The Demographics of Urban Commuters: How do Drivers Differ From Public Transit Riders?

Today, the prevailing wisdom seems to point to widespread demographic differences between people who commute by public transit and people who commute by car. A quick google search reveals strong evidence suggesting both economic and racial disparities between the two groups, a trend that’s backed by substantive research. I’ll talk about why one might expect each of those disparities between those who drive and those who take public transit, and then dig into some American Community Survey data to see how measurable these differences are.

One might expect a large economic gap between car commuters and public transit riders. The reason for that is pretty simple; there are clear cost advantages to public transit. In 2018, a new car had an average sale price of around $37,000. Taking into account maintenance and gasoline costs, AAA found that owning a car in 2018 cost owners an additional $8,849 per year on average. And that doesn’t even include high parking costs in large cities. In contrast, transit passes in large US cities cost only $67 per month, or $804 annually, according to ValuePenguin. Given this reality, it makes sense that transit riders would be more economically disadvantaged than car commuters. This difference probably gets reflected through a number of economic markers, including lower homeownership rates and higher poverty rates among transit riders.

In addition, there is evidence to suggest that immigrants are disproportionately likely to rely on public transit relative to cars. A study by the HUD’s journal, Cityscape, found that immigrants were less likely to own cars and more likely to use alternate modes of transportation. To approximate immigration status for the purposes of this article, I measured the percent of commuters who are not United States citizens.

Finally, one might also expect racial disparities between car commuters and public transit commuters. A report from the American Public Transportation Association, Who Rides Public Transportation, found that 24% of transit riders are African American. In comparison, African Americans make up only 12% of the general population. The overrepresentation of African Americans in mass transit can be explained in part by the racial wealth gap, but it may also be attributable to 20th-century housing policies. Douglas Rae’s book, Urbanism and its End, describes how New Deal policies prevented nonwhite residents from secure home loans and and buying in more desirable neighborhoods. Post WWII, federally-subsidized development paid for white city residents to move to newly constructed suburban neighborhoods, while the aforementioned policies, combined with private sector discrimination, denied nonwhite residents that same opportunity. These generations of government policies reinforced racial segregation in urban areas, which in turn contributed to the de facto segregation of mass transit. Based on this history, it is likely that the racial gap between urban and suburban residents will be reflected in commuter demographics.

I’m interested in understanding how these economic and racial disparities look across different cities. To do that, I compared income, poverty rates, homeownership rates, race, and citizenship status across 100 US cities. These cities had the largest total number of car and public transit commuters, as measured by the 2017 American Community Survey. The following five box plots show different demographic variables, for each city, for car commuters and public transit commuters. Each set of box plots allows the reader to sort each of the columns.

1. Median Earnings:

2. Poverty Rates:

3. Homeownership Rates:

4: Percent African American:

5. Percent Noncitizen:

For each of the five variables I looked at, the demographic difference between car commuters and public transit commuters was statistically significant. In general, the data from these 100 cities conformed to the expectations that public transit commuters are more likely to be economically disadvantaged, African American, or a citizen of another country. In some of the cities, particularly large cities in the Northeast and West Coast, the size of the disparity between car commuters and public transit commuters is more modest. There are even a handful of cities that do not conform to expectations, cities where car commuters actually had slightly lower earnings, higher poverty rates, etc. than public transit commuters.

The data I’ve presented aligns with existing studies, finding that public transit commuters are, as a group, disadvantaged compared to car commuters. And that disadvantage is compounded by policies that neglect or even undermine improvements and expansions of existing transit systems.

Historical development patterns as well as contemporary policies have shaped how accessible public transit is to commuters in different cities. Decades of disinvestment and political resistance to mass transit have limited transit infrastructure and had devastating impacts on urban commuters. This history played out miserably in cities like Atlanta and Tampa Bay, as an article from StreetsBlogUSA, “How Racial Discrimination Shaped Atlanta’s Transportation Mess,” and an article from The Tampa Bay Times, “A long way to go,” explain.

In any discussion of public transportation and who rides it, it’s worth noting that not all public transit systems are equal. While many cities on my top 100 list still don’t have light rail in 2019, Boston opened an operational streetcar in 1897. And today, Boston joins San Francisco, New York, and Washington, as home to one of the best public transit systems in the country. In those cities and others, taking public transportation every day sometimes means being able to afford living in expensive downtown neighborhoods that are transit-accessible. In a housing boom that prices residents out of urban centers and into the surrounding metro areas, daily public transit use in some cities is becoming a privilege. As housing costs continue to rise and urban neighborhoods gentrify, I would expect the demographic gap between public transit riders and car commuters to shift.

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Data sources: United States Census Bureau

Programs: MS Excel, Tableau

The Racial Income Gap in American Metro Areas

Just how bad are the current racial poverty and income disparities in cities? The map and scatterplot below show the gap between African American and white poverty rates in 104 metro areas in the US.

If the poverty rates among African American and white residents were equal, that trendline would be at 45 degrees. These metro areas tend to have significant racial disparities among their lowest-income residents. Most of the metro areas with the largest disparities in this map are in the north. The most extreme case of this is Boise, ID Metro Area, where 12% of white residents live in poverty, compared to 61.3% of African American residents.

In addition to poverty rates, income can also help illustrate how black and white residents fare in and around large cities. In this country’s largest metro areas, the disparities are significant and ubiquitous. The following chart shows the average distribution of earnings among black and white residents in the largest US metropolitan areas. The disparity in earnings is consistent and statistically significant; however, the extent of the it varies significantly by metropolitan area. The menu and sliding scale above the chart shows the distribution of White and African American earnings in each of the country’s 104 largest metro areas:

These 104 metropolitan areas are ordered by population size as of 2017. Most of these metro areas follow a similar pattern. In general, a higher percentage of African American residents occupy the lowest two income brackets. For virtually all metro areas in this dataset, the largest racial gap exists among top earners, with white residents typically twice as likely to earn $100,000+.

These extreme gaps in earnings among large-metro-area residents are symptomatic of even more entrenched racial inequalities. In 2015, an analysis by the Federal Reserve Bank of Boston revealed that, while the average net worth of white households in the Boston metropolitan statistical area was $247,500, the median net worth of black households of Greater Boston was only $8. African-American residents were significantly less likely to own homes, cars, and other assets than white residents, and much more likely to be in debt. Last year, the National Bureau of Economic Research released a report suggesting that even after controlling for education, neighborhood effects, and family characteristics, African American residents continued to trail white residents on income mobility. The statistics I’ve presented on the poverty and income gaps are troubling, but they are also only the tip of the iceberg when looking at racial economic inequality in metropolitan areas.

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Data sources: United States Census Bureau

Programs used: MS Excel, Google Fusion Tables, Google Sheets, Tableau

Population Size, Urban Density, and Party ID: How Liberal is Massachusetts?

According to a Pew study, in 2014 Massachusetts was ranked the second most liberal US state, with 56% of adults identifying with or leaning towards the Democratic Party. Even though the majority (54%) of the Commonwealth’s voters in are unaffiliated with any party, registered Democrats still outnumber Republicans 3:1, and Hillary Clinton received twice as many votes as Donald Trump did in 2016.

What I’m trying to understand is whether and how this widely-recognized liberal identity varies across different cities and towns. There is good reason to think that geography and political affiliation are not independent from one another; in fact, there’s an extensive literature on the relationship between the two. Democrats cluster in cities, and they do it so much so that, as The New York Times put it, “Republicans don’t even try to win cities any more.” Books like The Politics of Resentment by Katherine Cramer document a deeply ingrained political, cultural, and economic divide between rural and urban areas.

It’s pretty clear that Massachusetts has some degree of an urban-rural political divide. Somerville — my hometown, the densest city in Massachusetts and the 16th densest city in the country — is a widely hailed as a liberal haven. Somerville is part of MA’s 7th congressional district, which recently held an election that WBUR identified as a “battle between progressives and Democrats.” MA-7 is so thoroughly blue that whoever won the primary was virtually guaranteed to also win the House seat. However, west of Somerville, Massachusetts does have a handful of towns that vote Republican, as the Boston Globe observed in late 2016. And perhaps the clearest signal of a geographic political divide is how the 2016 Democratic primary turned out. Bernie Sanders outvoted Hillary Clinton in rural counties across the country, and MA was no exception. As the Boston Business Journal reported, “the results [in Massachusetts] generally fell along urban versus rural lines.”

In order to illustrate the urban-rural divide in Massachusetts, I made an interactive map of statewide voter enrollment data, shown below:

Those who are familiar with maps of Massachusetts will notice that this one looks pretty similar to the state’s population density map.

Using the following two scatterplots, I show this association between the party ID of voters and the population size of those voters’ municipalities. This dataset contains all cities and towns in Massachusetts, omitting statistical outliers:

A few things jump out about these graphs. First, yes, the cities and towns tend to have a much higher percentage of Democrats than they do Republicans. Second, the data points in the Democrat plot are upward sloping. A higher percentage of Democrats is associated with a larger population size. To be clear, causation can’t be inferred from these data. Others have tried to link party ID to population size using much larger datasets, and it’s hard to determine which way the causation runs.

Although population size can be a useful way to categorize rural areas, exurbs, suburbs, and urban areas, it is not the only relevant metric. Population density, too, is an important tool used to distinguish urban areas from rural ones. As The Atlantic’s City Lab summarized in an article in 2012, “politics are inseparable from density.” Let’s take a look:

In the first graph, the percentage of Democrats in MA cities and towns is positively correlated with population density. In the second graph, the data are essentially flat and the correlation coefficient is near zero, indicating that there is almost no association between the percentage of Republicans and the population density.

The first scatterplot is also hereroskedastic, meaning that, when a trend line is fitted, the size of the error is correlated with population density, the x-variable. This simply means that there’s more variation at the left end of the distribution than there is on the right. Areas of medium-to-high population density follow the pattern better than areas of low density. As with the Democrat scatterplot, the Republican scatterplot suggests that low-density cities and towns have more variation in party ID than high-density ones.

In summary, the differences between these two graphs indicate that Massachusetts municipalities’ party enrollment data is highly related to how urban or rural each municipality is. And, in general, cities and towns in Massachusetts conform to expectations that denser areas are more liberal and less dense ones are more conservative.

Of course, I wouldn’t be me if I didn’t qualify these statements with their potential limitations.

City or town-level population density assumes uniformity of composition within each municipality, which isn’t always the case. My measure of density is the number of people per square mile in each municipality. Although widely-used, this is an imperfect measure of how people actually live, since the composition of cities and towns varies. For example, a municipality with a dense town center surrounded by low-density agricultural land would have a much lower overall density rating than an otherwise similar town without the surrounding open space. As there is no clear way to avoid this measurement issue, assessments of any individual city or town’s population density should be viewed as an approximation rather than an exact measure of how urban or rural the municipality is.

So, how liberal is Massachusetts? I think the simple answer is, pretty liberal, but it depends on where you live. My hope is that, when trying to understand the geography of political attitudes, we can more away from the “red state, blue state” narrative. State-level statistics matter enormously in studies of political representation in the Senate or Electoral College. However, when trying to understand the geography of political attitudes, maybe the more important question isn’t which state you live in, but rather, what does the building with your closest polling place look like?

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Data sources: Mass GIS, Secretary of the Commonwealth of Massachusetts Voter Registration Statistics, United States Census Bureau

Programs used: MS Excel, Google Fusion Tables, Tableau