[3] Most of the southern part of the route in Montgomery County passes through suburban areas around Rockville and Gaithersburg that are home to many biotech firms.

North of the Gaithersburg area, the road continues through the northern part of Montgomery County, passing Germantown and Clarksburg as a six- to eight-lane highway with an HOV lane in the northbound direction only.

Increasing traffic levels led to a $200-million (equivalent to $496 million in 2023[4]) widening of the road in Montgomery County to its current configuration.

I-270 heads northwest from an interchange with I-495 (Capital Beltway) and MD 355 (Rockville Pike) in suburban Bethesda, Montgomery County, as a six-lane freeway.

[5] Past MD 28, the freeway passes numerous business parks and some wooded areas before coming to an interchange with Shady Grove Road.

This highway provides access to a park and ride lot at the Shady Grove station, which serves Washington Metro's Red Line, and MD 200 (Intercounty Connector), which heads east to I-95 in Laurel.

[5] I-270 continues past more suburban development before coming to a northbound exit and southbound entrance with MD 117 (West Diamond Avenue).

[2][5] Past MD 124, the northbound local lanes continue to the diamond interchange at Watkins Mill Road, after which they end.

A short distance later, after passing by some business parks, I-270 comes to an interchange with MD 27 (Ridge Road) and Father Hurley Boulevard.

[2][5] Past MD 27, I-270 heads into more rural areas of woods with some farm fields, paralleled by a set of power lines.

[2][5] A short distance past the MD 109 interchange, I-270 crosses into Frederick County, continuing northwest through rural woodland and farmland.

[2][5] The highway briefly runs along the east side of MD 80 past this interchange before continuing through agricultural areas.

[30] The construction of the Washington National Pike resulted in suburban growth along the corridor between Washington, D.C., and Frederick, with several federal agencies including the United States Atomic Energy Commission and the National Bureau of Standards moving their headquarters to Montgomery County.

[31] I-70S was projected to continue past the Capital Beltway into Washington, D.C., on the North Central Freeway to connect directly to I-95.

[36] In 1975, the Federal Highway Administration (FHWA) officially approved withdrawal of plans to construct the road south of the Capital Beltway.

[37] Similar opposition also affected I-70N's eastward extension into Baltimore proper, although a small part of its planned spur, I-170 (now US 40, as the freeway never connected to any other Interstate), was built and opened in 1979.

This widening led to growth of residences and businesses along the I-270 corridor as far north as Germantown and increased traffic counts along the road.

[43] In Montgomery County, an interchange was constructed to allow northbound and southbound I-270 access both to and from Watkins Mill Road in Gaithersburg.

[53] In June 2021, the project was omitted by the Washington Council of Government's Transportation Planning Board, calling its future into question.

[54] In June 2022 Montgomery County Executive Marc Elrich asked the U.S. Department of Transportation (USDOT) to extend its review period for the I-270 toll lane proposal, citing the need for additional time for local agencies and the public to review the environmental analysis.

[55] In July 2022 advocates of public transit questioned the validity of Maryland DOT's traffic models for the proposed highway widening project, in a letter submitted to USDOT.

[57] In August 2023, Maryland DOT applied for a federal grant to support improvements on the American Legion Bridge and a portion of I-495 leading to the junction with I-270.

[2][6][59] I-270 Spur continues north through wooded suburban areas to an interchange with Democracy Boulevard adjacent to the Westfield Montgomery shopping mall.