Interstate 19 (I-19) is another intrastate interstate (as are I-2, I-4, I-12, I-14, I-16 and I-17). I-19 is one of two intrastate interstates in Arizona (the other being I-17, which was recently profiled in another FreightWaves Classics article).

How can I-19 (as well as the other interstates listed above), be an interstate if it doesn’t cross one state’s border into another? According to an article by David Rookhuyzen of the Arizona Department of Transportation (ADOT) that quotes the Federal Highway Administration (FHwA), “the Federal-Aid Highway Act of 1956, which created the system, called for uniformity in construction standards. These standards were developed by the American Association of State Highway Officials (now the American Association of State Highway and Transportation Officials, or AASHTO).” These standards are the “benchmark” that a highway must be built to in order to be deemed an interstate. “Metrics include having controlled access, design speeds of 50 to 70 miles per hour depending on terrain, a minimum of two travel lanes in each direction, 12-foot lane widths, a 10-foot right shoulder, and a 4-foot left shoulder.”

This map shows the route of Interstate 19, which is entirely within Arizona. (Map: Google Maps)


Therefore, I-19 is a north-south intrastate interstate highway. It runs from Nogales (and starts about 300 feet from the Mexican border), to an intersection with I-10 in Tucson. The highway also passes through the smaller Arizona cities of Rio Rico, Green Valley and Sahuarita.

At just over 63 miles (101 km), I-19 is the sixth-shortest primary (two-digit) interstate highway in the contiguous 48 states. The only interstates that are shorter are I-97, I-86 (western), I-14, I-11 (which is set to replace most of, if not all of I-19 in the future) and I-2.

While short, the highway is a very important corridor, serving as the fastest route from Tucson and Phoenix (via I-10) to the Mexican border. Interstate 19 is also a section of the United States route of the CANAMEX Corridor, a trade corridor that stretches north from Mexico across the United States to the Canadian province of Alberta.

In Nogales, I-19’s southern end is at West Crawford Street, which is adjacent to the international port of entry. Southbound travelers can continue into Nogales, Sonora, Mexico, via state-maintained surface roads, and then connect with Mexico Federal Highway 15 either to the south or west of Nogales, Sonora.

An aerial view of I-19’s original construction in the 1960s. (Photo: Arizona Department of Transportation)

I-19’s route

Starting from its southern end at kilometer post 0 (not milepost 0; see information below for an explanation), I-19 initially heads briefly south then west on surface streets, navigating its way through the town of Nogales for 0.2 miles before becoming an interstate-grade freeway and making the turn to head north toward Tucson.

Interstate 19’s interchange with State Route (SR) 189 at exit 4 funnels traffic to bypass around Nogales, Arizona, and Nogales, Sonora, for travelers bound to or from Hermosillo or Mexico City. Exit 4 also provides for the continuous flow of freight and truck traffic through the larger Mariposa Port of Entry to Mexico’s Highway 15, which has its northern end at the U.S.-Mexico border with SR 189. Mexican Highway 15 runs from there to its southern end over 1,350 miles away in Mexico City. 

After exiting Nogales, I-19 northbound passes near and around several sparsely populated towns and retirement communities along the banks of the Santa Cruz River. Prior to entering Tucson, I-19 passes through the eastern portion of the San Xavier Indian Reservation; it crosses the Santa Cruz River at that point. I-19 then enters the city limits of Tucson; it ends at an interchange with I-10.

Along nearly its entire route I-19 follows, or is adjacent to, the former route of U.S. Route 89 and the Santa Cruz River. The river flows northward from Mexico, through Tucson and usually disappears in the desert between Marana and the Gila River, southeast of Phoenix. The river bed is usually dry; however, heavy storms can cause it to spill over its banks, flooding farmland before reaching the Gila River.

One of the original signs on Interstate 19, erected by the Arizona Department of Transportation. (Photo: ADOT)

Kilometers, not miles

In addition to being a very short intrastate interstate, I-19 is unique among all U.S. interstates. What sets it apart? The highway’s distances are signed in meters or kilometers instead of miles. However, the speed limit signs give speeds in miles per hour, not kilometers per hour. 

Why? According to the Arizona Department of Transportation (ADOT), metric signs were originally placed on the highway because of the push to adopt the metric system in the U.S. at the time I-19 was built.

The United States is one of only three countries that have not officially adopted the metric system (the other two countries are Liberia and Myanmar). There have been attempts to convert the U.S. to the metric system, though.

In 1975, the U.S. Congress passed the Metric Conversion Act, which stated that metric was the preferred system in the U.S. However, the act did not contain any compulsory provisions. That same year, President Ford established the U.S. Metric Board to implement the conversion. 

A public information campaign began to educate the American public to the benefits of the metric system. One of the very first decisions of the Metric Board was to use the nearly completed Interstate 19 as a pilot program for converting all of America’s mileposts into kilometer markers. Road crews installed metric signs on the interstate ahead of its 1978 completion. It is the only highway in America with distances posted solely in kilometers.

In 1980, ADOT awarded a contract to install new metric unit signage and to provide bilingual signing in select locations. The 1980 signing project also provided for speed limit and advisory speed signs that used metric units. However, the advisory signs were cancelled and not installed.

How far is Nogales? An 1-19 road sign with distances in kilometers. (Photo: Arizona Department of Transportation)

The metric experiment in the U.S. did not last long. The Metric Board was dismantled by President Reagan in 1982, seven years after it was established. More importantly, it was dismantled without the U.S. making any serious effort to convert to the metric system. (Those of us old enough to remember saw road signs in both mileage and metric measurements.) 

Nearly 50 years later, the U.S.  remains committed to measuring distance in feet, yards and miles. Nonetheless, I-19 continues to be marked with kilometers for its entire 102-kilometer length. Why is that?

While I-19’s signs could easily be changed to use miles instead of kilometers, it has maintained its unique identity. Why? Those who live, work and travel along I-19 seem to have a special reverence for the highway’s unique markings and have fought to preserve them since the first mention of conversion.

Therefore, in 1999, ADOT awarded two new contracts (administered as a single construction project) to renew the metric signs along the full length of I-19. 

And they have stayed that way ever since…

One of I-19’s road signs that uses kilometers for measurements. (Photo:


The first two sections of I-19 that were opened to traffic include a three-mile section from I-10 to Valencia Road (in 1962) and a two-mile section in Green Valley in 1963. More sections came online in the mid- and late 1970s. According to an ADOT project report in 1978, the entire I-19 project was completed by that point in time. Of course, like all interstate highways, sections of I-19 have been rebuilt and/or improved. 

The highway is a very heavily traveled corridor in the Tucson metro area, which has grown from its 1970 population of 297,000 to more than 554,000 in 2020. I-19 is currently two lanes in both directions for its entire length (the exception is its interchange with I-10, where it is four lanes). Current plans call for widening the highway to three lanes in each direction. Future construction plans call for its expansion to up to five lanes in each direction by 2030 from I-19’s crossing with I-10 to San Xavier Road.

While Interstate 19 is not very long, and doesn’t leave Arizona, it has a unique history and heritage…