A map of every intersection and dead-end inside the Beltway, color-coded by type. Image by the author.

Recently, Bloomberg Citylab featured my maps of intersections in US metro areas. Their article didn’t include maps of the Washington region, so here’s a comparison of intersections in different parts of our area.

Why intersection types matter

Street networks are an important part of urban form, both for pedestrians and for public transit. In traditionally gridded cities, like New York, a regular pattern of long, straight roads provides natural routes for buses and good connectivity for pedestrians to get where they’re going without taking indirect routes. When three-way intersections predominate, buses often have to follow winding, indirect routings to serve communities — for example the A22 in Prince George’s County — or many routes overlap on the small number of straight through streets, such as Rte 7 and US-1 in Northern Virginia.

Suburban areas built after World War II, on the other hand, tend to have curving streets that rarely meet at four-way intersections, and also have such a large number of cul-de-sacs that DC-area cartoonist Richard Thompson’s nationally-syndicated comic strip about suburban life was called Cul de Sac.

This change in street networks was a consequence of Federal policy: the Federal Housing Administration, established during the New Deal to underwrite loans for the construction of new residential subdivisions, promoted residential areas made up of “neighborhood units” that discouraged through traffic by not having complete grids. Then, in 1965, the Institute of Transportation Engineers published recommendations that subdivisions should avoid four-way intersections and should use cul-de-sacs and curving streets with only three-way intersections to slow traffic.

Today, many local governments are trying to encourage a return to better-connected street networks with few dead-ends and more four-way intersections. For example, Fairfax County is encouraging better connectivity in Tysons and Prince George’s County regulations now require new developments to connect their internal road networks to neighboring areas better. However, it can be an uphill climb. Besides the obvious difficulties of retrofitting existing street networks to add connections, residents in many areas are still pushing to have intersections closed to reduce traffic in their neighborhoods.

Intersections in DC and the suburbs

A look at the intersections inside the Beltway shows that the urban core of the DC region doesn’t so much have one rectilinear street grid as several of them. In the original L’Enfant core of the city — the area bounded by the Potomac, the Anacostia, Rock Creek, and Florida Avenue — most intersections are four-way, though clusters of three-way intersections can be seen at circles, along K Street (because the OpenStreetMap road network data I used treats the service lanes as separate roads), and where major avenues cut across the street grid.

A map of every intersection and dead-end inside the Beltway, color-coded by type. Image by the author.

Some areas of the District outside the L’Enfant core, which largely developed as streetcar suburbs in the late 19th and early 20th centuries, also have their own street grids, but they aren’t well connected to each other. The largest of these, north of Florida Avenue and bounded to the west by Rock Creek Park and the east by the Old Soldiers’ Home and Rock Creek Cemetery, provides a regular street network along some of the region’s busiest bus routes on 16th Street and Georgia Avenue.

The Brookland-Langdon area along Rhode Island and Michigan Avenues is the largest area with a regular street grid in Northeast, but it is largely separated from other areas with connected street networks by New York Avenue and the institutional land uses along North Capitol Street and the Metropolitan Branch. West of Rock Creek, Chevy Chase, Friendship Heights, and Tenleytown form a similar area, separated by Rock Creek and other stream valleys from much of the L’Enfant City.

East of the Anacostia River, we do see grids of four-way intersections along East Capitol Street and in historic Anacostia, but much of the area is much more broken-up into post-War-style street networks.

Finally, Georgetown — which, along with Old Town Alexandria — had a street grid predating the establishment of the District and the city of Washington is visible as a relatively isolated section of closely-spaced four-way intersections, with three-way intersections visible along Wisconsin Avenue.

A map of intersections and dead-ends in Montgomery County, color-coded by type. Image by the author.

Looking north, to Montgomery County, we see a definite gradient. Inside the Beltway, and just north of Veirs Mill Road, we see areas with street networks that largely consist of four-way intersections, stitched together by areas where three-way intersections dominate. Further north, though, cul-de-sacs become at least as common as three-way intersections, especially in the Gaithersburg area.

A map of intersections and dead-ends in Prince George's County, color-coded by type. Image by the author.

Prince George’s County has a much less connected street network on a large scale: even inside the Beltway, there are barriers, such as Kenilworth Road, the Baltimore-Washington Parkway, US-50, Central Avenue, and Suitland Parkway that have very few connections across them. Traditional rectilinear street networks are most visible in the several streetcar suburbs in the Route One corridor running as far north as Beltsville: this area was home to the county’s only long-lived streetcar line. A number of small gridded areas are visible along the Northeast Corridor rail line and along the former Baltimore, Washington, & Annapolis interurban line.

A particularly noticeable plan can be seen in the portions of Bowie that were part of the Levitt and Sons’ “Belair at Bowie” subdivisions built around 1960. This area has unusually closely-spaced intersections for Prince George’s County outside the Beltway, and it is very notable that they are nearly all three-way intersections: both cul-de-sacs and four-way intersections are very rare.

A map of intersections and dead-ends in Fairfax County, color-coded by type. Image by the author.

Several areas of Arlington County and Alexandria seem to have relatively gridded street networks, including Old Town Alexandria, of course, but also Del Ray, Aurora Highlands, and the Ballston-Rosslyn corridor.

The Northern Virginia suburbs are more densely filled with intersections outside the Beltway than the Maryland ones are, though — as usual — the Difficult Run valley is a notably sparse area. Cul-de-sacs seem to be significantly more common than they are in Maryland, though, with significant numbers of them even inside the Beltway.

Intersections in Baltimore and Richmond

Expanding our view to include other nearby metro areas shows that Baltimore and Richmond look a bit different from Washington, with more consistent rings of street network types: gridded areas in the center, mixtures of three and four-way intersections further out, and finally a belt of cul-de-sac country.

A map of intersections and dead-ends in Baltimore, color-coded by type. Image by the author.

Baltimore’s core area of tightly-spaced, gridded streets is similar in size to the L’Enfant City, which is not really surprising in light of the observation that pre-streetcar cities rarely grew more than three miles across, since few people were willing to commute more than about a mile and a half — a thirty-minute walk — to get to work.

A map of intersections and dead-ends in Richmond, color-coded by type. Image by the author.

Richmond’s densely-gridded core is a bit smaller but, as in Baltimore, it is surrounded by less-dense areas with more three-way intersections that likely had their origins as streetcar suburbs. Neither city, however, has the many satellite grids that our region has.

Comparison to cities outside our region

It may come as no surprise that New York’s street network is unique in the country. Not so much for having regular grids as in the size of the area covered by densely urban grids: not only Manhattan but nearly all of Brooklyn and much of Queens and the Bronx as well. Still, the particularly-dense network in lower Manhattan — which developed as a traditional pedestrian city before the Manhattan street grid was plotted out in 1811 — stands out from the larger-blocked grids in the portions of the city that developed later along streetcar, elevated, and finally subway lines.

A map of intersections and dead-ends in New York, color-coded by type. Image by the author.

On the other hand, the Boston area — which has nearly the same urban area population as Washington according to the Census — has far fewer four-way intersections than our region. This doesn’t mean that the area’s street networks aren’t well-connected: much of the region developed as streetcar suburbs, but with three-way rather than four-way intersections as the norm.

A map of intersections and dead-ends in Boston, color-coded by type. Image by the author.

It’s notable, however, that the areas with the most regular rectilinear grids, such as South Boston and the Back Bay were built on landfill in the 19th Century, while the old pedestrian-city core of Boston, which is notoriously ungridded, is largely made up of three-way intersections.

A map of intersections and dead-ends in Atlanta, color-coded by type. Image by the author.

Atlanta, another city with a very similar urban area population to Washington, but with a much more car-oriented and unwalkable urban form, shows that intersection density can matter as much as the types of intersections. Atlanta has larger areas of primarily four-way intersections than Boston but, except for a small core in downtown, few areas with many densely spaced intersections. Furthermore, as soon as one leaves Atlanta proper, cul-de-sacs seem to become nearly as common as intersections of any type.

These differences in street grids can be more persistent than density patterns, because road networks, once built out, are so rarely changed, even as population and employers move in or out of an area. This makes it particularly important that new developments be built with walkable, transit-friendly grids.

At the same time, the quality — or repairability — of the street grid needs to be taken into account in plans to densify areas and improve their transit. Areas with low density, but strong street grids, such as the portion of Northwest west of Rock Creek Park are particularly appealing areas for increased densification.