Interstate 15 in Utah - History

History

The southwest-north alignment followed by I-15 was a major transportation corridor in the early 20th century, followed by auto trails such as the Arrowhead Trail (south from Salt Lake City), Evergreen National Highway (entire length), and Banff-Grand Canyon Road (north from Nephi). In 1926, when the numbered system of U.S. highways was created, this route was signed U.S. Route 91

The route of I-15 from St. George to Brigham City was built along the corridor of what was U.S. Route 91 from 1926-1974. In 1974 I-15 was reasonably intact and US-91 was deleted, with the unfinished segments signed as Temporary I-15. The route north of Brigham City was built along the corridor of an old route of U.S. Route 191. This highway was also deleted once I-15 was reasonably intact. However, a different route in Utah was re-designated US-191 in 1982.

Prior to 1977 the route of I-15 (and US-91 before) was also designated State Route 1 by the state of Utah, but no portion of the route was signed with this number.

In January 2009, the speed limit on two sections of I-15 together totaling 34 miles was raised to 80 mph as a "test," making Utah the second state to currently have speed limits greater than 75 mph.