US 1 is the easternmost of the main north-south U.S. Highways, all of which end in one, but there are areas where it is not the easternmost route of the system, with large portions of US 9, US 13, US 17, and US 301 occupying corridors closer to the ocean. When the system was laid out in the 1920s, US 1 was mostly assigned to the existing Atlantic Highway, which followed the fall line between the Piedmont and the Atlantic Coastal Plain north of Augusta. At the time, the highways further east were of lower quality and did not serve the major population centers.
US 1 travels along the east coast of Florida, beginning at 490 Whitehead St. in Key West and passing through Miami, Fort Lauderdale,West Palm Beach Fort Pierce, Melbourne, Titusville, Daytona Beach, Palm Coast, St. Augustine, and Jacksonville. The southernmost piece through the Florida Keys, about 100 miles (150 km) long, is the two-lane Overseas Highway, originally built in the 1930s after the Florida East Coast Railway's Overseas Railroad was ruined by the Labor Day Hurricane of 1935. The rest of US 1 in Florida is generally a four-lane divided highway, despite the existence of the newer I-95 not far away. State Road A1A is a mostly-continuous beachfront alternate to US 1, cut only by inlets and the Kennedy Space Center. North of Jacksonville, US 1 turns northwest in order to reach the fall line at Augusta, Georgia; US 17 becomes the coastal route into Virginia, where US 13 takes over. Until the 1990s, US 1 used high contrast markers (white text on a red background).
Through Virginia, US 1 is paralleled by Interstates: the remainder of Interstate 85 to Petersburg, Interstate 95 through Richmond and Fredericksburg to Alexandria, and Interstate 395 into Arlington. Within Virginia, US 1 is called Jefferson Davis Highway by state law, although local communities have renamed it without consequence. It is best known as "Jeff Davis Highway". US 1 crosses the Potomac River with I-395 on the 14th Street Bridges, and splits to follow mainly 14th Street and Rhode Island Avenue through the District of Columbia. After exiting the District into Maryland, US 1 follows the Baltimore-Washington Boulevard, the first of several modern highways built along the Baltimore-Washington corridor; I-95 is the newest, after the Baltimore-Washington Parkway. The route bypasses downtown Baltimore on its west and north sides and exits the city to the northeast on Belair Road, gradually leaving the I-95 corridor, which passes through Wilmington, Delaware, for a straighter path towards Philadelphia. Around and beyond Bel Air, US 1 is a two-lane road, crossing the Susquehanna River over the top of the Conowingo Dam before entering Pennsylvania. (US 1 bypasses Delaware, unlike I-95).
The two-lane US 1 becomes a four-lane freeway, officially known as the John H. Ware III Memorial Highway, upon crossing into Pennsylvania. This bypass extends around Oxford and Kennett Square, merging into the four-lane divided Baltimore Pike just beyond the latter. At Media, US 1 again becomes a freeway - the Media Bypass - ending just beyond Interstate 476. After several name changes, the road becomes City Avenue, the western city limit of Philadelphia, at the end of which a short overlap with Interstate 76 leads to the Roosevelt Expressway and then the twelve-lane Roosevelt Boulevard. US 1 again becomes a freeway after leaving the city, bypassing Penndel and Morrisville and crossing the Delaware River into New Jersey on the Trenton-Morrisville Toll Bridge.
After crossing into New Jersey, US 1 continues on the Trenton Freeway through the city of Trenton, becoming a divided highway with frequent jughandles once the freeway ends. The highway bypasses New Brunswick before merging with US 9 in Woodbridge, beyond which the US 1/9 concurrency continues through the rest of the state. The divided highway remains through Rahway and Elizabeth, though largely without jughandles, until it reaches the Newark Airport, where it becomes an eight-lane freeway around downtown Newark. The historic Pulaski Skyway takes US 1/9 into Jersey City, and the route exits the freeway at Tonnele Circle to head north into Bergen County. After joining the almost-freeway US 46, the three routes run northeast to the George Washington Bridge plaza, where they merge into I-95. US 46 ends in the middle of the bridge, which crosses the Hudson River into New York, and US 9 exits just beyond onto Broadway in Manhattan, but US 1 stays with I-95 onto the Cross-Bronx Expressway, exiting in the Bronx onto Webster Avenue. Two turns take US 1 via Fordham Road to Boston Road, which it follows northeast out of the city to the state line, never straying far from I-95.
The short portion of US 1 in New Hampshire follows the historic Lafayette Road, staying close to I-95, before leaving the city of Portsmouth on the Memorial Bridge over the Piscataqua River. Within Maine, US 1 begins as a parallel route to I-95 near the Atlantic Ocean. At Portland, I-95 splits off to the north, and I-295 heads northeast with US 1 to Brunswick. There US 1 turns east as a mostly two-lane road along the coast to Calais; this portion is advertised as the "Coastal Route" on signs. North from Calais, US 1 follows the Canadian border, crossing I-95 in Houlton and eventually turning west and southwest to its "north" end at the Clair-Fort Kent Bridge in Fort Kent. The short Route 205 extends north on the New Brunswick (Canada) side of the bridge to Route 120, a secondary east-west route from Edmundston, New Brunswick west to Saint-Alexandre, Quebec.
The direct predecessor to US 1 was the Atlantic Highway, an auto trail established in 1911 as the Quebec-Miami International Highway. In 1915 it was renamed the Atlantic Highway, and the northern terminus was changed to Calais, Maine. Due to the overlapping of auto trail designations, portions of the route had other names that remain in common use, such as the Boston Post Road between Boston and New York, the Lincoln Highway between New York and Philadelphia, Baltimore Pike between Philadelphia and Baltimore, and the Dixie Highway in and south of eastern Georgia. North of Augusta, Georgia, the highway generally followed the fall line, rather than a more easterly route through the swamps of the Atlantic Coastal Plain.
When the New England road marking system was established in 1922, the Atlantic Highway within New England was signed as Route 1, with a Route 24 continuing north to Madawaska; New York extended the number to New York City in 1924 with its own Route 1. Other states adopted their own systems of numbering, and by 1926 all states but Maryland had signed the Atlantic Highway as various routes, usually changing numbers at the state line. In 1925, the Joint Board on Interstate Highways created a preliminary list of interstate routes to be marked by the states, including Route 1 along the Atlantic. This highway began at Fort Kent, Maine and followed the existing Route 24 to Houlton and Route 15 to Bangor, beyond which it generally followed the Atlantic Highway to Miami. In all states but Georgia that had numbered their state highways, Route 1 followed only one or two numbers across the state. The only significant deviation from the Atlantic Highway was between Augusta, Georgia and Jacksonville, Florida, where Route 1 was assigned to a more inland route, rather than following the Atlantic Highway via Savannah.
One of the many changes made to the system before the final numbering was adopted in 1926 involved US 1 in Maine. The 1925 plan had assigned Route 1 to the shorter inland route (Route 15) between Houlton and Bangor, while Route 2 followed the longer coastal route via Calais. In the system as adopted in 1926, US 2 instead took the inland route, while US 1 followed the coast, absorbing all of the former Routes 24 and 1 in New England. Many local and regional relocations, often onto parallel superhighways, were made in the early days of US 1; this included the four-lane divided Route 25 in New Jersey, completed in 1932 with the opening of the Pulaski Skyway, and a bypass of Bangor involving the Waldo-Hancock Bridge, opened in 1931. The Overseas Highway from Miami to Key West was completed in 1938, and soon became a southern extension of US 1.
With the construction of the Interstate Highway System in and after the 1950s, much of US 1 from Houlton to Miami was bypassed by Interstate 95. Between Houlton and Brunswick, Maine, I-95 took a shorter inland route, much of it paralleling US 2 on the alignment proposed for US 1 in 1925. Between Philadelphia and Baltimore, I-95 leaves US 1 to pass through Wilmington. Most notably, I-95 and US 1 follow different corridors between Petersburg, Virginia and Jacksonville, Florida; while US 1 followed the fall line west of the coastal plain, I-95 takes a more direct route through the plain and its swamps. Although some of this part of US 1 was followed by other Interstates - I-85 between Petersburg and Henderson, North Carolina, and I-20 between Camden, South Carolina and Augusta, Georgia - the rest remains an independent route that has been four-laned in many places. By the late 1970s, most of I-95 had been completed, replacing US 1 as the main corridor of the east coast and relegating most of it to local road status.