Complex sentence: Similar lake communities popped up all over the state in the late 1920's and the early 1930's when automobile travel exploded and city dwellers were able to easily "retire" to the country for weekends and the summer.
Split into simpler sentences: Similar lake communities popped up all over the state in the late 1920's and the early 1930's. Automobile travel had exploded and city dwellers were able to easily "retire" to the country for weekends and the summer.