How Things Work: The Mail

One of the great boons our country has enjoyed in modern times has been the speedy delivery of mail. This has been made possible by the steady advance of transportation technology, processing techniques, and the United States Postal Service?s pledge to provide ?prompt, reliable, and efficient services to patrons in all areas.?
Origin-to-destination delivery times for transcontinental mail traveling via the railroad and Pony Express took 13 to 14 days in 1860. Today, coast-coast deliveries are as short as 6 hours when sent by air. This may seem long compared to the instant correspondence of e-mail, but until Scotty is sending packages through the transporter buffer, there will be a need for physical transport of mail. Recognizing this need, the U.S. Postal Service has pioneered advancements in mail processing.
To best understand how mail is processed, we can follow a letter?s route from start to finish. Consider Tom Swift, who wishes to write to his family across the country. Tom takes his correctly addressed envelope and drops it in his curbside mailbox, in one of over 300,000 street collection boxes, at one of over 37,000 Post Offices, or into the bag of his local letter carrier. These carriers bring Tom?s letter to the local Post Office.
Once at the Post Office, the letter enters what is called receiving. Letters can go two ways: local or out-of-town delivery. This preliminary sorting is done either by the carriers or by Mr. Swift himself, who can drop his letter in the Out-of-Town slot. Mail divided here goes through separate processing lines, but the steps for each automated line are the same.
The first step, known as culling, removes mail with dimensions unsuitable for follow-on machines. Examples of mail culled from letter-sorting machines include packages, large envelopes, and magazines. Magazines and large envelopes, which make up a class of mail known as flats, have their own dedicated machines. Packages don?t usually undergo automated sorting at local branch offices. Culling is accomplished by drums and rollers that segregate by thickness, width, and stiffness and use light sensors to determine length.
Next, Tom?s letter is brought by air suction or high-speed rollers to the facing/canceling machine. This machine uses a variety of detectors to determine on which side the postage stamp is and void it with a mark identifying the date and Post Office. Facers/cancelers detect postage with fluorescent ink detectors, chromatic and contrast sensors, and serrated edge recognition. They are also programmed to recognize pre-paid or franked postage, which isn?t cancelled.
From here, letters proceed to the optical character recognition/video coding system machine (OCR/VCS). Each piece of mail speeds by an electronic eye which uses heuristic algorithms to recognize characters, both handwritten and typed. The digitally rendered address is standardized with correct abbreviations and then compared against a national database. This national database assigns an 11-digit ZIP code to each piece of mail.
An 11-digit ZIP code, you ask? While most people are accustomed to seeing 5-digit ZIP codes, all ZIP codes are actually ZIP+4 codes, or 9 digits long. The first digit represents a large region of the country ? zero, for example, represents the Northeast ? and each successive digit represents a smaller and smaller area, some representing individual floors of buildings. An extra two digits are tacked on after the ZIP+4 code for further specification.
Once this 11-digit number is known, the video coding system prints a 62-line barcode on the letter that shows where the mail is to be sent. To avoid interference from the existing address, the barcode is usually printed on UV or invisible ink, which are easily distinguished by machines.
On about 15 percent of addresses, the electronic eye cannot determine the 11-digit ZIP. These letters are redirected to a standby loop before the bar-coder is reached. Technicians at computers examine unread addresses and manually enter the information so each letter can pass through the bar-coder. With 650 million pieces of mail being handled a day, almost 100 million letters have to be manually entered. One can see the need for automation in mail-handling.
Finally, letters pass through a sorting machine. This machine reads the newly-printed barcodes on each envelope and sorts each letter into one of many different bins for processing. If Tom?s letter were locally bound, it would be sorted into a bin that represented some local mail route. However, since Tom?s family is across the country his letter will be sorted into a bin destined for a large regional processing center. At these centers robotic arms group large amounts of mail into gigantic transportation containers awaiting travel by tractor-trailer, airplane, or even rail.
On arriving at its destination, Tom?s letter will be unpacked and sent through even more sorting machines, sorting mail into smaller and smaller groups. Soon, the friendly neighborhood mailman is dropping a letter in the Swift family mailbox. While the U.S. Postal Service boasts that it delivers mail through rain, shine, and snow, perhaps it should add ?through Herculean feats of logistics.?