income”. Household income values less than $15,000 and greater than $150,000 were not included in the deciling process in order to avoid skewing the data with outliers. The second, third, and fourth deciles constituted the “low income” category, and the seventh, eighth, and ninth deciles made up the “high income” category.

Thus, a walkability-income quadrant, shown in table 1, was created. For each participant, therefore, a walkability index score is available as well as a walkability quadrant.

Neighborhood selection for the PLACE study was similar, but certain issues did arise. More details to follow shortly. For more information contact kcain@projects.sdsu.edu