With just weeks remaining to election day, Texas Governor Abbott reduced the number of ballot drop boxes across the state to one per county. The change reduces ballot drop-off locations from 12 to 1 in Harris County, which includes Houston and is the third-most populous county in the US with over 4.7 million people. Advocates for voting rights are now suing the state because the latest changes in Texas disenfranchise voters who already faced barriers to voting by forcing them to travel farther to vote. The team at the Center for American Progress used isochrones and census data to show how these changes dramatically reduced access to voting for millions of people across the county.
Prior to Abbott’s order, 87 percent of Harris County residents could access a drop site within 20 minutes’ drive; now, only 29 percent of residents can drop their ballots off within that timeframe.
Isochrones calculate how long the travel time is from any given point, in any direction. They are more accurate than circular ‘as-the-crow-flies’ buffers because they account for the actual route to a given location. When it comes to adding, removing, or changing locations for voting, isochrones show areas of accessibility.
There are several parameters to set when creating isochrones, but the most important is ‘contour size’, which sets a travel time in minutes. Set this to an appropriate accessibility threshold for the analysis and choose a routing profile of driving, walking, or bicycling. For this ballot drop-off use case, the team used a 20-minute drive time, which represents a 40 minute round trip to vote during a lunch break not including wait times or last-minute traffic. Here is an example query showing a 20-minute drive-time isochrone around NRG Arena in Houston, the last remaining drop-off location in the county:
In CAP’s analysis, they clipped this shape to the Harris county boundary because voters in Texas can’t cross county lines to access a ballot drop-off box.
Another analysis by ProPublica in 2018 examined how changes to early voting rules in North Carolina impacted voters across the state. And in states and counties that are trying to expand access to safe voting options, travel time analyses could help identify populations that are still underserved.
What accessibility and equity questions would you answer with isochrones? Learn more on how to get started with the Isochrone API: https://docs.mapbox.com/help/tutorials/get-started-isochrone-api/.
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