Complex sentence: As cars became cheaper and began to fill the region's roads in the 1920s, the Pacific Electric both lost ridership and slowed to a crawl; traffic congestion soon threatened to choke off the region's development altogether.
Split into simpler sentences: As cars became cheaper and began to fill the region's roads in the 1920s, Pacific Electric lost ridership. Traffic congestion soon threatened to choke off the region's development altogether.