Route 66 was designated from a series of other routes throughout the seven states from Illinois to California.  In Illinois, the primary route that became Route 66 was Route 4.

Routes were marked with markers on bridge abutments and signs. While most of these are long gone due to bridge replacements, one Route 4 bridge marker still exists in Pontiac, Illinois.

This Route 4 marker has been restored by the Route 66 Association of Illinois. A 1926 bronze plaque still exists on this bridge, as well as a 1998 plaque to recognize this rare marker.