The Red Line Metro passes over Rockville Pike by Matthew Straubmuller licensed under Creative Commons.

The Washington region’s population continues to grow, albeit a bit slower than in previous years. Montgomery County is no exception with more than 200,000 more residents expected to be added within the next 30 years.

We can draw insights on how we could grow from remote sensing data products, an often overlooked tool in planning. GGWash contributor Leah Brooks and her colleagues at George Washington University wrote about the National Land Cover Database (NLCD), a dataset generated from satellite imagery. It classifies land within 30-meter (100-foot) squares into one of 16 categories. Four of these categories describe development in terms of land use intensity, or how much of each square is covered by constructed, impervious surfaces instead of vegetation.

I’d like to focus on what the NLCD tells us about development near rapid transit corridors. Transit corridors make the most sense to examine because there are fewer political, environmental, and capital obstacles to creating density near existing transit networks. We can enable growth while containing sprawl. Growth along existing corridors was emphasized in current drafts of Montgomery County’s Thrive Montgomery 2050 plan.

Land within walking distance of the Red Line is not fully utilized

Let’s take a look at the Red Line first. The figure below shows land use classification within a half-mile radius of all stations along this line. In planning, half a mile is chosen because it corresponds to a roughly 10-minute walk. To make this plot, I simply add up all the categorized areas in the NLCD dataset within half a mile of each station. I recategorize everything that isn’t labeled “Developed” or “Open Water” to “Undeveloped.”

In Silver Spring, one of Montgomery County’s most urbanized areas, 69% of land falls under high and medium-intensity development. In contrast, the quiet residential neighborhoods surrounding the neighboring Forest Glen and Takoma stations fall at 20% and 30%, respectively.

Only seven of the 12 stations with surrounding land in Montgomery County have more than 50% high and medium-intensity development.

Development along the Red Line in Montgomery County is constrained by zoning

Much of this land use intensity is constrained by zoning. Let’s focus on just Montgomery County, dividing the Red Line into east and west corridors. This comparison omits Rockville, where all of the surrounding land is governed by a municipality rather than the county.

We can compare the land use with the actual zoning map of Montgomery County by adding up all the zoned land area within half a mile of a station according to broad zoning categories. Doing so reveals the unsurprising relationship between land use intensity and zoning.

The total land area with medium and high development intensity is negatively correlated (r = –0.79) with the total area zoned for single-family detached housing. For example, Twinbrook and White Flint, which are almost entirely surrounded by mixed-use zones, outpace all other stations in the county in terms of land use intensity.

Forest Glen, which is famously deep underground because of local resistance to building a station, retains single-family zoning and low land use intensity in its surroundings.

Of the seven stations with more than 50% high and medium land use intensity, only one (Medical Center) has more than half of its surrounding land area zoned for single-family housing.

The Purple Line has even more room for transit-oriented development

Land use in Montgomery County has plenty of room to grow along the Red Line and the intensity of land use is closely related to zoning. What about along the Purple Line? I’ve repeated the analysis in the half-mile walksheds of Purple Line stations in the figure below. The NLCD dataset shows that land along this corridor is much less developed. Only three of the 12 stations with walksheds in Montgomery County have more than 50% high and medium land use intensity.

In Montgomery County, the relationship with zoning is clear. Eight of those 12 stations have more than 50% of land in their walksheds zoned for single-family detached housing.

Growth along corridors can be encouraged by amending zoning codes

We’ve looked at urbanization and zoning within walking distance of the major transit corridors of Montgomery County, and seen how zoning constrains growth. That may change soon. On December 8 2020, Montgomery County Councilmember Will Jawando proposed an amendment to the county zoning code, ZTA 20-07, which would loosen restrictions on building intermediate-density housing on land zoned R-60 within a mile of Red Line stations.

R-60 sites within half a mile of these stations would be loosened even further. R-60 single-family housing zones cover over 17,400 acres of county land. The table below shows the extent of R-60 zones near stations.

Total R-60 zoned land near Red Line stations (acres)

Metro Station

½ Mile

1 Mile
Friendship Heights 165 825
Bethesda 188 1031
Medical Center 355 1023
Grosvenor-Strathmore 115 399
White Flint 0 160
Twinbrook 1 10
Takoma 128 569
Silver Spring 37 767
Forest Glen 265 1350
Wheaton 93 853
Glenmont 160 565

Note: Some areas overlap with multiple stations.

ZTA 20-07 would ease one of the biggest constraints on housing in the region. The proposed amendment would allow for missing-middle housing in over 6,400 acres in Montgomery County. Missing-middle housing includes intermediate-density housing, such as duplexes, townhouses, and courtyard apartments.

These represent an incremental improvement over single-family detached housing. Enabling missing middle housing near transit stations potentially means tens of thousands of new housing units with access to jobs and amenities anywhere in the region via rapid transit.

This bill is set for a public hearing on February 11.

Montgomery County’s rail corridors are an ideal place to accommodate its coming growth. The satellite data we’ve examined here show that there is plenty of room along these corridors. Zoning is currently the main obstacle to growth near transit stations. Amending the county zoning code to enable missing middle housing will be an important first step in ensuring future residents can live near transit.

Ryan Hardy is a geoscientist who lives and works in downtown Silver Spring. He conducts research in geodesy, the study of Earth's precise shape and gravity field. He holds a Ph.D. in aerospace engineering sciences from the University of Colorado Boulder.