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Existing comment: Collection Box:
In addition to delivering the mail, carriers collect mail from collection boxes along their routes at pre-determined times. This mail is carried by vehicle back to the post office and placed into the mail stream.
Collection boxes first appeared in the early 20th century as package boxes that were placed alongside post-mounted letter boxes. In 1931, package boxes were renamed large collection boxes and began to appear on sidewalks without a separate letter box. They slowly replaced post-mounted boxes. By the 1960s, this now-familiar shape could be spotted on street corners across the U.S.

A New Design on Mail Volume:
After the end of the Second World War in 1945, the Post Office Department was finally able to purchase 3,247 new trucks. Post-war booms in business and employment fueled an increase in mail volume. More mail flowing into individual mailboxes meant more mail toted by the carriers.
Growing suburban sprawl added to the complexity of the challenge facing the Department after the war. City mail delivery systems were strained to the point of breaking and letter carriers to the point of exhaustion. A new approach was needed to keep mail moving quickly and efficiently.
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