serves the southwestern Baltimore County community of Halethorpe and connects US 1 with full-access interchanges with Interstate 95 (I-95) and I-695.

The Caton Avenue portion of the alternate route was improved and expanded in the 1930s to serve as a rerouting of US 1 in southwest Baltimore.

heads northeast as Washington Boulevard, a four-lane undivided highway that immediately crosses over Amtrak's Northeast Corridor railroad line, which carries MARC Train's Penn Line, and Herbert Run.

; that movement is made via a ramp from eastbound I-695 to Sulphur Spring Road, which intersects US 1 Alt.

continues northeast to an intersection with Caton Avenue and Hammonds Ferry Road just south of the Baltimore city limits.

The alternate route leaves Washington Boulevard to turn north onto four-lane divided Caton Avenue and enters the independent city.

heads north as a six-lane divided highway through a seven-ramp partial cloverleaf interchange with I-95.

The highway reduces to a four-lane undivided highway and passes by Seton Keough High School, the former Cardinal Gibbons School, and St. Agnes Hospital before reaching its northern terminus at US 1 (Wilkens Avenue).

The highway from west of the Pennsylvania Railroad (now Amtrak's Northeast Corridor) to the city limits of Baltimore at Gwynns Falls was reconstructed as a 16-foot-wide (4.9 m) concrete road by 1915.

[6] The highway, which was marked as US 1 in 1926,[7] was widened to 40 feet (12 m) with the addition of a pair of 10-foot-wide (3.0 m) concrete shoulders, and resurfaced with sheet asphalt in the late 1920s.

was expanded to a divided highway in 1972 in conjunction with the completion of the western half of I-95's interchange with US 1 Alt.