The western half of the arch is less traveled and almost entirely two-lane, passing through the Great Basin Desert, Sevier Lake, Delta, Eureka, and the Tintic Standard Reduction Mill.

According to the National Highway Traffic Safety Administration, there were 519 fatal and serious injury crashes between Spanish Fork and Green River (the non-freeway portion of the eastern half of US-6) from 1996–2008, leading the stretch to be considered one of the deadliest roads in the U.S.[5] Most of the route in Utah is part of the National Highway System, including the 120-mile (190 km) section referenced above as well as the concurrencies with I-15, I-70, and US-50.

After passing through Goshen, the highway curves around the north side of Warm Springs Mountain and into Utah Valley, where it enters Santaquin.

After about 13 miles (21 km) together in the Utah Valley, I-15 and US-6 separate in Spanish Fork, the latter turning southeast onto a short two-lane expressway.

US-6 begins climbing the Wasatch Plateau, cresting at Soldier Summit, where it finally leaves the Great Basin into the watershed of the Colorado River.

Relatively flat land continues as US-6 parallels the Book Cliffs to the southwest and west, crossing the Price River at Woodside.

The eastern half in Utah, from Colorado to Spanish Fork, overlapped US-50, but, after a short segment on US-91 to Santaquin, it followed a route that was new to the U.S. Numbered Highway System into Nevada.

[11] US-50 was moved farther south in 1976, due to the completion of I-70 across the San Rafael Swell, separating the two routes between Delta and Green River.

Initially it was to follow the present US-6 via Woodside between Price and Green River, but an amendment changed it to the existing state road through Buckhorn Flat (east of Castle Dale).

[22] A. L. Westgard of the National Highways Association praised the improvements to the road since the previous year, singling out the Price Canyon segment as "almost beyond comprehension".

[23] Although it was hoped that it would become part of the Lincoln Highway, the high mountain passes in Colorado convinced that association to designate a route farther north through Wyoming in September 1913.

At the urging of Grand County, the route that corresponded to the Midland Trail was realigned to the longer but more scenic road along the Colorado River between Moab and Cisco, including the new Dewey Bridge.

[33] Also that year, the Bureau of Public Roads approved Utah's seven percent federal-aid system in accordance with the Federal Aid Highway Act of 1921, including the Springville–Colorado segment of the Midland Trail.

The state completely reconstructed US-50 east of Spanish Fork in 1930 and 1931, eliminating most curves and railroad grade crossings and shortening it by 14 miles (23 km).

[47] The Eureka Commercial Club posted a billboard in Santaquin in July, advertising the "shortest and best all year route to California".

[56] The road again received attention in 1932, when the Roosevelt Highway Association was looking for a path for a westward extension of its trail—which had survived the 1920s by being identified with US-6—from Greeley, Colorado, to the West Coast.

The Delta Lions Club had suggested this alignment for the same reasons that the highway had become popular in the 1920s: cooler weather than the Arrowhead Trail (then US-91).

BusinessWeek described the original route as "nothing but a wagon trail-rutted, filled with dust [...] one of the worst chunks of federal road in the country".

The night before the rebuilt US-6 opened, the highway stubs at either side of the landslide were filled with tens of miles of trucks, the drivers tired of the lost revenue from the long detours.

The landslide remains the most costly in the history of the U.S.[62] Prior to 1995, US-6 passed directly through the cities of Payson, Salem, and Spanish Fork along what is now SR-198, and the four-lane expressway that connected I-15 in northern Spanish Fork southeast with Moark Junction was known as State Route 214 (SR-214), from I-15 to East 800 North, and State Route 105 (SR-105) from East 800 North to Moark Junction.