Two sites, same city. Site A sits in a dense residential neighborhood: 25,000 people within a 10-minute drive. Site B sits near an office park: only 9,000 residents nearby. The radius report says A wins easily. Then you open a sandwich shop at A and discover the neighborhood is empty from 8am to 6pm — everyone commutes out.
The number the radius report didn't show: Site B has 40,000 people who work within that same drive time. At lunch, B's trade area is four times larger than A's.
Where the data comes from
The Census Bureau's LODES program (LEHD Origin-Destination Employment Statistics) publishes job counts by census block — effectively a map of where people are during working hours. Cross-reference it with residential population and you get the daytime/residential ratio for any trade area.
A ratio above 2.0 means the area more than doubles during business hours — classic office district, great for weekday lunch and coffee. A ratio under 0.5 means a bedroom community — better for dinner, weekend, and family-oriented concepts.
Matching the ratio to your concept
Coffee shops and fast-casual lunch concepts live and die on weekday daytime traffic; they want high ratios. Sit-down dinner restaurants, daycares, and gyms draw from where people live; they tolerate — or prefer — low ratios. Retail splits the difference depending on category.
This is why our scoring weights daytime population more heavily for cafés and restaurants than for, say, a salon. Same data, different business, different answer — which is exactly how site selection should work.