Exploring Better Density Maps and Link Station Areas

Let's talk about density maps.

The concept of a population density map seems easy: take the population and divide it by the area to retrieve the density. Unfortunately a major problem stands out: what exactly is the area to divide by and how to easily count the population? Traditional density maps suffer from unevenly-sized census blocks that can inflate or deflate the density. Newer density maps created using satellite imagery can be much more accurate with evenly-sized areas calculated throughout the world. They're much better at highlighting both the density clusters and where transit should go throughout the Seattle metro area.

Old Density MapsCity of Seattle density map using census tractsSeattle Metro Area 2020 Census Tract Density Map

https://maps.geo.census.gov/ddmv/map.html

The older density maps shown above are based on census tracts, which are of different sizes and were arbitrarily defined before recent population trends. Small census tracts can accidentally be calculated as overly dense, or they can leave out two apartment buildings just outside the boundary and thus understate the density. Large census tracts can hide pockets of density within them implying lower density, or misleadingly imply higher density throughout the entire tract when it was just one block that housed apartments.

Modern Density Maps

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• Above WorldPop 100m blocks: https://www.arcgis.com/home/webmap/viewer.html?webmap=f9cb5e481ecc48bfbdd6486c42e19a24&find=seattle
• WorldPop Less granular 1km blocks: https://www.arcgis.com/apps/mapviewer/index.html?layers=38b7672a1977426caa6cf4c62e37b777
• Other Population density map using GHSL: https://luminocity3d.org/WorldPopDen/#10/47.6455/-122.3053
• Job Density map of USA using census: https://www.rclco.com/wp-content/uploads/2020/10/USA-Employment-Map-2017_Layers_v3.html

Nowadays using a combination of satellite imagery + existing census data we can see density granularity down to the 100m² to 1km² range. All these blocks are the same size, allowing us to compare city blocks rather than just mile-wide areas. For instance, Lake City has a low-income apartment cluster at 145th & 30th NE, a commercial cluster on Lake City Way, and single-family blocks around them. If you just take the average of the entire census block, it can fail to indicate how dense those apartments are, or what people in them can walk to, or what's in the walkshed of bus stops along Lake City Way. The newer mapping granularity is useful to calculate walk sheds for transit ridership.

The 100m² map reveals denser townhouses or apartments that are obscured by averaging in the larger 1km map. In contrast, the 1km² map is better suited for comparing larger regions. For instance, consider comparing one city that has denser townhouses (4k people per square kilometer) with double/quadruple the density versus a city that just has single family homes (1~2k people per square kilometer). Overall density is difficult to compare with the 100m² map's green gradient. While with the 1km² map one can click on the individual square to bring up the estimated people per square kilometer.

For most of the examples below both maps will be used to highlight certain aspects, the luminocity3d map's color gradient is slightly easier to tell where apartments are in red, while the WorldPop map is easier to compare townhouse vs single family home density for a neighborhood as a whole.

East LinkEast Link Job Density Map

East Link connects many job clusters with Downtown Bellevue, the Microsoft Campus, and in Bel-Red. However, connecting jobs is only useful when also connected with residents. Hopefully that gap to Seattle residents will be bridged after East Link fully opens across I-90.

Population Density Map of Bellevue in 100 meter square segments. Less to more density from white, blue, redPopulation Density Map of Bellevue in 100 meter square segments. Less to more density from yellow, green, blue

On the other hand, while the light rail will definitely have higher ridership when trains crosses Lake Washington, its potential to serve Bellevue residents is hampered by not running on Bel-Red Road or NE 8th Street. As shown above, the light rail alignment misses almost every pocket of existing apartment density from NE 8th Street to Crossroads. Only the apartments in downtown Bellevue (shown in red left image) are reached.

Lynnwood Link along the freewayLynnwood Link Extension density map. Less to more density from yellow, green, blue

The Lynnwood Link extension is built along i-5 rather than Aurora Avenue, so most stations miss the pockets of density between Northgate station and Lynnwood City Center station.

Lynnwood Job Density Map

But of course Lynnwood City Center station does give access to jobs and retail, using the above job dot map (red) as a proxy for places where one can shop. Note that we can see some errors in the above map. For instance, Alderwood Mall appears as an empty (vacant) area.

Lynnwood Link bus connections to Aurora Avenue and Lake City

When the full Link 2 Line (Lynnwood-Seattle-Redmond) is open, the Lynnwood Link Connections bus restructure will be fully implemented. Sound Transit plans to redirect the Route 522 from Lake City Way NE (and terminating at Roosevelt station) to head west on N 145th and terminate early at North Shoreline station. Metro responded by adding more routes in the area the 522 will abandon. Some of these have raised doubts among some transit activists, such as the east-west part Route 72 or the L-shaped Route 77. While 145th, 130th, and Lake City Way must be served somehow, are these the optimal route alignments? We've covered this in a previous article.

North Seattle density map

The above density map shows Seattle north of Lake Washington Ship Canal. Most know about the large number of people living around UW as well as clusters in Ballard and Fremont. Some have argued Route 72 and 77 as being duplicative of Lynnwood Link and the new Route 522 and argued for their truncation or elimination. However, there are other corridors of moderate density along Lake City Way NE and Greenwood Ave that are important to serve as well.

North Seattle density map annotated with Lake City Way bus routes

The new Route 522 (in yellow) terminating at Shoreline South Station actually misses most of the density along Lake City Way NE. Most of the density is south of NE 145th St. The Route 72 (in pink) connects those apartments on Lake City Way NE and then heads south down 25th Avenue. Maintaining Route 72's duplication on NE 145th with Route 522 is necessary to connect Lake City residents to a Link station without transferring immediately. Route 77 (in green) traveling east across on N 130th St connects the apartments on Greenwood Ave near Bitter Lake to future 130th Link station and Lake City.

Federal Way Link and RapidRide A South King density map annotated with RapidRide A route

There is a slight misconception among some that along Pacific Highway from Seatac to Federal Way there isn't much density. The Pacific Highway corridor has many apartments shown in red along the corridor.

Riverstone Apartments from Google Street View

Though note, the apartments are not five-over-one's but the older design of 2-3 story stacked units with surface parking in front (aka garden apartments").

Federal Way Link Extension with density overlayed

One can also see how routing Federal Way Link along I-5 misses most of the apartments in the area, which are along Pacific Highway South. The freeway alignment minimized construction and guideway impacts in the area, but the light rail stations are now a lot less useful for the community, and it's missing potential stations at 216th and 240th. Transit riders along Pacific Highway will have to continue using RapidRide A to get to these neighborhoods rather than directly using an elevated light rail line along the corridor.

Future ST3 Link lines

The next three examples are about future ST3 Link projects: the Everett extension, Tacoma Dome extension, and the Issaquah-South Kirkland line. We'll take a brief look at the density near the stations as well as the opportunities or drawbacks of each one.

Everett LinkEverett Link Map

Everett Link's detour to Paine Field for manufacturing jobs has been criticized by some for lengthening the travel time between Everett and Seattle. However, the detour alignment actually reaches a sizable amount of density around the stations of Mariner, Airport Road, and Evergreen.

Everett Link Extension with density map

This makes the route stronger than expected at first glance. Even if the alignment was chosen for originally the wrong" reasons, the sizable number of residents near each station will greatly increase ridership.

Tacoma Dome Link StationTacoma Dome Link ExtensionTacoma density map annotated with Tacoma Dome Station

You can see how Tacoma Dome Link Station manages to fall just short of the residential density in downtown Tacoma. Instead the line ends east of the I-705 freeway spur. Troy Serad recognized this problem wrote about some ways to bring Link into downtown Tacoma.

South Kirkland to Issaquah LinkIssaquah Link corridor map annotated with station locations in circles

Above shows the South Kirkland to Issaquah Link light rail and its relatively poor alignment. The proposed line fails to reach any residents between Issaquah and downtown Bellevue. The line manages to avoid not only apartments but even all single family homes.

ST Express Route 554 map

In comparison, the existing ST 554 bus reaches downtown Issaquah as well as the Issaquah Highlands. While technically a truncated bus route could end at Issaquah Link light rail stations, it's unlikely anyone would desire to transfer twice to reach Seattle. Instead of Issaquah Link, it might be more useful to add an HOV ramp to Issaquah or add tolling to the existing I-90 HOV lane.

Lower Density Suburbs

Of course not everywhere is an exception. For many cities farther from Seattle, the density maps validates the conventional assumption of low-density single-family sprawl that' are's hard to serve with transit. The darker green patches represent density around 2K per square kilometer, light green around 1K per square kilometer, and the bright yellow below that.

Conclusion

The new modern density maps, using satellite imagery and census data, help both planners and advocates better understand where people live, work, and play. By focusing on dense areas, transit systems can maximize their impact and better serve the community. Hopefully future Link alignments, BRT and RapidRide projects, and bus restructures can take advantage of the knowledge shown here and try to avoid the kinds of mistakes made in the past.