With just weeks left until 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 more than 4.7 million people. Voting rights advocates are now suing the state because Recent changes in Texas disenfranchise voters who already face barriers to voting because it forces them to travel further to vote. The team at the Center for American Progress used isochrone and census data to show how these changes dramatically reduced voting access for millions of people across the region.

“Prior to Abbott’s order, 87 percent of Harris County residents could access a drop-off location within a 20-minute drive; now, only 29 percent of citizens can submit their ballots within that time period.”

Isochrones calculate how long it takes to travel from a certain point, in any direction. These buffers are more accurate than ‘on-the-fly’ circular buffers because they take into account the actual route to a given location. When adding, removing, or changing polling locations, the isochrone indicates accessibility areas.

There are several parameters to set when creating an isochrone, but the most important is the ‘contour size’, which specifies the travel time in minutes. Set this to an appropriate accessibility threshold for analysis and select a driving, walking, or cycling route profile. For this ballot delivery 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 last-minute wait times or traffic. Here’s an example query showing a 20-minute drive time isochrone around the NRG Arena in Houston, the last remaining drop-off location in the region:

https://api.mapbox.com/isochrone/v1/mapbox/driving/-95.4068219375%2C29.6818331875?contours_minutes=20&polygons=true&generalize=0&access_token=YOUR_MAPBOX_ACCESS_TOKEN

In CAP’s analysis, they truncate this shape to the Harris county line because voters in Texas cannot cross county lines to access ballot drop boxes.

Another analysis conducted by ProPublica in 2018 examined how changes to North Carolina’s early voting rules impacted voters across the state. And in states and counties seeking to expand access to safe voting options, travel time analysis can help identify populations that remain underserved.

What accessibility and equality questions will you answer with isochrones? Learn more about how to get started with the Isochrone API: https://docs.mapbox.com/help/tutorials/get-started-isochrone-api/.

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