Old school D3 from simpler times
A quick-and-dirty approximate visualization of distance from the U.S. border or coastline, whichever is closer. Unlike the coastal distance map, this is based on geographic distance rather than graph (topologic) distance.
This map is an approximation because distances shown are measured in projected coordinates (10px) rather than on the surface of the Earth, and no geographic projection preserves distances equally between all pairs of points. A more accurate approach would compute the border buffer shapes in spherical coordinates.
(Also, I’d rather have this map just show distance from the coast rather than the border, but that requires a bit more work, too!)