TUCSON, AZ — Former Scripps News Tucson traffic reporter Allen "Big Al" Kath points out a large sign on a recent drive along Interstate 19.
He reads it out loud: "Metric signs in use on I-19."
The interstate is a major artery for goods coming out of Mexico, heading into the United States.
It runs 63 miles from Nogales to Tucson. That is 102 kilometers, which is important because I-19 is the only metric interstate in the nation.
ADOT uses kilometers on signs, not miles, when noting the distance between Tucson and Nogales.
It all starts in Tucson at The Crossing, the flyover bridge that connects Interstate 10 to I-19.
Then it's onto the oldest portion of this highway.
"This was built between I-10 and Valencia in 1962," Kath said. "Long before the metric portion came into play."
The rest of the interstate was built in the 1970s, finally completed in 1978.
That's also when President Jimmy Carter was pushing for the country to convert to the metric system.
Those in school at the time may remember learning about the attempted move to metric in class.
"They wanted to push us into metric," Kath said.
Students took crash courses on the measuring system. But then nothing happened.
"We didn't do it because there (was) so much push back," Kath said.
The Carter Administration did get one interstate highway to adopt the metric system. The 63-mile long I-19 in Southern Arizona became a 102-kilometer long interstate.
It soon became a major corridor between Northern Mexico and the U.S. Today, it remains this nation's only metric interstate; although it was nearly converted back about 10 years ago. But those using I-19 on a daily basis opposed the ADOT plan.
"And so ADOT pushed back and said we're going to leave it metric," Kath said.
While the signs marking distances and exits are in kilometers, the interstate still features speed limit signs in miles per hour.
There is also a little known secret along the metric interstate.
Kath explained that the nation's only metric interstate highway is also signed with hidden mile markers.
"Right here is 56," pointed out Kath. "It's not looking toward you and none of them are."
That's right, along the nation's only metric highway are imperial units -- or mile markers -- turned at a 90-degree angle from the kilometer markers. ADOT and DPS use the mile markers to keep things standard throughout the entire state highway system.
"All signed in metric, but it's also signed in imperial units," Kath said.
Now you know the secret of I-19.
So what's the future look like? ADOT's proposed Sonoran Corridor will divert some of the heavy truck traffic off of the northern part of I-19. The proposed I-11 would move even more of those semis off of I-19.
But for now, Interstate 19 enjoys the designation as America's only metric highway.
The United States, Myanmar and Liberia are the only nations in the world not using the metric system, instead using the imperial system.
