Interstate 19

Metric-unit advance guide sign on I-19, installed as part of the Valencia Road interchange renovation

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 US at the time of the original construction of the highway.[5]

I-19 had originally been signed as it was constructed, in a series of small signing contracts that used customary units.[6] In 1980, ADOT awarded a single contract to install new signs which used metric units, to overlay customary-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" (1.2 mi) for distances over one kilometer (0.62 mi) and "500 m" (0.31 mi) for distances under one 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 km/h (55 mph), which is a close soft-conversion of the then-existing 55 mph (89 km/h) national maximum speed limit. As Arizona's current maximum speed limit is 75 mph (121 km/h), the metric equivalent would most likely read 120 km/h (75 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.[7]

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.

As was the case in 1980, the signing plans were dimensioned in feet and inches.[8] However, a number of signs near the Valencia Road interchange were replaced or amended when it was converted from a partial cloverleaf interchange to a single-point urban interchange in 2000. One of these signs has a fractional kilometrage greater than one kilometer (0.62 mi), rendered as "1500 m" (0.93 mi), while others use "m" rather than "meters" as the unit expression. Metric-unit expressions on the advance guide signs installed or modified as part of this contract appear on the same baseline as the metric values, rather than on a raised baseline as on other I-19 advance guide signs. Again, the plans were dimensioned in feet and inches.[9]

Citing motorist confusion arising from the metric signs on I-19, ADOT's Tucson district announced that new signs on I-19 would use US customary units. To avoid the cost of replacing the metric signs all at once, signs would be replaced in specific areas of the freeway during construction projects in those areas.[10] New signs were put into place between exit 99 (Ajo Way) and exit 101 (I-10) in 2004 after the completion of the new I-10/I-19 interchange.

As of 2010, the remainder of the project has been stalled due to local opposition, particularly from businesses that would have to change their directions.[11]

A reconstruction project at the Interstate's northern terminus with I-10 in Tucson (at the interchange commonly called the Crossing) began in 2002 and was completed in August 2004.[12]

I-19 Bus. (Grand Avenue) junction with SR 82 in Nogales

Interstate 19 Business (I-19 Bus.) begins at the Mexican border in Nogales near the southern end of the Interstate. I-19 Bus. follows Grand Avenue in Nogales and has intersections with SR 82 and SR 189 before terminating at I-19 just north of Nogales.[1] The route follows the former alignment of US 89, SR 93, and SR 789, back when all three highways traversed through southern Arizona.[18] The route has been commissioned as I-19 Bus. since US 89 was decommissioned from southern Arizona in 1992.[17]

Major intersections
The entire route is in Santa Cruz County.

Location	mi[1]	km	Destinations	Notes
Nogales	0.00	0.00	Fed. 15 south – Mexico	Dennis DeConcini Port of Entry; international border with Mexico; continues south as Mexican Federal Highway 15
	0.14	0.23	I-19 north (Crawford Street) – Tucson	ADOT defines this intersection as southern terminus of I-19
	1.66	2.67	SR 82 east (Patagonia Highway) – Patagonia
	2.77	4.46	SR 189 south (Mariposa Road) to I-19
​	5.88	9.46	I-19 north	Northern terminus; no access to I-19 south
1.000 mi = 1.609 km; 1.000 km = 0.621 mi       Incomplete access

Interstate 19 Business (I-19 Bus.) began at I-19 exit 69 in Green Valley at West Duval Mine Road and South Nogales Highway. I-19 Bus. followed Nogales Highway north through unincorporated Pima County and Sahuarita into Tucson. In Tucson, I-19 Bus. curved from Nogales Highway onto South 6th Avenue where it served as the eastern terminus of SR 86 at West Ajo Way, before terminating at I-10 exit 261 and South 6th Avenue in South Tucson.[1] The route follows the former alignment of US 89, SR 93, and SR 789, back when all three highways traversed through southern Arizona.[18] The route was commissioned as I-19 Bus. since US 89 was decommissioned from southern Arizona in 1992.[19] In 2002, the state handed maintenance of I-19 Bus. between Los Reales Road and I-10 to the city of Tucson.[21] The rest of the route was handed over to Pima County and the city of Sahuarita in 2004. This also meant the designation was retired from the state highway system completely.[20]

Major intersections
The entire route was in Pima County.

Location	mi[2]	km	Destinations	Notes
Green Valley	43.88	70.62	I-19 / West Duval Mine Road – Nogales	Southern terminus; I-19 Exit 69; road continues west as West Duval Mine Road
Sahuarita	48.15	77.49	East Sahuarita Road to I-19
Tucson	60.41	97.22	East Valencia Road to I-19 / I-10 / BL 10 – Tucson International Airport
	63.50	102.19	SR 86 west (West Ajo Way) to I-19 / East Ajo Way – Ajo	Eastern terminus of SR 86
South Tucson	64.12	103.19	I-10 / South 6th Avenue – Casa Grande, El Paso	Northern terminus; I-10 Exit 261; road continues north as South 6th Avenue
1.000 mi = 1.609 km; 1.000 km = 0.621 mi