Since its inception as part of the National Road in the mid-1800s, the Illinois section of U.S. 40 has undergone changes in both alignment and surfacing materials. Improvements in the road surface progressed from dirt to macadam and from brick to concrete as public usage and demand dictated. Hard-surfacing of the road in the late 1910s and early 1920s precipitated an increase in automobile traffic, replacing the horses, wagons and carriages which crowded the route when it was known as the National Road. Improvements in the internal combustion engine combined with assembly line production provided cheaper and faster automobiles. Increasing numbers of automobiles lead to congestion in areas where the road passed through town centers, and their acceleration in speed generated an increase in accidents at sharp curves and turns. These problems were often rectified with newly constructed by-passes and realignments. As the road and the automobile evolved, so evolved the built environment which lined the road. As the automobile became more affordable, an increasing number of middle-income families took to the road and these families needed food, gas and shelter for the night. Enterprising land owners along the route began to provide these amenities, while providing an increased income for their own families. These small businesses were generally housed in vernacular buildings, often built by the owners themselves. By-passes, realignments, and later the advent of the franchise, often meant the dramatic reduction of these family businesses and abandonment of the their unique buildings and structures.This study attempts to answer the following three questions. First, what was the original alignment of U.S. 40 through Illinois? Second, to what degree is the original road configuration still in existence today? Third, how much of the automobile-related built environment of the earliest route presently remains? Results reveal that significant sections of the historic road surface combined with numerous and varied vernacular motels and gas stations provide a visual experience of the automobile era during the fifty year period between 1920-1970. / Department of Architecture