People drive about 31 billion miles on Illinois interstates, making it the 5th busiest highway system in the nation.
The Federal Highway Administration released the 2011 findings on Monday.
California, Texas, Florida and Ohio round out the top five.
U.S. Transportation officials said the data allows cities and states to more efficiently target congestion and help people get home from work faster.
Overall, vehicles traveled almost 3 trillion miles on U.S. roads in 2011.