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How do they measure distance markers? It's complicated. 

Long road trips can be mind-numbing experiences. But there comes a time in every drive when finally, at long last, you get a physical marker of just how far you are from your destination. Way before we had GPS, we had highway distance signs.

When I was a kid, family trips up I-95 from D.C. to New York were broken up by those signs. On the way back down, somewhere just south of Baltimore, a sign let us know we were in the home stretch. Washington was just 30 miles away. 

But where, exactly, in Washington? It's a big place, after all. And I-95 actually goes around the city, not through it. This lifelong question plaguing me, I called up some state departments of transportation to see what actually goes into these signs.

Road signs along highways include both closer destinations, usually the next place that has a post office or a railroad station, and "control cities," the major destinations along a route. This is why New York pops up several states early on the drive up I-95.

But a sign indicating how close you are to New York doesn't exactly reflect how far it is to the city's border. The distance being measured is to a "center point" within these control cities, decided on a "on a case-by-case basis" guided by either state policies or the local jurisdiction that owns the road, according to the Federal Highway Administration's 800-page Manual of Uniform Traffic Control Devices. 

The guidelines for choosing a center point are fairly loose: It just needs to be a "well-defined central area," a "central business district," or, even less obviously, "where it appears that most drivers would feel that they are in the center of the community in question." It's not, as some have believed, necessarily where the city hall is. 

David Buck, a spokesperson for the Maryland State Highway Administration, says that highway engineers try to use "what most people consider to be the center of the city." In Baltimore, it actually is City Hall. In D.C., it's the U.S. Capitol (not, sad to say for District-pride boosters, the actual City Hall a mile away). 

As for New York, local lore has it that these distances are measuring to Columbus Circle.  (Robots think differently, to make matters more confusing, a Google Maps trip from "Washington, D.C." to "New York" starts in D.C.'s Scott Circle and ends at New York's City Hall in Lower Manhattan.)

NYC's City Hall, where Google Maps uses to define "New York City" as a destination, is almost five miles from Columbus Circle, which many people have long assumed is the trip's endpoint.

To calculate what mileage should appear on signs leading into a city, highway engineers generally assume that drivers take the first exit for a major road that will lead them to that central point. 

"Very infrequently, someone will call up and question the validity of the distance on the sign," Buck says. "They'll say, 'I measured it and I sat in my car and it was only 12.3 and you told me it was 13.' It’s not perfect." (Also remember that a sign's estimation won't take into account your back-road shortcuts.) 

The system also doesn't always work exactly as planned. U.S. Route 50 cuts clear across the country, beginning in Ocean City, Md., and running straight through to Sacramento, Ca. It's 3,073 miles from either direction, but for a few years there, those last two digits were reversed on one California sign, allegedly cutting 36 miles off the final trip. 

And, because these highway policies are set by country and state officials, they obviously differ across national borders.

For London, a plaque just south of Trafalgar Square marks the spot, once home to a 13th century memorial cross for Queen Eleanor, to which all distances in the city have been measured historically. 

In Ontario, distances use the "seat of government," or, for smaller jurisdictions, whatever the maps indicate to be the geographic center of the built-up area. As Spacing Toronto's Marcus Bowman points out, this can lead to some awkward confusion:

On Highway 401 west, a sign in Whitby states that you are still 51 km away from Toronto. In less than half that distance however you will cross the Rouge River and be welcomed to "Ontario’s Capital" raising the question of just what exactly that distance is measuring.

So just explain all of this to the kiddies when they start asking that inevitable car-trip question.

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