Complex sentence: US-91, before the formation of I-15, was the major thoroughfare between California and the Canadian border in Montana, and connected the town of Clearfield, and residents along the new interstate highway to Hill Air Force Base.
Split into simpler sentences: US-91, before the formation of I-15, was the major thoroughfare between California and Montana. It connected the town of Clearfield, and residents along the new interstate highway to Hill Air Force Base.