Detroit radius map: 5, 10, 25 and 50 mile circles

If you want one landmark to calibrate against, use Hamtramck. It’s 4.2 mi north of the centre, so it sits almost exactly on the inner circle. At the other end, Chatham is 44.3 mi out to the east, near enough the edge of the 50 mi ring. Everything in between scales from those two.

One caution before you lean on the table. A GeoNames centre is a single point, usually somewhere near the historic core, and a city isn’t a point. Windsor is listed at 2.6 mi because that’s the gap between two reference points. Its nearer edge is closer than that, probably by a good margin.

Place names, coordinates and populations come from GeoNames, which the site uses under CC BY 4.0.

Here’s the thing people get wrong about a radius. Double it and you quadruple the ground underneath. Going from 5 miles to 50 multiplies the area by a hundred, so 79 mi² becomes 7,854 mi². The outermost band on its own, the gap between the 25 and 50 mi circles, accounts for 5,890 mi². Three quarters of everything inside the big circle, in one band.

Put the two tables together and you get a crude density. Inside the 25 mi circle there are about 3,029,580 listed people spread over 1,963 mi², so roughly 1,543 per mi². Crude is the right word. It counts GeoNames settlements only, so villages and open country vanish, and it treats a harbour or a reservoir as populated ground.

These are straight-line rings. That’s the right tool for a coverage question and the wrong one for travel time. A 30-minute drive out of Detroit comes out as a lopsided blob, an isochrone, racing down a motorway and stopping dead at a bridge. Drawing one properly takes a routing server. This site hasn’t got one, deliberately, because everything runs in your browser with no account and nothing uploaded.

Plenty of sites draw a plain circle, label it drive time and leave it there. Those are worth ignoring. A proper isochrone needs a road graph, turn restrictions and some idea of traffic, and it changes shape by the hour. What a circle gives you instead is a bound that doesn’t move: nothing outside the 25 mi ring is within 25 miles of Detroit, whatever the roads do.

Where it all comes from. Names, coordinates and populations are GeoNames cities15000, which covers settlements of roughly 15,000 people and up. Anything smaller is simply absent. City subdivisions get stripped out on purpose, so a page about Detroit doesn’t fill with its own neighbourhoods instead of its neighbours. Distances are haversine on a 6,371.0088 km sphere, areas are spherical caps.

One oddity you’ll notice if you compare pages. Population counts come from GeoNames, which records what the national statistics office last published, so some are a few years old and a couple are more. They’re good for ranking and for rough sizing. The figure carried for Detroit is 645,705, which for some cities means the municipality and for others the whole urban area, so compare it with care.

What people actually use these rings for, roughly in order of how often it comes up: delivery and service areas, finding somewhere to live within a sensible distance of work, sizing a catchment for a shop or a clinic, checking how far an event’s noise and traffic will carry. Straight-line distance is the right first pass for all of them.