An approximately 33 mile long north/south freeway spur of Interstate 70 connecting the northwestern side of Washington, D.C.'s beltway with Frederick, passing through the large suburbs of Rockville and Gaithersburg along the way. The freeway runs through both Montgomery and Frederick Counties, labeled on many maps as Dwight D. Eisenhower Memorial Highway and Washington National Pike. Interstate 270 is one of the oldest freeways in the state of Maryland, serving now as a major road for the Montgomery County suburbs.
Originally US Highway 240, the highway became Interstate 70S with the arrival of the United States Interstate System. There was also an Interstate 70N, which is now the section of Interstate 70 between Frederick and the western side of Baltimore. Like Interstate 70N, Interstate 70S was changed to Interstate 270 to eliminate suffixed interstate numbers. There are still some remaining signs within Baltimore City that have never been updated to reflect the removal of the 'N' and 'S' suffixes. The southern end of I-70S originally ran to Connecticut Avenue, and was originally intended to extend down into Washington, D.C. roughly parallel to the red line of the metro rail system. This plan was killed, like many DC metropolitan area freeways, by local opposition. This small section of I-70S was eventually absorbed into Interstate 495.
In most of Montgomery County, Interstate 270 exists with two persisting local and four persisting express lanes in both directions, consisting of twelve lanes of traffic in most locations. The far left lane of the express lanes is reserved for HOV-2 (High Occupancy Vehicle, Two+ People) traffic during rush hour periods. The local and express lanes are separated by a jersey wall with occasional exit ramps allowing local traffic out onto the express lanes, and express traffic into the local lanes to local exits.
The southern terminus of Interstate 270 divides into two separate highways carrying traffic to the different loops of Interstate 495, also known as the Capital Beltway. The eastern spur heads toward the inner loop of the beltway, carrying traffic east to Silver Spring, College Park, and Baltimore. The western spur takes traffic to the outer loop of the beltway, south into Northern Virginia. Both the east and west spurs have a lane on the left in both directions dedicated to HOV-2 traffic during rush hour periods. The eastern spur is designated just plain Interstate 270, with the western spur carrying Interstate 270 Spur signage.
The northern terminus of Interstate 270 ends in a junction with US Highway 40 and Interstate 70, as an interchange in Frederick currently under heavy reconstruction. North of this interchange, Interstate 270 becomes US Highway 15 and heads toward Gettysburg.
Interstate 270 suffers from horrible gridlock traffic due to rapid development in Montgomery and Frederick Counties, following along with the long commute many people make into Washington, D.C. from as far as Frederick.