Frederick Road splits off from US 40 as a county highway about east of the eastern end of the Mount Airy – Ellicott City portion of MD 144. This county highway parallels US 40 to the south as it passes through residential subdivisions, crossing the Little Patuxent River, intersecting Centennial Lane and St. John's Lane, and passing the historic home MacAlpine. Frederick Road passes under US 29 (Columbia Pike) before intersecting Toll House Road. The county highway intersects Rogers Avenue and descends into downtown Ellicott City as Main Street. Within the Ellicott City Historic District and nearby are the Howard County offices and the Howard County Circuit Courthouse, the remains of the Patapsco Female Institute, and the Ellicott City Station, which preserves the oldest remaining passenger train station in the U.S. as a railroad museum. The highway passes under the Oliver Viaduct, which carries CSX's Old Main Line Subdivision, before crossing the Patapsco River into Baltimore County.