Despite the growing transport accessibility offered by the airlines, the Post Office Department in 1930 still moved the bulk of its domestic mail by rail, massing, re-sorting, and redistributing it for long distance hauling through the major railroad hubs of the nation. More than 10,000 mail-carrying trains crisscrossed the country, moving round the clock into virtually every village and metropolitan area.
The railroads' peak year may have been 1930. By 1963, fewer trains, making fewer stops, carried the mail. In these same years, 1930-1963, the United States underwent many changes. It suffered through a prolonged and paralyzing depression, fought its second World War of the 20th century, and moved from an agricultural economy to a highly industrial one of international preeminence. The character, volume, and transportation of mail also changed.
The social correspondence of the earlier century gave way, gradually at first, and then explosively, to business mail. By 1963, business mail constituted 80 percent of the total volume. The single greatest impetus in this great outpouring of business mail was the computer, which brought centralization of accounts and a growing mass of utility bills and payments, bank deposits and receipts, advertisements, magazines, insurance premiums, credit card transactions, department store and mortgage billings, and payments, dividends, and Social Security checks traveling through the mail.
In June 1962, the Presidentially appointed Advisory Board of the Post Office Department, after a study of its overall mechanization problems, made several primary recommendations. One was that the Department give priority to the development of a coding system, an idea that had been under consideration in the Department for a decade or more.
Over the years, a number of potential coding programs had been examined and discarded. Finally, in 1963, the Department selected a system advanced by department officials, and, on April 30, 1963, Postmaster General John A. Gronouski announced that the ZIP Code would begin on July 1, 1963.
Preparing for the new system was a major task involving realignment of the mail system. The Post Office had recognized some years back that new avenues of transportation would open to the Department and began to establish focal points for air, highway, and rail transportation. Called the Metro System, these transportation centers were set up around 85 of the country's larger cities to deflect mail from congested, heavily traveled city streets. The Metro concept was expanded and eventually became the core of 552 sectional centers, each serving between 40 and 150 surrounding post offices.
Once these sectional centers were delineated, the next step in establishing the ZIP Code was to assign codes to the centers and the postal addresses they served. The existence of postal zones in the larger cities, set in motion in 1943, helped to some extent, but, in cases where the old zones failed to fit within the delivery areas, new numbers had to be assigned.
By July 1963, a five-digit code had been assigned to every address throughout the country. The first digit designated a broad geographical area of the United States, ranging from zero for the Northeast to nine for the far West. This was followed by two digits that more closely pinpointed population concentrations and those sectional centers accessible to common transportation networks. The final two digits designated small post offices or postal zones in larger zoned cities.
ZIP Code began on July 1, 1963, as scheduled. Use of the new code was not mandatory at first for anyone, but, in 1967, the Post Office required mailers of second- and third-class bulk mail to presort by ZIP Code. Although the public and mailers alike adapted well to its use, it was not enough.