Route 66 may have its kicks, but a dusty, mostly hidden and sporadic trail winding its way from Mexico City to Santa Fe, N.M. is still king, as the oldest of the Southwest highway systems. For 400 years, El Camino Real de Tierra Adentro, the nation’s oldest and longest “highway,” was the only road into New Mexico and the Southwest, bringing thousands of settlers from Mexico and Spain into the region.