Every four years the President of the United States drives (and sometimes walks) 1.5 miles from the U.S. Capitol to the White House as a part of the inaugural fanfare. The president is accompanied by an entourage of hundreds of towncars, limousines and motorcycle police, all arranged in tight V formations.

To accommodate this procession, the National Park Service gets to work weeks ahead of time clearing the path. Normal streetscape obstructions like bike lane bollards are unscrewed from the asphalt and put into temporary storage. In recent years, to keep the parade lined up, a thin blue line was painted down the center of Pennsylvania Avenue. During the parade, the lead vehicle tries to keep centered on this target.

The temporary paint will wear away in a few months after inauguration, but it’s still visible into the springtime. The practice appears to have started with President Obama; George W. Bush didn’t have a line.