During its nearly 60-year existence, US 66 was under constant change. As highway engineering became more sophisticated, engineers constantly sought more direct routes between cities and towns. Increased traffic led to a number of major and minor realignments of US 66 through the years, particularly in the years immediately following World War II when Illinois began widening US 66 to four lanes through virtually the entire state from Chicago to the Mississippi River just east of St. Louis, and included bypasses around virtually all of the towns. By the early to mid-1950s, Missouri also upgraded its sections of US 66 to four lanes complete with bypasses. Most of the newer four-lane 66 paving in both states was upgraded to freeway status in later years.