Interstate 19 between Nogales and Tucson got caught in the middle of this "voluntary" conversion. A lot of their signs are still in metric (km/m). Unfortunately Arizona decided a while ago that any signs that needed replacement would not be replaced with metric signage.President Ford signed a law calling for voluntary conversion to the metric system in December 1975. Various conversion-related activities were started. Some states passed laws requiring it to be taught in schools by 1980.