Visas som om det var en tidssituation när den byggdes, under en kort lagändring.
Gizmodo har en artikel om det :
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.