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Located only in the state of Arizona is Interstate 19. It begins in Tucson at Interstate 10 and ends about 63 miles later at the Mexican border in Nogales.

An interesting thing about this freeway is that since it was build during the 1970s when the United States was in the process of trying to convert the nation to the SI/metric system, they have the entire route marked off in metric. Cool, huh?

One big American peeve is the push the US government has been making towards metric roadway signage for over 35 years. A token vestige of metrification still exists in the form of Arizona Interstate I-19, from Tuscon to Nogales, Mexico.

I-19 spans 101 km (63 mi), and has served a critical function as one of the main arteries from Mexican border areas into Arizona. The highway was signed in the late 70s, during the fiasco that was US highway conversion to metric. Arizona has declined to plaster the exit signs over with US measurements because of the convenience to Mexicans, yet everyday Americans largely unfamiliar with metric use the road as well. Compromises have been made.

A driver entering I-19 is greeted with "THIS ROAD IS SIGNED IN METRIC", though the road is really an amalgam of the two systems. I-19 has both mileposts and, um, kilometer posts. The kilometer posts face traffic while the mileposts are at 90 degree angles to the roadway. Hazard signs (as for construction) use US measurements. Speed limits are posted in the familiar American design and in US measurements. In my opinion, having speed limit signs in US measurements only without the acronym "MPH" is potentially dangerous to metric drivers. There is an inherent contradiction in having distances in km and speeds in mph, but I guess some legislator thought it more important that the majority American drivers on the road not speed and set the tempo for the metric drivers. Keep in mind that the speed limit is 75 mph (120 km/h) for the greater part of the highway, so there's a high probability that drivers are routinely going 90 mph (140 km/h), rendering any signage meaningless. Yet, a metric driver would be forgiven for seeing "75" and interpreting that figure as 75 km/h. I wonder what the driver accident rate is on a road that combines measurement systems in this fashion.

Though the predictions for a new halcyon metric era have passed, I think I-19 raises the question of how to deal with America's roadway borders with Canada and Mexico. The most apparent solution is a new push for full bi-measurement within 160 km (100 mi) of the borders, but many states have prohibitions against funding of metric signage. Many states with Canadian borders routinely post dual speed limit signage within 80 km (50 mi) of the crossings, with exit distances posted in mileage only. This may be a nice solution that prevents accidents but serves the needs of the majority of drivers. It's one thing not to understand where you're going, but it's another thing to be going 45 mph on a four lane interstate.

for more information try http://www.arizonaroads.com/interstate/i19.htm

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