How dense is Seattle? It depends on what geographic area is meant by “Seattle” and also temporal factors like day of the week and hour of the day. For instance, the Downtown Seattle Association’s 2014 economic report estimates nearly 60,000 residents in the “greater” downtown area (roughly, Mercer to SODO and Elliott Bay to Broadway) with a weekday population exceeding 230,000. That implies a pretty impressive density. And downtown Seattle isn’t alone. Redmond, per the census, has one of the largest increases in weekday population in the country. It’s interesting to see what effect this daytime concentration has on the distribution of people in the region and on overall measures of density.

Let’s look at downtown Seattle first. Using the Census Bureau’s 2014 LODES data on primary employment locations, I count about 225,000 jobs in greater downtown. The geographic distribution of these jobs by census block group is shown below. GIS files defining census boundaries came from the PSRC’s public data.

The area adjacent to Westlake Park dominates the map with nearly 40,000 jobs in its block group alone. And of course, most of those workers don’t live downtown; they had to come from somewhere else. The LODES data set comes in handy here because it estimates the home locations, as well as the employment locations, for all workers in the state. After the jump, I’ve mapped the percentage of the population that has a job downtown for every census block group in the metro area based on the 2014 LODES data.

You can see similar maps for workers in downtown Bellevue, UW, Overlake, and Paine Field by clicking through the album.

So what does this mean for population density on an average weekday? For that, I built a simple model that simulates the morning and evening commute process using population data from the ACS 2010-2014 estimates and LODES employment data. I should note here that LODES data are derived from administrative records that employers submit to States and, as such, will not always list the correct location of home or employment. Student commutes to/from schools are also excluded. So, there is a certain amount of uncertainty in population shifts inferred from LODES data. To add to that uncertainty, my model has a free parameter, a commute rate, which I calibrated via comparison with the Downtown Seattle Association’s estimates of weekday population. Given those caveats, the final result is an estimate of the population and density for every block group in the Puget Sound Region at any time of day.

To condense this information into a more manageable format, it’s useful to average over a typical 24-hour weekday. The map below shows the results of that calculation for the Seattle metro, with the major employment districts of Seattle, Bellevue, and Overlake clearly visible.

The main point of this post is to report some representative numbers, so I’ll further average the density over some geographic regions of interest.

The second column of the table reports a delightfully wonky quantity called “population-weighted density” which is basically the density experienced by an average person in the given region. Martin wrote a nice piece on weighted density back in 2012 and the census has some interesting material on the topic too. The third column in the table reports the weighted density averaged over a typical 24 hour weekday. As these weekday averages depend on my model estimates, they should be taken with some salt (orders-of-magnitude are probably more relevant). Still, the results here show the reality we are all used to living with: the Seattle metro area consists of high density islands in a sea of sub/exurban sparsity. This contrast is even more clear when employment is considered with population density.

To put those numbers in context, consider the densities (as of 2014) for some other cities – New York, San Francisco, and Nashville.

Note the stupendous density of the five NYC boroughs; the weighted density is more than five times that of Seattle! And the NYC number includes “suburban” Staten Island. If you’re feeling inadequate at this point, just make a comparison with a typical sunbelt city. Nashville is a similarly sized city to Seattle (pop: 680,000 in 2015), but has a fraction of the density. San Francisco is also a peer in terms of population, but is, of course, far more compact.