605 ILCS 5/4-405

The Department shall maintain all highways in the State highway system either with its own forces or pursuant to an agreement or contract entered into pursuant to this Code.

In the course of its other maintenance work, the Department shall paint and maintain a line not less than 2 1/2 inches in width on the edges of uncurbed roadways of all State highways having an Illinois or U.S. route traffic marking and which carry an average daily traffic of more than 1000 vehicles. The Department may paint and maintain such lines on such other State highways as it deems desirable.