The Washington National Pike, US-240, was completed from Frederick, Maryland to Connecticut Avenue near Chevy Chase, Md. in the late 1950's, in conjunction with the US-40 bypass of Frederick. These highways were four-lane freeways.

In the early 1960's, US-240 became I-70S, and the Frederick bypass became I-70. I-70 enters Maryland at Hancock, and split into two routes at Frederick, I-70N to Baltimore, and I-70S to Washington. The easternmost two miles of US-240 (through Rock Creek Park) was upgraded and became part of I-495. That explains the winding alignment of I-495 today in that area. With the construction of I-495, the west wye of I-70S was built and designated I-270. The two wyes are in a long-distance sense the same road, as it takes both of them to interchange I-495; the east wye interchanges I-495 east and the west wye interchanges I-495 west/south. In a local sense, since each wye has an interchange with a local thoroughfare, they are separate roads, so the two wyes are kind of an unusual highway feature. There were the I-270 roadside trailblazers on the west wye, but the big overhead signs signed it southbound as "TO I-495 SOUTH" and northbound as "TO I-70S NORTH".

In February 1975, the I-70S/I-70N designations were changed; I-70N was changed to I-70, and I-70S was changed to I-270. The west wye was changed to I-270 Spur. The upgrade of the four-lane US-40 Baltimore National Pike east of Frederick was not completed until 1975; this became six-lane I-70. The 10 miles of I-70N between US-40 and I-695 was completed in the late 1960s. The rest was completed after the route number change.

I-70N never existed in a complete Frederick-to-Baltimore route. The 7-mile-long US-40 four-lane freeway bypass of Frederick was completed in the late 1950s, and in the early 1960s it was designated with portions of I-70 and I-70N, with the easternmost 2 miles being I-70N. The four-lane-divided US-40 Baltimore National Pike was completed between Frederick and Baltimore in the mid-1950s. The 10 miles of I-70N between US-40 near West Friendship, and I-695, was completed in the late 1960s, and extended 2 miles to Security Boulevard at the Baltimore city line about 1970. The lane widths are, four lanes from US-40 to US-29, six lanes from US-29 to I-695, and eight lanes from I-695 to where the Interstate was truncated at the city line. Those were the original lane widths and the highway has not been widened. The rest of the Frederick-Baltimore Interstate highway was completed after the 1975 route number change. The 22-mile upgrade of the four-lane-divided US-40 Baltimore National Pike from Ijamsville Road east of Frederick to the I-70/US-40 junction near West Friendship was completed in 1975; this became six-lane I-70. The timing would indicate that all sections of this 22-mile-long section were under construction by the time that the route number changed in 1973. The last 6 miles of I-70 between the east end of the Frederick Bypass and Ijamsville Road was built on new location with six lanes and was completed in 1986, and the US-40 highway that it bypassed was an at-grade expressway.

Why did Baltimore get the main-line I-70 designation and Washington the spur I-270? Some cities may feel more status with a main-line interstate, but with D.C. being the center of power that it is, most residents probably couldn't care less whether they got I-70 or I-270. And, the decision was probably a Maryland decision rather than a D.C. decision, naturally, Maryland would tend to favor Baltimore. Besides, I-70 is an east-west route, so it probably makes more sense to continue east to Baltimore than southeast to D.C.

In 1972, I-270 was widened to six lanes from the wye split near Tuckerman Lane to MD-118, a distance of 13 miles. In 1989, I-270 was widened to twelve lanes (2-4-4-2 configuration) from the wye split to I-370 (9 miles), and eight lanes from there to MD-118, a total of 13 miles. In 1997, widening to six lanes was completed from MD-118 to MD-121, a distance of 3 miles. In 1997, widening to six lanes was completed on both wyes, with upgraded interchanges with I-495. This completes the expansion of I-270 from I-495 to MD-121. These projects cost over $400 million to construct. Studies are underway to widen I-270 to six lanes from MD-121 to Frederick.

I-370 is unusual in two respects. One is the fact that it is an interstate spur off another interstate spur (I-270). The second is the fact that it was built as a spur into a rapid rail transit station. The Metrorail Red Line ends at the Shady Grove Station, and I-370 is the direct expressway link from the station to I-270 northwestward for interfacing cars, van pools, and busses. I-370 is 3 miles long, and has four lanes. I-370 was opened around 1990. The proposed Inter-County Connector would extend from I-370 to the Baltimore-Washington Parkway at Laurel, Md., but to my understanding, would not have an interstate design or designation.

One of its main purposes of I-370 was to be a spur into the rapid rail transit station. However, it extends west of I-270 also, as an arterial (non-Interstate) road. The whole corridor serves as a relief route for busy Shady Grove Road. Also, I-370 was developed with the idea that the Inter-County Connector would eventually branch from I-370, about 3/4 mile north of the Metro station, and head east about 18 miles to the Baltimore-Washington Parkway at Laurel.

(Completed 9-1997, updated 1-20-2002)