Interstate 19 (I-19) is a north–south Interstate Highway located entirely within the US state of Arizona. I-19 travels from Nogales, roughly 90 meters (300 ft) from the Mexican border, to Tucson, at I-10. The highway also travels through the cities of Rio Rico, Green Valley, and Sahuarita.

Having a total length of just over 102 kilometers (63 mi), I-19 is the seventh-shortest primary (two-digit) Interstate Highway in the contiguous 48 states, where only I-87 (North Carolina), I-97, I-86 (Idaho), I-14, I-11, and I-2 are shorter.

While the highway is short, it is a very important corridor, serving as a fast route from Tucson and Phoenix (via I-10) to the Mexican border. The highway is a portion of the United States section of the CANAMEX Corridor, a trade corridor that stretches north from Mexico across the United States to the Canadian province of Alberta.

I-19 is unique among US Interstates, because signed distances are given in meters (hundreds or thousands as distance-to-exit indications) or kilometers (as distance-to-destination indications), and not miles. However, the speed limit signs give speeds in miles per hour. According to the Arizona Department of Transportation (ADOT), metric signs were originally placed because of the push toward the metric system in the United States at the time of the original construction of the highway.

I-19 had originally been signed as it was constructed, in a series of small signing contracts that used customary units. In 1980, ADOT awarded a single contract to install new signs which used metric units, to overlay English-unit expressions on some existing signs with metric-unit expressions, to install kilometer posts, and to provide bilingual signing in select locations. The signing scheme used in 1980 provided explicit units on advance guide signs, but not on interchange sequence signs or post-interchange confirmation (distance) signs.

The expressions on advance guide signs were of the form "2 km" for distances over 1 kilometer (0.62 mi) and "500 m" for distances under 1 kilometer (0.62 mi), with no provision for fractional kilometrages. On advance guide signs, the metric unit expressions "km" and "m" were placed on the baseline where "MILES" would otherwise have gone but were sized so that their lowercase loop height matched the uppercase letter height of "MILES" on customary-unit signs.

The exception was a handful of advance guide signs for the SR 86 (Ajo Way) exit, which used "KM" (in uppercase, contrary to SI nomenclature) on the same baseline and at the same letter height as "MILES". The 1980 signing plans also provided design details for speed limit and advisory speed signs using metric units, with the limit values enclosed in a red circle on the speed limit sign and a black circle on the yellow-background advisory signs. These speed signs all had explicit units, with "km/h" below the circle enclosing the limit value. However, the advisory signs were canceled by change order and not installed.

Had the metric speed limit sign been installed, the signed speed limit on I-19 would have been 88 kilometers per hour (54.7 mph), which is a close soft-conversion of the then-existing 55 miles per hour (88.5 km/h) national maximum speed limit. As Arizona's current maximum speed limit is 75 miles per hour (121 km/h), the metric equivalent would most likely read 120 kilometers per hour (74.6 mph). Information signs, to three distinct designs, were also placed at various locations on or near I-19 to advise motorists that the highway was signed in metric. Notwithstanding the metric legends, the signing plans were dimensioned entirely in feet and inches.

In 1999, ADOT awarded two contracts (administered as a single construction project) to renew the signs along the full length of I-19. The general approach toward metric signing differed from that taken in 1980. Explicit units were given not just on advance guide signs, but also on interchange sequence signs, post-interchange confirmation signs, and community interchange signs (the last-listed had not been used in 1980). On the distance signs, "km" appeared after each kilometer measurement except when one or more of the distances was a fractional kilometer.

In such cases, all the distances were given in meters with "meters" (written out in full, not "m") after each distance value. On distance signs in general, "km" or "meters" appeared on the same baseline and with the same letter height as the distance values, while advance guide signs were formatted as in 1980. Since a typical ADOT freeway guide sign rehabilitation contract also replaces surface road signing near those roads' interchanges with the freeway, metric-unit signs also appeared on local roads near I-19, giving distances in kilometers to tourist attractions such as Mission San Xavier del Bac.

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