From the Census Bureau: "Primary roads are generally divided,
limited-access highways within the Federal interstate highway
system or under state management. These highways are distinguished by the presence of interchanges
and are accessible by ramps and may include some toll highways. Secondary roads are main arteries,
usually in the U.S. highway, state
highway, or county highway system. These roads have one or more lanes of
traffic in each direction, may
or may not be divided, and usually have at-grade intersections with many other roads and driveways.