National Scenic Byway 12 "A Journey Through Time" All-American Road: Explore some of the most diverse, remote and ruggedly beautiful landscapes in the country. Travel through the remains of ancient sea beds and across Boulder Mountain, one of the largest high-elevation plateaus in the United States. Because of the high terrain and winding road, the speed limit across Boulder Mountain is 50 mph in the lower elevations and 40 mph in higher elevations. Be sure to plan enough time to view all of the sites at pullouts provided along the way. Most of all - slow down - enjoy the scenery - and have a safe journey!