Interstate 15 partially coincident with Interstate 80
Utah
I-15 continues through Utah for 401 miles (645 km). It is the main north–south connection for the state. The highway approximately follows the old alignment of US 91 from St. George to Brigham City. The highway passes through the fast-growing Utah's Dixie region in the southwestern part of the state, which includes St. George, Cedar City, and eventually most of the major cities and suburbs along the Wasatch Front, including Provo, Orem, Sandy, Salt Lake City, Layton, and Ogden. Near Cove Fort, I-70 begins its journey eastward across the country. The Interstate merges with I-80 for about 3 miles (5 km) from South Salt Lake to just west of Downtown Salt Lake City and also merges with I-84 from Ogden to Tremonton. Along nearly its entire length through the state, I-15 winds its way along the western edge of a nearly continuous range of mountains (the Wasatch Range in the northern half of the state). The only exceptions are north of Cove Fort and when it passes between Cedar City and St. George, known as the Black Ridge, a transition zone of drastic change in elevation and climate, an area where the eastern Great Basin, Colorado Plateau, and Mojave Desert converge.
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