This brings me back to the days of working at a planning & architecture firm where I edited Illustrator maps with 500,000 objects in them.

The thick white outlines do a good job of ensuring that the letterforms aren't impeded by other design elements.

The cleared-away areas around cities are a good use of "white space" to increase the hierarchical dominance of major metropolitan areas.

There are some nice detailed observations here. Bookmarked.