How Dense is California?
By Mike Stajura and Devin Lavelle
California is an incredibly diverse state, in more ways than one. The nation’s most populous state includes sparsely populated areas in the state’s sprawling deserts, mountains, forests, and agricultural regions as well as some of the densest areas in the country, in San Francisco and Los Angeles, making it, on balance, the 11th most densely populated state. Many statewide policy issues (e.g., water, recycling, utilities, and insurance) are described in terms of their differential impact on the binary and imprecise typology of “urban” versus “rural” counties (alternatively, “small, medium, and large,” etc.). Closer analysis, though, reveals that it is not quite so cut and dry. Every state Senate district, nearly every Assembly district, and most counties, contains a mix of high-, medium- and low-density tracts. Despite that, the bulk of California’s population is concentrated in relatively compact areas.
In fact, 10% of California’s population is concentrated in just 0.1% of its geographic area, while 80% is concentrated in just 3.0% of its geographic area, and 90% is concentrated in just 5.1% of its geographic area. These realities likely impact the efficiency and impacts of policies implemented across such a vast state.
The California Research Bureau developed these insights when analyzing population at the census tract level for an issue for which there was a very acute sense of “urban versus rural” jurisdictions in the stakeholder discussions. We heard things like, “there’s no one-size-fits-all policy.” It turns out there’s no one-size-fits-all jurisdictional typology, either. In most cases, population thresholds to differentiate among small, medium, and large counties (for example) are based on conveniently round numbers, like “under 100,000,” or “500,000 to 1,000,000”).
In order to help better inform policy discussions about regional efficiencies and impacts, we developed the Population Density Interactive. The interactive shows how density varies around the state and by Senate and Assembly districts. It allows users to see the portion of the state needed to reach customized shares of the population and it provides a walkthrough to help understand how density plays out in our state.
Our methodology started with examining census tract data. Nominally, a census tract is meant to represent “about 4,000” people, with a range of somewhere between 1,000 to 8,000 people. The interactive shows that tracts are generally in this range, but with variation on both ends.
The mean and median census tract populations (4,350 and 4,200, respectively) are on target with expectations, California has 115 census tracts with a population of under 1,000 (32 of them have a population of zero). It also has 42 tracts with a population over 10,000, totaling more than half a million Californians. The most populous tract, in northern San Diego County, has 37,892 residents, nearly 10 times the target.
The most compelling takeaway is how much of California’s population resides in densely populated areas. Over 60% of California’s population lives in areas with a density of at least 5,000 people per square mile. Another 26% live in areas with 1,000 to 5,000 people per square mile. This leaves less than 13% of the population in areas with less than 1,000 people per square mile – but those areas make up more than 95% of the state’s land area.
Using the Density Slider tool, you can see where those populations live, selecting any cutoff that may prove useful. This could be helpful when designing a policy intended to efficiently reach a given percent of the population. Here we see that 90% of California’s population lives in just 5.1% of the state’s land area. Nearly all of this is concentrated in the San Francisco Bay Area, Los Angeles area, San Diego area, or along the Interstate 5/Highway 99 corridors through the inland areas.
We focused our analysis on census tracts rather than political boundaries. While many policies focus on counties, most counties (and districts) include both denser and less dense areas. Further, some of the densest tracts were in counties that are traditionally “rural” areas. The population centers within these lightly populated counties are portioned off as a separate tract from the other tract(s) in the county, making them extremely dense relative to other areas. Further, many of these population centers have state colleges, prisons, or other factors that increase the number of people living there.
When calculating summary data, we learned that when using counties with an overall density of 500 or fewer people per square mile as a cut-off between “large” and “small,” 841 of the 3,654 census tracts with over 8,000 people per square mile (23.0%) were in the 45 least populous counties (out of 58). When changing the threshold to counties with a density of less than 250 per square mile, the 40 least populous counties still accounted for 559 of the 3,654 densest census tracts (15.3%).
Likewise, 939 of the 1,377 (or 49% of those with 500 or fewer people per square mile) smallest census tracts (those with a population less 2,000 people) were in the 13 most densely populated counties. Almost all counties have both very high- and very low-density areas within them, but the feature that is not yet accounted for here is the relative or proportionate areas which are accounted for by these high- and low-density areas. On that point, the largest census tract in the state has an area of 6,591.8 square miles, and the smallest census tract with residents has an area of 0.01 square miles (22 census tracts have a population of zero).
Moving to legislative districts, we find that Senate Districts 11 (89% high-density), 26 (80%), 28 (81%) and Assembly Districts 17 (92%), 54 (86%), and 57 (95%) have the largest number of high-density tracts. By contrast, Senate Districts 1 (64% low density) and 4 (55%) and Assembly Districts 1 (77%), 34 (54%), 2 (57%), and 8 (52%) have the most low-density tracts.
The tool allows users to parse the data in numerous ways to better understand the distribution of California’s population around the state. We hope this tool is helpful to California policymakers and their staff as they work to implement policies most efficiently and invite them to look to the California Research Bureau to dive deeper as further questions arise.