Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The primary highway system makes up over 9,000 miles, a mere 8 percent of the U.S. state of Iowa's public road system. The Iowa Department of Transportation is responsible for the day-to-day maintenance of the primary highway system, which consists of Interstate Highways, United States Highways, and Iowa state highways. Currently, the longest primary highway is U.S. Route 30 at 332 miles. The shortest highway is Interstate 129 at 0.27 miles.