The red lines indicate roads that the Wisconsin Department of Transportation has closed due to flooding.
(Via Google Maps.)
Things are looking bad in Jefferson County, which is under the big red line in the lower right side of the picture. Apparently the Rock River has continued to rise, and some roads in Rock County are closed as well. The Rock County Emergency Management has listed places in the towns of Beloit, Fulton, Janesville, and Milton where sandbags and sand are available at no cost.
FEMA has provided sandbagging instructions via this helpful graphic. (What does the second frame represent? Open the bag and fold its top over?)
If you haven’t seen it, the state DOT has a list of all closures of state roads. Good luck, everyone!