By Any Metric

...Interstate-19 is different than any other stretch of interstate in the United States.  It is the only  one demarcated in kilometers.

The road starts at the Mexican/US Border in Nogales, AZ (the other side of the border is Nogales, Sonora, MX) at Exit 1.  It comes to an end at the junction of I-10 in Tucson at Exit 101.  As with every highway, the exits signify how far (in Km or Mi) along you have traveled.

So let's look at the distances marked on I-19:
The city mark represents 101 Km (or 64 mi) from Mexico.
I take Exit 65 to get home in Green Valley which is 36 km from the city (22 mi).
That leaves 65 Km (40mi) to the southern border from my home.

I stood in the middle of the road to get this photo. 
Oh, what we do for a blip!