Drive time vs radius: why the circle on your map overstates your reach

The three-mile ring survives because it is easy. Every GIS tool draws one in a click, and every property pack contains one. It also assumes customers travel in straight lines at uniform speed, which is false in every market with a river, a motorway or a rail corridor.

Where the two diverge most

• Severed geographies. A ring that crosses water or a limited-access motorway counts population that cannot reach you without a long detour.
• Dense urban cores. A ring understates reach on transit corridors and overstates it across congested grids.
• Highway-adjacent suburban sites. Real reach stretches along the corridor into an elongated shape a circle cannot represent.

What changes in the numbers

In practice, swapping a three-mile ring for a ten-minute isochrone commonly changes captured population by 20–40% in either direction, and it changes competitor counts too. Because both the demand numerator and the supply denominator move, saturation metrics can flip sign entirely.

When a ring is fine

For a first-pass screen across hundreds of markets, rings are acceptable because errors are roughly consistent and you only need relative ranking. Move to drive time before any site-level decision, and always before a lease.

Practical guidance

Pick the travel mode your customers actually use. Use peak-hour travel times for commuter-led formats and off-peak for weekend destinations. Keep the same isochrone definition across every candidate so scores stay comparable, and state the definition on the report — a trade area without its method is not evidence.