Figure 1. Grid cell dimensions and ¼ mile radius from cell center.
The Walking Accessibility Index (Table 1) examines the potential of study area residents to walk to surrounding destinations. The analysis begins by combining three factors that influence the potential for walking to nearby destinations – 1) the density of services within a walkable distance (¼ mile radius) of a grid cell, 2) street network (intersection) density, and 3) sidewalk length. Next, this combined score is penalized by subtracting a barriers index, sidewalk gap index, and the estimated average walking time to the four adjacent cells. Recognizing that these factors impede, but generally do not completely prevent walk access, each penalty index is discounted by 50% to reduce its overall impact on the score. The barriers index removes areas covered with water, or with a major roadway that would be difficult to cross. The sidewalk gap index penalizes areas lacking sidewalks. Walking time is estimated using the Google Directions API, and averaged across four of the nearest cells. This measure provides an estimate of how connected a cell is to neighboring cells. The longer the walking time, the higher the penalty. A cell with high network density, good sidewalk coverage, and services within a quarter-mile, along with few sidewalk gaps or barriers, and low walking time is assigned a high potential index for walking accessibility to the surrounding area. An example calculation for the Walking Accessibility Index is provided in Table 2.
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