Distance signs - how accurate are they?

Distance signs - how accurate are they?

By David Woodfill / ADOT Communications
April 14, 2021

We passed them every day without giving them much consideration.

They're the big, green guide signs on the side of the road telling you the distance between you and your destination, whether it be Tucson, Flagstaff or the tiny census-designated area of Why.

Have you ever wondered how accurate they are?

The answer would be "very."

The measurements are determined by something literally named a Distance Measurement Instrument, or DMI.

"DMIs are more accurate versus an odometer." said John Roberts, ADOT's Systems Technology Manager. "Odometers are considered accurate within 10 percent."

In other words, if you travel 100 miles, your odometer can be off by up to 10 miles.

DMIs, however, are accurate to within one foot, Roberts said.

So, does that mean that every city listed on a guide sign is exactly as far away as the sign says it is - down to within one foot?

Not quite.

If that were the case, there wouldn't be enough room on the signs to accommodate all the decimal spaces that would be required. 

Instead, the distance displayed is rounded to the nearest mile to the city's "center."

How is the "center" of a municipality determined? It's anything determined to be significant near the center of that community.

As we explained in an earlier blog, the center could be the town hall, post office or a railroad crossing.

So, next time your road-tripping down a dusty Arizona highway with some friends or family and you see one of these green signs, take a moment to impress your travel mates with your new knowledge. 

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