Veteran air travelers believe that people on their way to hell have to change in Atlanta -- though some insist that the real junction on the route to perition is O'Hare Airport in Chicago. But Atlanta is cleaning up its act.

Milling around an airport will never be a positive experience, but from now on it should be a lot less hassle here than ever before. On Sept. 21, all commercial operations were shifted from the disorganized old terminal building eight miles south of this city's center to a brand new, $750-million complex a mile to the southwest, in the middle of Hartsfield International Airport.

Already the world's second-busiest commercial aviation hub, Atlanta's big field looks forward to the day when it will wrest from O'Hare Airport the questionable distinction of handling more airline traffic than any other terminal on earth. That day probably is not far off. If recent trends continue, Atlanta could be No. 1 in passengers handled by 1982, and in commercial aircraft movements a year or so later.

Plans already are cooking to expand the passenger area by 25 percent and runways by 33 percent. Airport officials say the new layout already is the world's biggest by three measurement criteria:

Number of passenger gates -- 138, compared with 94 at O'Hare and 70 at the existing Atlanta terminal.

Acreage covered by the terminal proper -- 367 acres, or more than half a square mile (not counting runways and other parts of the airport complex).

Enclosed floor space -- 2.2 million square feet, or more than 50 acres, 1 million square feet in the central terminal area and 1.2 million in outlying concourses.

At $750 million (including new air cargo facilities), the terminal probably also is the world's most expensive, especially since no new land acquisition was involved. It was built between the parallel sets of east-west runways that already handle nearly 600,000 takeoffs and landings a year.

(Only a few hundred thousand dollars of federal money was included in the funds used to build the new terminal, airport spokesmen are quick to point out. Equally quick with statistics is Eastern Airlines, which reportedly kicked in more than $100 as its share of the project.)

The new terminal complex was designed with Atlanta's unusual role in jet-age aviation in mind. It is predominantly a place where people change planes: For every three passengers who originate or terminate here, seven come from somewhere else and make connections to an onward destination.

Most airports generate the bulk of their passenger traffic. La Guardia's passenger load in New York is only 6 percent transfer traffic and National's in Washington is about 18 percent, and even O'Hare's heavy transit business represents only 48 percent of its total passenger volume. But at Atlanta the percentage is 70.

This means that of 47 million travelers who will land at Atlanta in the next 12 months, 33 million will be changing planes. For this reason, the terminal's planners concentrated on minimizing the mixing of transfer and origin-destination traffic and on keeping through-passengers out of areas where Atlanta-passengers must go.

It is possible, of course, for transit passengers to go into the main terminal for a meal or a look-see at some of the $450,000 worth of avant-garde art that decorates the place, but airport officials believe that most of them won't bother.

In laying out the complex, planners also sought to strike a balance between two conflicting requirements: (1) the desire of passengers to walk as little as possible inside the airport buildings and, (2) the need of large airliners for plenty of room to move around outside. What emerged was a layout quite unlike any other commercial terminal in the world: six major buildings arrayed along an axis more than a mile long, all interconnected by a "pole-moving" corridor with a robot train and moving sidewalks.

Passengers may have to travel a mile or more from the check-in area, but airport officials say travel time within the terminal should be no more than five minutes or so.

All federal standards for facilities for the handicapped are met or exceeded here, and even to the selection of shades of red and green hues distinguishable to the color-blind in the color-coding of various parts of the layout.

The complex is divided into "landside" and "airside" sectors. On the landside are two terminal buildings, almost mirror images of each other, joined back to back, where outgoing Atlanta passengers check themselves and their luggage in and incoming passengers claim their bags and get local transportation. Vast multilevel parking lots accommodating 12,000 cars flank the twin terminals.

Also adjacent is a transportation feature lacking at most big-city airports.

It is a station for Atlanta's fledgling rail rapid-transit system, MARTA, which is scheduled to reach the airport by about 1985.

Only four U.S. airports have rail rapid-transit connections to the central city, and only one (Cleveland's Hopkins) has a station right at the terminal, as Hartsfield Airport will. Atlanta's Aviation Department decided early on to buld the station years in advance of MARTA'S arrival so heavy construction would not interfere with normal operations later.

Like Los Angeles (the world's thirdbusiest commerical field), Atlanta has all its landside facilities on one level. Ticket counters are at the east end of each terminal and baggage carousels are at the west end. Both areas are only a few yards from curbside, where outgoing baggage can be checked directly to its destination.

The bulk of Atlantic's traffic is carried by Eastern and Delta, so each of these airlines dominates one terminal, with other lines sharing lesser blocks of space. The north side of the terminal complex (Eastern's) contains most of the peripheral facilities such as restaurants and shops, but these are easily accessible from the south side.

Except for an internatinal concourse adjourining the north terminal, there will be no flight operations in this part of the complex. Incoming overseas passengers get quickie, "one-step" clearance through immigration and customs simultaneously -- a new government service now in effect only at Los Angeles and Albuquerque, N.M.

Domestic airside operations take place in four separate concourses arrayed parallel to one another at 1,000-foot intervals east of the terminal buildings.

A single security checkpoint controls access to the entire domestic airside.

Friends and relatives are allowed to accompany ticketed passengers, or meet them at the gate, as long as they undergo security screening.

Although a single checkpoint might seem a narrow bottleneck for a terminal handling 130,000 passengers a day from the outset, officials explain that only about 15 percent of this number (the departing half of the non-transit 30 percent, or about 20,000 a day) will need to be screened.

From the security gate an escalator leads to a 1.1-mile passage to the concourses. At the foot of the escalator is one of 10 stations on a 2 1/2-mile subway loop in which rubber-tired, two-car robot trains operate at intervals of 1 minute 43 seconds. The trains stop at each concourse outbound and then again inbound, and also take arriving passengers to a station just below the baggage-claim area.

Baggage handling at the new terminal is streamlined so that (theoretically, at least) if a passenger gets to the airport in time to catch his plane, his checked baggage will make it, too. The inbound baggage-handling goal is to have luggage coming onto the carousels by the time passengers get in from the arrival gates.

Moving sidewalks, already in use at several U.S. airports, offer an alternative to the automated train. But, because there are breaks in the sidewalks every 500 feet or so, some walking is involved. For the truly athletic, there is an ordinary walkway.

At each people-mover station, UP and DOWN escalators and elevators for the handicapped connect with the departure gates two levels above. There are 32 to 36 gates on each 2,200-foot-long concourse.

The termnal's airside is where long walks are most likely to be necessary. While airport spokesmen say that the average walk between the people-mover and the gate will be only about 550 feet, it is easy to see how nearly half a mile of trudging could be involved in a plane change.

But a half-mile hike was not out of the ordinary at the old terminal or at many other bit airports. And at least in Atlanta, everything is enclosed and the inteline transportation is free -- which is more of a break than the traveler gets, for instance, at New York's John F. Kennedy or Boston's Logan Airport.

The view from a concourse window brings into focus the reason for the vast distances involved. With 1,000 feet -- about the length to two city blocks -- between concourses, two jumbo jets can taxi past each other while two more are parked at gates on either side.

This will not eliminate operational delays. When bad weather closes in, flight operations slow down at any airport, and Atlanta is no exception -- although it is almost never gripped by winter extremes. As far as takeoff delays are concerned, nothing is likely to change simply because the new terminal is in use. In fact, airlines go ahead with plans to expand their Atlanta service, the rush-hour lineup at the end of the runway could get worse.

But what will change, airport officials say, is the need for long, agonizing waits for access to a gate after an incoming plane makes its landing. These delays have been the cause of missed connections and frayed tempers in the past and have made the name Atlanta synonymous with "the pits" in the minds of countless travelers.