GREEN VALLEY, Ariz.  It is not the mountains or the cactuses that stand out most while driving Interstate 19 from the Mexican border to Tucson. It’s the signs.

Distance along I-19 is measured in kilometers, just as it is in Mexico. That means highway markers advise that there are three kilometers until the next gas station, four until the next rest stop, seven until the next desert town.

But the distinctive signs’ days may be, well, numbered.

The Arizona Department of Transportation says the 400 signs along the I-19’s 100 kilometers are too old and need to be replaced. The new signs, officials say, would be like all the others in the state and would indicate distance in miles. Exit numbers would be reconfigured as well.

A throwback to an American experiment with the metric system in the early 1980s that did not get far off the ground, the signs not only indicate the number of kilometers to the next exit, but also the exit numbers themselves coincide with the number of kilometers from Nogales, which abuts the border.