In the late 16th century Spanish exploration and colonization had advanced from Mexico City northward by the great central plateau to its ultimate goal in Santa Fe. Until Mexican independence (1821) all communications of New Mexico with the outer world was restricted to this 1,500-mile (2,400 km) trail. Over it came ox carts and mule trains, missionaries and governors, soldiers and colonists. When the Santa Fe Trail sprang up, traders from the United States extended their operations southward over the Chihuahua Trail and beyond to Durango and Zacatecas. Superseded by railroads, the ancient Mexico City-Santa Fe road was revived as a great automobile highway of Mexico. The part in New Mexico, State Highway 85, pioneered by Franciscan missionaries in 1581, may be the oldest highway in the United States.