US 340 is a north/south route extending from Greenville, Virginia, to Frederick, Maryland. It enters Maryland from Virginia via a two-lane bridge over the Potomac River. On the other side, it widens to become a four-lane expressway. At first, it continues the name from Virginia, Jefferson Pike. After a few exits, Jefferson Pike splits off to become MD 180, while US 340 continues northeast as Jefferson National Pike. In Frederick, Jefferson National Pike merges with Catoctin Mountain Highway (US 15) and continues northeast with the designation of US 15. US 340 is signed as being multiplexed with US 15 until US 40, but it is listed as officially ending at the merge in state documents.
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