U.S. Route 6 (US 6) or U.S. Highway 6 (US 6), also called the Grand Army of the Republic Highway, honoring the American Civil War veterans association, is a main route of the United States Numbered Highway System. While it currently runs east-northeast from Bishop, California, to Provincetown, Massachusetts, the route has been modified several times. The highway's longest-lasting routing, from 1936 to 1964, had its western terminus at Long Beach, California. During this time, US 6 was the longest highway in the country.

In 1964, the state of California renumbered its highways, and most of the route within California was transferred to other highways. This dropped the highway's length below that of US 20, making it the second-longest U.S. Route in the country. However, since US 20 has a discontinuity through Yellowstone National Park, US 6 remains the longest continuous U.S. Route in the country.

US 6 is a diagonal route, whose number is out of sequence with the rest of the U.S. Route grid in the Western U.S. When it was designated in 1926, US 6 only ran east of Erie, Pennsylvania. Subsequent extensions, largely replacing the former U.S. Route 32 and U.S. Route 38 (US 38), have taken it south of US 30 at Joliet, Illinois, US 40 near Denver, Colorado (past the end of US 38), US 50 at Ely, Nevada, and US 70 near Los Angeles, California, due to its north–south alignment in that state.

US 6 does not serve a major transcontinental corridor, unlike other highways. George R. Stewart, author of U.S. 40: Cross Section of the United States of America, initially considered US 6, but realized that "Route 6 runs uncertainly from nowhere to nowhere, scarcely to be followed from one end to the other, except by some devoted eccentric".

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Lengths

	mi	km
CA	40.51	65.19
NV	305.65	491.90
UT	373.96	601.83
CO	467.28	752.01
NE	373.07	600.40
IA	319.60	514.35
IL	179.88	289.49
IN	149.00	239.79
OH	248.00	399.12
PA	403.00	648.57
NY	78.09	125.67
CT	116.33	187.21
RI	26.50	42.65
MA	118.00	189.90
Total	3,198.87	5,148.08

The modern US 6 in California is a short, two-lane, north–south surface highway from Bishop to the Nevada state line. Prior to the 1964 state highway renumbering, US 6 extended to Long Beach along what is now US 395, State Route 14 (SR 14), Interstate 5 (I-5), I-110/SR 110, and SR 1. Despite the fact that the renumbering removed all freeway portions, it is still part of the California Freeway and Expressway System. US 6's former routing included a short segment of the famous Arroyo Seco Parkway.

Currently, US 6 begins at US 395 in Bishop and heads north between farms and ranches in Chalfant at the base of the 14,000-foot (4,300 m) western escarpment of the White Mountains. After about 30 miles (48 km), Benton is reached, which has a cafe and gas station. SR 120 begins here, heading west past Mono Lake through Lee Vining, over Tioga Pass, and through Yosemite National Park to the San Joaquin Valley. US 6 continues north to the Nevada state line.

From the California border, US 6 heads northeast through the semidesert Queen Valley with Boundary Peak, Nevada's highest summit, and Montgomery Peak in California on the right. These twin peaks are the northernmost high summits of the White Mountains, both over 13,000 feet (4,000 m). The highway then climbs into pinyon–juniper woodland and crosses Montgomery Pass (7,167 ft or 2,185 m).

From the pass, US 6 descends into barren shadscale desert, passing Columbus Marsh on the left, then merging with US 95 from Coaldale Junction to Tonopah. The Nevada Test and Training Range begins about 15 miles (24 km) southeast of Tonopah.

Just east of Tonopah, US 6 continues east across a series of desert mountain ranges and valleys, including the Monitor Range. At Warm Springs, SR 375, also known as the "Extraterrestrial Highway", departs to the southeast and US 6 assumes a northeasterly alignment across the Reveille, Pancake, Grant and White Pine ranges. Rainfall increases eastward, so valleys become less barren and peaks over 11,500 feet (3,500 m) add scenic interest.

Ely is the largest city on US 6 in Nevada. US 50 joins US 6 at Ely. East of Ely, US 6/US 50 cross the Schell Creek Range, known for verdant forests and meadows and for a large mule deer and elk population. The highway descends to Spring Valley, then crosses the Snake Range at Sacramento Pass, north of Nevada's second-highest mountain, Wheeler Peak, where a branch road accesses Great Basin National Park. Beyond the pass, US 6 passes just north of Baker, a Mormon farming community, and reaches the Utah state line.

US 6 enters Utah concurrently with US 50 in a remote portion of the Great Basin Desert; the routes separate at Delta. US 6 then proceeds on a northeast course toward the Wasatch Front serving the Tintic and Eureka historic and mining districts along the way. Upon entering the Wasatch Front, at Santaquin, the route joins I-15 to Spanish Fork, which is where US 6 reverses course on a southeastern path away from the Wasatch front, also joining with US 89 for the journey through Spanish Fork canyon. After cresting the Wasatch Range via Soldier Summit, the route descends into Utah's coal country, which is where it joins US 191. At Green River, those two routes join I-70 and rejoin US 50. From Green River east the routes travel concurrently using the route of I-70 following the southern rim of the Book Cliffs. US 191 leaves the concurrency at Crescent Junction but the other three routes remain concurrent as they continue to follow the Book Cliffs toward the Grand Valley into Colorado.

US 6 is basically parallel to, or runs concurrently with, I-70 for a significant portion of its length as it generally heads east from the Utah state line thru about half of Denver. It is unsigned while it is overlapped. It separates to become I-70 Business (I-70 Bus.) through the south side of Grand Junction, rejoining I-70 just east of Palisade. It again separates west of Rifle and runs through that town, Silt, New Castle, and Chacra until it again joins I-70. It separates from and runs parallel to, I-70 at Dotsero, where the Eagle River joins the Colorado River. It again joins I-70 just east of Avon, the location of the Beaver Creek Resort, just a bit west of Vail. A significant departure from I-70 occurs at Silverthorne where it veers a bit south then north, avoiding the nearly two-mile-long (3.2 km) tunnel on I-70 as it goes under the Continental Divide. It passes Keystone Resort and Arapahoe Basin before a steep climb to the highest altitude along US 6, 11,990 feet (3,650 m), at Loveland Pass, where it crosses the Continental Divide. It continues down the Clear Creek valley as it again reaches I-70 at the Loveland Ski Area straddling the eastern mouth of the Eisenhower Tunnel. Slightly east of Idaho Springs, it separates to go through the scenic Clear Creek Canyon. At Golden, it combines with 6th Avenue to head south until it again crosses paths with I-70. Then, continuing east, it passes through Lakewood and west Denver as the 6th Avenue freeway. From near the eastern terminus of the 6th Avenue freeway, US 6 takes a convoluted route through Denver. First, it turns north on I-25, then heads east on I-70 to Vasquez Boulevard through Elyria-Swansea. The route emerges from the freeway alongside US 85 through Commerce City, where the pair briefly join State Highway 2 (SH 2) before separating at Colorado Boulevard. SH 2 represents the historic alignment of US 6 northeast to Brighton, while US 6 combines with I-76, heading northeast, until east of Brush. It then separates to join I-76 Bus. until it crosses I-76 east of Sterling, where it changes direction from the Interstate. It continues east until it reaches Nebraska. The last town in Colorado that it passes is Holyoke.

From the Colorado state line, US 6 starts going southeast. The first town it goes into is Imperial. US 6 runs concurrently with US 34 near Culbertson, passing through McCook. US 6 then moves to the northeast, through Hastings. At Hastings, US 34 diverges and moves north. US 6 parallels I-80 north of Milford until it reaches Lincoln. At Lincoln, US 6 becomes West "O" Street, Sun Valley Boulevard, and eventually Cornhusker Highway as it moves north of I-80 outside of the city, paralleling I-80 to Gretna. There US 6 moves due north and becomes the West Dodge Expressway and Dodge Street in Omaha. It passes through Downtown Omaha on parallel one-way streets and runs concurrently with I-480 in Omaha on its last Nebraska segment. East of Omaha, it crosses the Missouri River to enter Iowa at Council Bluffs, Iowa, on a girder bridge completed in 1966 that replaced the Ak-Sar-Ben Bridge, which was the first road bridge to connect the two cities.

US 6 enters Iowa at Council Bluffs, across the Missouri River from Omaha where it intersects I‑29 within the first mile. Through Council Bluffs, it travels south concurrent with I‑29, then turns east to run concurrent with I‑29 and I‑80. Where I-29 and I-80 split, US 6 continues northeast concurrent with I-80. The concurrency with I-80 ends at the interchange with East Kanesville Boulevard, on the northeast side of Council Bluffs, at which point US 6 turns east to run through rural Pottawattamie County. Further east, the highway briefly overlaps US 59 near Oakland and continues east until Lewis. There, it turns sharply north-northeast to Atlantic where joins US 71 until I‑80. It overlaps I‑80 between US 71 and US 169 at De Soto. US 6 travels north with US 169 to Adel, then turns east to go toward Des Moines. It enters the Des Moines metro area along Hickman Road in Waukee and then forms the border between Urbandale to the north and Clive and Windsor Heights to the south. In Des Moines, Iowa Highway 28 (Iowa 28) joins US 6 at 63rd Street, and the two highways head east and north on Merle Hay Road. US 6 splits away at Douglas Avenue and it continues east. Near the Des Moines River, Douglas Avenue becomes Euclid Avenue. In the northeastern part of the city, it intersects US 69 and I‑235. The highway turns onto Hubbell Avenue and heads into Altoona where it meets US 65 and rejoins I‑80.

East of the Des Moines metropolitan area, US 6 again overlaps I‑80 until they reach Newton. There, it splits away to the north and runs parallel to the Interstate. It passes through Grinnell, Marengo, and the Amana Colonies before arriving in Coralville. In Iowa City, it passes through the University of Iowa campus on the banks of the Iowa River. The highway heads southeast from Iowa City toward West Liberty and then more easterly to Wilton. At Wilton, the highway heads north to rejoin I‑80 until they reach Davenport. After a short overlapping of I‑280, US 6 enters the city along Kimberly Road, on which it remains until I‑74. The two routes then run south to cross the Mississippi River into Illinois on the I-74 Bridge.

In Illinois, US 6 parallels I-74 and I-80, mostly along its original routing, overlapping with I-74 for its first five miles (8.0 km) and I-80 for the final two miles (3.2 km) of its routing in Illinois. US 6 directly serves the downtowns of many cities for its length, including Oak Forest, Tinley Park, Moline, Geneseo, Atkinson, Annawan, Princeton, Peru, La Salle, Ottawa, Channahon, and Joliet—unlike US 20, which, in Illinois, mainly consists of freeway sections that bypass the cities US 6 serves. Like nearby US 30 and US 52, US 6 skirts the Chicago city limits.

US 6 crosses the state line and shares the same Borman Expressway with I-80 and I-94 through Hammond and Gary, until State Road 51 (SR 51, exit 15); it then runs south for about two miles (3.2 km) and turns east until it meets US 421 in Westville, then runs south for a mile (1.6 km), then east until it meets US 31 and US 35, and it shares the same road with US 33 for about five miles (8.0 km) until Ligonier, where US 33 breaks south toward Fort Wayne. From there, it is mostly two lanes through Indiana until it meets the Ohio state line just east of Butler. Before the Borman Expressway was completed, US 6 was on Ridge Road, portions of which were signed US 6 Bus.

US 6 enters Ohio from Indiana in Williams County. It travels through Edgerton, then just south of Bryan before it passes through Napoleon, Bowling Green, and Fremont, before turning northeast toward Sandusky Bay and Lake Erie. After passing through Sandusky, the highway follows the southern shore of Lake Erie, passing through Huron and Vermilion. After crossing the Charles Berry Bridge in Lorain, it passes through the western suburbs of Greater Cleveland as Lake Road in Sheffield Lake, Avon Lake, Bay Village, and Rocky River, and Clifton Boulevard in Lakewood and the West Boulevard–Edgewater neighborhood of Cleveland proper. US 6 follows the Cleveland Memorial Shoreway into Downtown Cleveland, entering downtown by crossing the Detroit–Superior Bridge. US 6 follows Superior Avenue through Public Square and the east side of Cleveland before turning east onto Euclid Avenue in East Cleveland and Chardon Road in the city of Euclid. US 6 continues eastward through Lake County, Geauga County, and finally into Ashtabula County before entering the state of Pennsylvania along the Pymatuning Reservoir causeway.

Ohio also has an alternate route of US 6 in Greater Cleveland.

US 6 runs for 403 miles (649 km) in Pennsylvania between its entrance point 20 miles (32 km) west of Meadville and its exit at Matamoras. From the Ohio border to US 322 in Conneaut Lake, the route runs in a southeasterly direction. US 6 then joins US 322 and heads east to Meadville, picking up US 19 west of the city. South of downtown, US 322 splits from the concurrency while US 6 and US 19 remain concurrent through Meadville. The two routes continue northward to Mill Village, where US 6 and US 19 split at a junction with US 6N.

For the remainder of its routing in Pennsylvania, US 6 runs roughly parallel to the New York–Pennsylvania border. Along the way, US 6 is concurrent with US 62 for a short distance near Warren. US 11 joins US 6 from the north in Factoryville. They run concurrently to Scranton, where US 11 continues south and US 6 east. At Milford, US 6 meets US 209. The two routes embark to the northeast, crossing the Delaware River from Matamoras to Port Jervis, New York.

The 79-mile (127 km) portion of US 6 in New York travels through Orange, Putnam, and Westchester counties, going across the southern part of the state through the northern suburbs of New York City. The route enters the state along with US 209 in Port Jervis. The two routes split just north of town, with US 209 taking a more northerly route to access Kingston. US 6, in contrast, runs primarily east–west through southern New York.

A section of US 6 runs concurrent with New York State Route 17 (NY 17; The Quickway) between Goshen and Harriman. The route is currently being upgraded to become part of I-86. At Harriman, US 6 passes through one of the largest interchanges in the state of New York, where US 6/NY 17 meet I-87 (New York State Thruway) and NY 32. NY 17 becomes an at-grade road and heads south, while US 6 remains a limited-access freeway as it heads east into Harriman State Park. Near the east side of the park, US 6 intersects the Palisades Interstate Parkway and runs concurrently with it to the historic Bear Mountain Bridge, where US 6 is joined by US 202 and is narrowed to a rural two-lane road as it crosses the Hudson River with scenic views of the Hudson Highlands.

On the other side of the river and exiting Orange County for Westchester, US 6 and US 202 run along the Hudson River to Peekskill and has a short concurrency with US 9 before the three routes split, allowing US 6 to continue to the northeast through northern Westchester County and into Putnam County. At Shrub Oak, US 6 has an interchange with the historic Taconic State Parkway, one of the first and most scenic long-distance freeways in the U.S. In Brewster, US 6 joins US 202 once again, with the routes running concurrently into Connecticut. US 6 and US 202 also have a large interchange with I-84, I-684, and NY 22 in Brewster.

US 6 runs for 116.3 miles (187.2 km) in Connecticut. It enters from the town of Southeast, New York, concurrent with US 202, shortly passes through begins the city of Danbury, and ends at the Rhode Island state line in the town of Killingly. In western Connecticut, US 6 either closely parallels or is concurrent with I-84, serving as the local route in the suburbs of Danbury, Waterbury, Bristol, and Hartford. It crosses the Connecticut River (overlapped with I-84 and US 44) on the Bulkeley Bridge. In eastern Connecticut, US 6 is one of the principal routes connecting Hartford and Providence, passing through the small urban areas of Willimantic and Danielson. The unsigned portion of the Connecticut Turnpike then meets with US 6 shortly before crossing the Rhode Island state line.

US 6 covers approximately 26.5 miles (42.6 km) in Rhode Island from Foster (western border with Killingly, Connecticut) to East Providence (eastern border with Seekonk, Massachusetts). In and around Providence, US 6 overlaps with Route 10, as well as US 1A, US 44, I-95, and I-195.