Enlarge Ohio Department of Transportation Signs along I-71 in Ohio were installed in 1973, when the nation flirted with the metric system. ON INTERSTATE 19, Ariz.  Expect drivers new to this 63-mile interstate that connects Tucson and the Mexican border to do a double take every time they pass a highway distance and exit sign: They are in kilometers. The speed limit signs aren't. Nor are the mile posts. But all of the 400 distance and exit signs along both sides of the interstate are. I-19 may be the only interstate in the USA to use kilometers on such signs along its entire length. The U.S. Metric Association, a group that supports U.S. adoption of the metric system, says other border areas, north and south, often add metric signs on segments of their roads to accommodate visitors. Long sections of I-89 in Vermont near the Canadian border, for example, use kilometers. Signs on sections of highway in California, Tennessee, Connecticut and other states have used miles and kilometers. Arizona is considering converting the signs to miles — a move that is stirring as much controversy as the signs have caused for more than 30 years. "We were a little surprised," says Linda Ritter, spokeswoman in the Tucson district of the Arizona Department of Transportation. "Through time, we have not received kudos for having kilometer signs … because drivers were confused." So why would anyone complain now that the state is ready to replace the signs? "The merchants are somewhat upset with that because a lot of them have the exit sign in their advertising and promotional materials," says Stan Riddle, president of the Green Valley Community Coordinating Council, a quasi-governmental body that helps run the unincorporated area along Exits 63, 65 and 69. Exit numbers reflect the distance — in kilometers — from the southern tip of I-19 in Nogales. Exit 63 is 63 kilometers from Nogales (39.1 miles). A change to miles would make it Exit 39. I-19 has the kilometer signs because it was built in the late 1970s when the nation was considering converting to the metric system after Great Britain, Australia and New Zealand made the switch. The United States never made the transition official because of resistance — mainly from groups that oppose government mandates and businesses worried about shouldering the cost of change, but many U.S. manufacturers eventually switched voluntarily to be in line with the rest of the world. Arizona planned on using federal stimulus money to pay for new signs, but when communities along I-19 — including Green Valley and Sahuarita — protested, Gov. Jan Brewer, a Republican, called for further study. Surveys were conducted and results were "all over the place," Ritter says. The state expects to have a plan by year's end. Guidelines: You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. Read more