Open any cheap demographics report and you'll get population counts inside a 1-, 3-, and 5-mile ring around the address. It's a fifty-year-old convention from the era of paper maps — and in most cities, it's wrong enough to change your decision.
A circle assumes customers travel as the crow flies. They don't. They travel on roads, around rivers, across (or not across) interstates, and through traffic lights.
Where circles fail hardest
Consider a site near a river with one bridge. The radius says half your trade area is across the water; in reality, a 5-minute detour to the bridge means those households shop somewhere else. The same thing happens with interstates, rail lines, large parks, and one-way street grids.
In Atlanta, a 10-minute drive northbound up a highway corridor can cover 7 miles, while 10 minutes through midtown surface streets covers 2. A circle treats both directions identically. Your customers won't.
Isochrones: the fix
An isochrone is a polygon of everywhere reachable within N minutes of actual driving, computed on the real road network. It's the standard for professional site selection because it answers the question you actually care about: who can conveniently get to me?
Once you have the polygon, demographics get computed by proportional allocation — if a census tract is 40% inside your drive-time area, you count 40% of its population. That avoids overcounting tracts that merely touch the edge.
What this means for your search
When you compare two candidate sites, compare their 10-minute drive-time populations, not their radius populations. Two sites a mile apart can differ by 30% or more once the road network has its say. The site with the worse curb appeal and the better isochrone usually wins over time.
Every analysis we run uses drive-time polygons by default. It costs more to compute than drawing a circle — and it's the single biggest accuracy upgrade in the whole report.