Does U.S. Census Data Predict MBTA Bus Ridership?

Guest poster Anna is an M.A. candidate in Tufts’ Urban and Environmental Policy and Planning program and is one of OPMI’s interns for this semester. The following is a post she wrote about a class GIS project she did using recently-available MBTA Rider Census data. We’re currently holding a Data Challenge using this data — see this post for more details.

The MBTA follows Title VI of the 1964 Civil Rights Act, which protects people from discrimination based on race, color, or national origin. This means that when the MBTA considers service improvements and changes in service, it must make sure that these changes do not have disparate impact on minority populations. In order to evaluate service changes for impacts on minority populations, the MBTA must understand the proportion of riders who are minority on all of its services.

There are currently two main ways to calculate minority ridership. The MBTA could either conduct a rider census survey to collect demographic information from a sample of riders on each MBTA bus route or station, or could use U.S. census data for people within the service areas of that service to estimate minority ridership. Conducting a rider census survey is the preferred method; however, this method is resource and time intensive. U.S. census data is easily accessible and easily analyzed so this method, if representative, would be useful.

In this project, I sought to understand if U.S. census minority data within MBTA bus service areas was representative of the minority bus ridership on MBTA buses. Essentially this is a question of whether the minority make-up of the people on bus routes matches the minority make-up of the neighborhoods the routes travel through.

The MBTA recently conducted a Rider Census, so I compared the rider census survey data to U.S. census minority population data in MBTA bus service areas.

How did I analyze the difference between the minority population riding the bus and living in bus service areas?

I examined all MBTA bus routes with a daily ridership of over 600 passengers, excluding the Silver Line. The analysis used MBTA Rider Census data, MassGIS bus stop and bus route data and 2010 U.S. census block and population data.

In order to find service areas for the key bus routes, I conducted a network analysis on the key routes’ bus stops using Massachusetts streets. I then selected the census blocks intersecting the service area polygons generated from the network analysis.

For all non-key bus routes with daily ridership over 600 passengers, I conducted a simpler analysis to find the service area. For these 121 routes, the census blocks within a quarter-mile Euclidian radius of the bus routes’ bus stops were selected to create the routes’ service areas. I chose this simpler analysis for these routes because this analysis was faster to conduct on a large number of bus routes. Network Analyst more accurately represents walking distances; however, the simpler analysis results varied little from the network analyst results, so I chose to use the more accurate network analyst method for key bus routes only.

After finding the service areas for key routes and for all bus routes, I calculated the average U.S. Census minority percentage living in each route’s service area. I then compared the U.S. Census minority percentage to the MBTA rider census minority percentage for each of these routes. For each bus route, I created a value by dividing the MBTA rider minority percentage by the U.S. Census minority percentage to show the rider census minority population as a percentage of the U.S. Census minority population living within the route service area. This value shows the difference between the percentage of riders who are minority and the percentage of the population living within each bus service area who is minority. I then mapped the difference values for each bus route, several individual high ridership bus routes and the bus routes with the highest difference values.

So, can U.S. Census data predict minority bus ridership?

The study found that the percentage minority riding MBTA buses is higher than the percentage minority living in MBTA bus service areas; t(115) = 12.38, p = .00. On 86% of bus routes, the minority percentage riding the bus was at least 115% of the minority percentage living in the service areas (Figure 1). Figure 2 below shows the 7% of MBTA bus routes in blue where the minority population percentage living in the bus service area is comparable to the minority population percentage taking the bus.

Pie chart titled 'Bus Lines: Minority Bus Ridership Compared to Minority Service Area.' 86% of bus lines had a higher percent minority than the service area. 7% had a similar percent minority. 3% had a lower percent minority. 4% not applicable. — Figure 1. 86% of bus routes have minority bus ridership higher than the minority population living within bus service area.

Map of the MBTA service area with the bus routes traced in different colors based on the rider minority proportion compared to the census minority proportion. Most routes are gold, representing routes whose minority proportion is 116%-366% of the census minority proportion. Standing out from there are eight blue routes, where the riders' minority proportion is comparable to that of the census (86%-115%). These routes are: 10, 26, 34, 36, 50, 132, 501, 505. — Figure 2. We identified eight bus routes where the percentage of minority bus riders is comparable (between 86% and 115%) to the minority population percentage living in the bus service area.

For example, on Route 1, the MBTA bus route with the fourth-highest weekday ridership, 36.7% of bus riders are minority; however, only 28.4% of people living in the Route 1 service area are minority.

A choropleth map of the Bus Route 1 service area with blockgroups color-coded by the percent minority. The areas where the percent minority is 81-100% are concentrated around Roxbury and Roxbury Crossing in the south. Along the Charles and into Cambridge there are mostly a mix of blockgroups in the 0-20% range and 21-40% ranges. — Figure 3. The minority population percentage living within the service area census blocks adjacent to route 1 bus stops.

This analysis not only demonstrates that minorities disproportionately ride the bus compared with the non-minority population, but also demonstrates where the difference between minority bus ridership and minority residents living in bus service areas is the highest. Figure 4 shows a map of routes where the minority bus ridership is greater (between 100% and 360%) than the minority population living in the bus service area.

A map of the MBTA service area with bus routes that are more minority than the surrounding census area highlighted in different shades of orange to represent the intensity of the difference. Roughly, lighter orange shades representing 115%-129% are closer to Boston and Cambridge, while darker shades tend to be further out on the edges, representing 245%-366% or routes that are a lot more minority than the surrounding census area. — Figure 4. Bus routes with a percentage minority ridership higher than the percentage minority residents in the service area. Darker orange bus routes have a higher difference.

The routes with the highest difference between minority bus ridership and minority population in the service areas are bus routes 350, 134, 76, 230 and 93. On these bus routes, the percentage minority of bus riders is more than three times the percentage of minority residents living in the service areas. Route 93 has the highest difference (Figure 5). 30.3% of bus riders on bus 93 are minority but only 9.9% of the population in the bus service area is minority. Route 93 runs through downtown between Haymarket and Sullivan Square.

Choropleth map showing the blockgroups adjacent to the Bus 93 route, color-coded by the census percent minority. The majority of the blockgroups are in the 0-20% minority range, with a handful coded in the darkest color, 80-100%, in the Charlestown/Bunker Hill area. — Figure 5. Route 93 bus route and minority population distribution within its service area.

In Conclusion…

Based on this analysis, the minority population living within a bus route’s service area is not representative of minority ridership on buses travelling along the route, so the MBTA should continue to use rider census data, instead of U.S. Census data, to estimate minority bus ridership.

This study’s methodology could be improved by automating the network analyst tool for creating bus service areas. This way, the analysis could be carried out quickly on all bus routes, not only high ridership routes. Also, further analysis could explore alternative ways to select census block data within a .25 mile walkshed of a bus stop. The current analysis selects full census blocks with parts within the .25 mile walkshed, in an effort to avoid assuming even distribution of the minority population. Future analysis could examine the distribution of minority populations within census blocks to improve accuracy.

Future studies may also generate a way to predict minority bus ridership using U.S. Census minority percentages, by weighting each U.S. census block minority percentage based on the number of boardings at the stop within the block. A future tool for predicting minority ridership based on U.S. Census minority population data could also identify and use other external factors that may affect minority ridership as well when generating a more accurate prediction.

Finally, this study gives the MBTA a general understanding of the minority bus usage in non-minority residential corridors. This could help the MBTA make service improvements to improve access for these riders.