A road sign from Arizona:

Arizona using metric?

Came across this from the web, found it curious. Traditionally the US, Burma and Liberia, I believe, don't use metric. But there are metric road signs in Arizona?

  • Comments are not for extended discussion; this conversation has been moved to chat. – Mark Mayo May 11 '18 at 14:39
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    You could try drawing a circle of radius 4km centered at Amado, a circle of radius 29km centered at Green Valley, and a circle of radius 67km centered at Tucson, and then (triangulating) see where all three circles meet. – Fixed Point May 11 '18 at 20:05
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    @FixedPoint Won't be precise since they're road miles, not straight-line miles. Amado is so small anyway, there's only one highway through it, and road signs only show what's in front of you, so it's I-19 4 km south of Amado. (Technically it's trilaterating since you aren't measuring angles) – user71659 May 11 '18 at 21:35
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    Burma went metric. – Harper - Reinstate Monica May 12 '18 at 0:00
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    @Harper Congrats, Burma – Hagen von Eitzen May 12 '18 at 12:11

That photo looks like I-19 in Arizona, America's only metric road (more from Atlas Obscura).

Congress passed the Metric Conversion Act in 1975, the last time the US had a serious metric system push, and various tests were carried out to start the conversion process. I-19 in Arizona was one of the test sites. After widespread opposition ("Forcing the American people to convert to the metric system goes against our democratic principles" is a quote from Chuck Grassley in 1977, and the guy has not mellowed out since then), the Metric Board was eventually disbanded.

But the locals like it:

Our bosses at the Arizona Department of Transportation have tried to change I-19's signs back to miles before. The signs have to be replaced periodically anyway. But businesses along the highway say doing so would make their shops harder to find, since the exit numbers would also change.

They tried to change the signs back to miles as recently as 2009, but there was local opposition that put the plan on hold. The issue came up again in 2014, still without any change.


Appears as though it was a timing situation when it was built, during a brief law change.

Gizmodo has an article on it:

That made sense in 1980, when I-19's signs first went up and when US was near the peak of its flirtation with the metric system. Five years earlier, President Ford had signed the Metric Conversion Act, declaring the metric system "the preferred system of weights and measures for United States trade and commerce" and establishing United States Metric Board to guide the conversion. Schoolchildren dutifully learned their kilograms and centimetres.

But the Metric Conversion Act was only voluntary, and there was far too much inertia to change every single label in the country voluntarily. Reagan disbanded the Metric Board in 1982. Instead of leading the charge into brave new metric system, Arizona's highway is a reminder of a failed experiment.


I'm willing to bet that this sign is from a place close to the Mexican border. I live in San Diego, CA, a city close to the Mexican border, and I've seen a few road signs in my area with metric units (actually, both metric and Customary units).

The reason why those metric signs exist is because frequently, people driving on those roads are often tourists from Mexico, driving cars that were built for the metric system (speedometer and odometer measure in metric units). This sign was put up for the benefit of people who are driving such cars, especially older cars that lack alternate scales for the Customary system.

You might even see similar signs in places close to the Canadian border, especially in areas frequented by Canadian tourists.

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    No, UK still uses miles/yards and MPH on their signs. Weight limits are in metric tons (<2% off a imperial ton anyway). Length and height limits are foot-inch but are now migrating to dual units. – user71659 May 11 '18 at 4:22
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    It's worth noting that the first sentence of this post (as it was at the time I commented, though it's since been removed) is literally correct. Whilst the UK still continues to use imperial units in certain contexts, the imperial and US Customary Unit systems are not the same, and confusion can arise if wrong assumptions are made. Myanmar also uses imperial units, though it's transitioning to SI. – MadHatter May 11 '18 at 7:59
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    @PLL But miles aren't miles :D Ask someone in the US to drive 400 miles, and they'd be like OK that's just going to the other end of the state, whereas in the UK they'd respond .. jeez you want me to drive half the length of the UK?!?!?!?! – Peter M May 11 '18 at 11:51
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    @PeterM, no, that's only halfway across the state. ;-) – shoover May 11 '18 at 15:23
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    @psmears Specifically, the difference is about 2 parts per million, so, in practice, no one cares which one they're using. Surveyors would probably care the most, but even they don't care in practice, since their equipment usually isn't that precise, anyway. – reirab May 11 '18 at 16:48

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