The primary highway system makes up over 9,000 miles (14,000 km), approximately 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. Highway 30 at 332 miles (534 km). The shortest highway is Interstate 129 at 0.27 miles (430 m).

The 20th century was a transformative time for vehicular transportation. In the early years of the century, roads were problematic at best – dusty dirt roads when dry and impassably muddy when wet. Over time, federal money was set aside and bonds were issued allowing the roads to be paved. The U.S. Highway and Interstate Highway Systems connected Iowa to the rest of the country and made national travel feasible. Periodically, new highway construction and changing driving habits have resulted in the obsolescence of local highways, to which the primary highway system has adapted. The former primary highways, turned over to counties and local jurisdictions, county highways, and farm-to-market roads make up the secondary highway system.

Early on, Iowa's registered routes were marked with hand-painted signs created by the group maintaining the highways. When the primary highway system was created, prisoners in Iowa's correctional system began making highway signs. Today, Iowa's highway markers are compliant with standards set forth in the Manual on Uniform Traffic Control Devices.

At the turn of the 20th century, roads in Iowa were suited for horse and buggy and ill-suited for automobiles. As more Iowans purchased automobiles, the Iowa legislature set up in 1904 a commission at Iowa State College in Ames to handle issues that arose concerning travel and safety. In 1904, less than 2 percent, or just over 1,000 miles (1,600 km) of public roads in Iowa had been improved with gravel or broken stone. The first task of the Iowa State Highway Commission, now the Iowa Department of Transportation, was to study Iowa's problematic roads. At the time, roads were merely dusty dirt trails when dry, and quagmires of mud when wet. By 1906, every county was maintaining its dirt road with drags. Designed by Missourian E. Ward King, road drags were a cost-effective way to smooth out dirt roads, costing as much as $3.00 ($107.00, adjusted for inflation) to build. In 1913, the Commission separated from Iowa State College and gained control over county and local transportation officials, who were responsible for road construction and maintenance. The Iowa Highway Commission did not gain jurisdiction over public roads until 1924.

The predecessor of the primary highway system was the registered route system. Organizations, such as the Lincoln Highway Association, volunteered to sponsor and register certain roads with the highway commission. Each organization chose their colors and designed route markers to guide motorists along the way. Eventually, confusion reigned and the highway commission took action. Beginning in 1920, primary road numbers were assigned to registered routes. Route numbers were assigned so they would match those of neighboring states – Primary Road Number 1 (No. 1) was assigned to the Jefferson Highway, the Lincoln Highway became No. 6, and the Red Ball Route became No. 40.

In 1925, the American Association of State Highway Officials approved a national numbering system for roads, which quickly replaced the registered route system. The new U.S. Highway system grew and improved for thirty years, until Interstate highways were created. Iowa renumbered some primary roads in 1926; marking them with a unique number as to not duplicate the new U.S. Highways. Iowa highways were signed with a circle with the route's number beneath the word Iowa.

In the late 1920s and early 1930s, highway paving had begun in earnest. In September 1929 alone, the Iowa State Highway Commission spent $3 million ($56.3 million, adjusted for inflation) on road construction, with two-thirds of that spent on highway paving. By the end of 1929, the first roads connecting two state borders neared completion. The Lincoln Highway from Ames to Clinton made travel to Chicago easier, while the Jefferson Highway from Lamoni to Ames facilitated travel to Kansas City. These two new roads also made travel easier between Iowa's two most populous cities, Des Moines and Cedar Rapids.

In the 1950s, the Iowa State Highway Commission planned an east-west toll road across the state, roughly along the current Interstate 80 corridor. When the Federal Aid Highway Act of 1956 created the Interstate Highway System, plans for the toll road were scrapped with great fervor. In Iowa, interstate corridors were designed to follow existing U.S. Highway corridors – Interstate 29 followed U.S. Route 75 and U.S. Route 275, Interstate 35 followed U.S. Route 65 and U.S. Route 69, and Interstate 80 followed U.S. Route 6. The first section of interstate highway in Iowa, a section of I-35 and of I-80 near West Des Moines, opened on September 21, 1958. Over the next 30 years, sections of interstate were completed and opened for traffic. As the interstates grew and expanded, many U.S. Highways were truncated at, relocated onto, or replaced by interstates. The last section of Interstate 380 to be completed, was opened on September 12, 1985.