Traveling in a southeasterly direction, it goes through the cities and towns of Williston, Blackville, Denmark, Bamberg, Branchville and St. George, where connects with I-95. Continuing east, it passes through Dorchester and begins to parallel with I-26 in a southeasterly direction through Summerville. It enters Charleston County at Lincolnville, where the highway skirts just inches south of the Berkeley County line. In North Charleston, US 78 shares a concurrency with US 52, as it connects with I-526. Entering Charleston, it splits with US 52 and goes south along King Street; its eastern terminus at Line Street, a block away from US 52's eastern/southern terminus and nearby to I-26/US 17 interchange.[7]
US 78 is a predominantly two-lane rural highway connecting much of the Lowcountry. Wider sections (four-lanes or more) are found between North Augusta–Aiken, St. George, and North Charleston. Though it provides the most direct route between Augusta (and points west, such as Atlanta) and Charleston (using US 278 and SC 781 to bypass Aiken), it is not a busy route because of the many cities and towns it goes through.[7]
By 1931, US 78 was rerouted north of Summerville and Lincolnville, leaving Richardson Avenue/Lincoln Avenue. Around 1939, US 78 was realigned from Meeting Street to Rivers Street, south of Durant Avenue in the Charleston area. Also in 1939, US 78 was fully paved in the state, its last unpaved section was between St. George and US 178. By 1952, US 78 was placed on new four-lane highway bypassing Clearwater, Burnettown, and Gloverville; also by same year a new bridge was constructed over the Savannah River, leaving Fifth Street Bridge. By 1967, US 78 was adjusted to bypass south of mainstreet Blackville, leaving a secondary road, but was later upgraded as a connector (signed as a business loop).[8][9]