Here is a description, supplied by the artist, of how the photographs were taken:
On the outward journey, Peter Spaans generally captured his images from the open window on the right side of the car, or by the windshield. On the return trip, he frequently photographed from the back of the car, either right or left side, depending on the position of the sun, and always with the window open. Day in, day out he focused on fixed points such as the grandness of the natural countryside, or electric and telephone wires, petrol stations, motels, churches, restaurants; he pointed his camera at houses and factories either empty, boarded up and abandoned, or inhabited.
And here is the map, charting their journey through the "fly-over states":