U.S. Route 15 (US 15) is a part of the United States Numbered Highway System that runs from Walterboro, South Carolina, north to Painted Post, New York.

US 15 is the descendant of a pair of turnpikes that connected Frederick with Emmitsburg to the north and Buckeystown to the south.

The highway crosses the river on the Point of Rocks Bridge, a two-lane, eight-span camelback truss bridge that also passes over the Chesapeake and Ohio Canal and CSX Transportation's Metropolitan Subdivision railroad line on the Maryland side of the river.

The U.S. Highway heads north as a two-lane expressway that passes through a mix of farmland and forest on the eastern flank of Catoctin Mountain.

US 40 exits the freeway at a partial cloverleaf interchange with Patrick Street, which heads east toward downtown Frederick and west through the heavily commercialized Golden Mile.

All three streets provide access to Fort Detrick, home of the US Army Medical Research Institute of Infectious Diseases.

Skirting the Frederick city limits, the highway crosses Tuscarora Creek and parallels the Monocacy River.

Just after passing the Frederick city limits, the highway has a superstreet intersection with Sundays Lane.

At the northern end of Auburn Road, MD 806 (Catoctin Furnace Road) begins to parallel the northbound side of the highway, which follows the eastern boundary of Cunningham Falls State Park just west of the village of Catoctin Furnace.

Access to the main section of the state park is provided by Catoctin Hollow Road, which intersects the southbound lanes of US 15 and follows Little Hunting Creek up the mountain.

On the southern edge of the town of Thurmont, the highway has a diamond interchange with unnamed and unsigned MD 15C that leads to the northern end of MD 806 and to Frederick Road, which serves a commercial area south of downtown Thurmont.

The highway heads northeast through farmland and meets Motters Station Road (also the northern end of MD 76) at a superstreet intersection before bisecting the campus of Mount St. Mary's University.

[1][7] North of Mount St. Mary's University, US 15 meets the southern end of US 15 Business (US 15 Bus.

North of the intersection, US 15 becomes a four-lane freeway that continues across the Pennsylvania state line as the U.S. Highway's bypass of Gettysburg.

[9] By the end of 1927, US 240 was reduced to a Washington–Frederick highway and US 15 connected Point of Rocks and Emmitsburg via Frederick as it does today.

[20][21] Around 1938, the U.S. Highway was resurfaced with asphalt from Emmitsburg to the Pennsylvania state line and from the south end of Frederick to Evergreen Point; in addition, a grade separation was constructed with the Western Maryland Railway in Thurmont.

[25] US 15 originally followed Commerce Street and Canal Road, which is now used to access the Chesapeake and Ohio Canal National Historical Park, through an at-grade crossing of the Baltimore and Ohio Railroad to the Maryland end of the bridge, which crossed the river immediately to the east of the modern bridge.

[26] The 1889 iron bridge was swept away in the March 1936 flood that also destroyed the Potomac River crossings at Harpers Ferry, Shepherdstown, and Hancock.

[27] In November 1936, the MDSRC put together a plan to replace the Potomac River bridges at Hancock, Shepherdstown, and Point of Rocks.

[29] The new bridge and its approaches, which crossed over the Baltimore and Ohio Railroad on the Maryland side, opened December 26, 1937.

[31] The highway's cloverleaf interchange with US 15, which would serve as the freeway's northern terminus for several years, was built in 1952 and 1953.

The adjacent portion of US 15 from Evergreen Point south to Lime Kiln was relocated, reconstructed, and widened in 1951 and 1952.

[40] The Frederick Freeway gained exit numbers in 1965, the same year the highway's interchange with 7th Street was added.

[53] The U.S. Highway was expanded to four lanes from MD 355 at Harmony Grove to Angleberger Road in Lewistown in 1973.

[56] As part of this project, the oblique intersection with MD 806 on the north side of Thurmont was removed.

[62] US 15 in Frederick is a subject of the I-270/US 15 Multi-Modal Corridor Study, which encompasses the corridor of the two highways from I-270's interchange with Shady Grove Road in Montgomery County to north of US 15's intersection with Biggs Ford Road north of Frederick.

[63] As part of another long-term project initiated by the I-70 Corridor Planning Study, Jefferson National Pike's interchange with I-70, which was constructed with two ramps in 1969, received two ramps from eastbound I-70 in 1997; that same year, access from northbound US 15 and US 340 to westbound I-70 was added at the Frederick Freeway interchange.

[70] In 2011, MDOT SHA completed a study to examine constructing an interchange at Monocacy Boulevard on the north side of Frederick to replace the present directional crossover intersection.

Design work then occurred on a diamond interchange with the municipal highway, which serves as a circumferential arterial in the northeastern part of the city.

As part of this project, US 15's intersection with Hayward Road was removed, resulting in the northern end of US 15's freeway section being extended from MD 26 to Tuscarora Creek.