Complex sentence: Combinations disappeared from general service in the late 1970s, when a downsized Cadillac commercial chassis appeared at the same time as changes in the federal ambulance regulations governing minimum width, headroom and equipment levels generally required larger vehicles.
Split into simpler sentences: Combinations disappeared from general service in the late 1970s, when a downsized Cadillac commercial chassis appeared. At the same time, changes in the federal ambulance regulations governing minimum width, headroom and equipment levels generally required larger vehicles.