In ArcGIS Community Analyst, you can map service areas using threshold areas.
Instead of creating a ring with a known distance, such as a ten-minute walking time or a two-mile radius from your site, you can map the distance needed to capture a variable, such as a population of 100,000 people. Threshold areas commonly use total population or households in an area, but you can also enrich your analysis using other variables from the data browser.
See more in my blog, Map service areas using thresholds in ArcGIS Community Analyst.
