One thing the folks who have planned Vancouver's upcoming world's fair, Expo 86, are emphatic about is that it is not simply a carnival, although one of its tallest and most visible structures is a wicked loopty-loop roller coaster called, appropriately, the "Scream Machine."

Yes, there will be plenty of fun things to see and do -- parades led by Royal Canadian Mounted Police, nightly fireworks, big-name pop and classical concerts, jugglers and other street entertainers, after-hours cabarets -- but the fair's primary purpose, according to these officials, is educational.

"We've tried to give it some usefulness," said Patrick Reid, commissioner general of the Canadian exposition, and "a deeper meaning."

A tour of the fairgrounds, opening May 2 for a five-month run through Oct. 13, confirms his remarks. Expo 86 will be a learning experience, but with lively colors and whimsical touches to make the lessons a pleasure.

This latest world's fair -- and the last scheduled in North America for this century -- is focused on the linked theme of transportation and communication -- "World in Motion/World in Touch" -- highlighting technological advances that are useful or entertaining or both. The idea is that nations can profit from each other's efforts in these fields.

This theme is illustrated in such varied exhibits as a walk-through Soviet space laboratory; a passenger-carrying, high-speed Japanese train that floats above its tracks by magnetic levitation; and a giant IMAX theater screen that will show the first-ever IMAX film in three super-sized dimensions -- for this Canadian presentation, you have to wear special 3-D glasses. The fair's mascot is Expo Ernie, a short, chubby two-legged robot who will roam the fairgrounds chatting with young' visitors.

In the whimsy category is a 200-yard-long, all-gray sculpture called "Highway 86," which is, indeed, a two-lane highway climbing in rolling leaps from the water into the sky. Headed up the road like earth's creatures marching into Noah's Ark are one each of every kind of vehicle imaginable, all life size: a bicycle, roller skates, a tank, a convertible, a submarine, a plane -- even a tow truck hauling a crumpled sedan.

Expo -- or at least most of it -- is splendidly located on False Creek, an inlet of the Pacific Ocean on the southern edge of Vancouver's downtown center. From anywhere you stand on the fair's sprawling 165-acre site, there are grand views of the water, the city's skyscraper skyline and the high, forested mountains that rise to the north. The pavilions are grouped around three main plazas -- Land Plaza, Marine Plaza and Air Plaza -- reflecting the transportation and communication theme.

You can get from one end of Expo to the other -- distance of several blocks -- via a pair of high-flying gondolas; a Disneyland-like 3.3-mile monorail that circles the fair in 20 minutes; or one of the seven False Creek ferries that ply the 2.5-mile shoreline, docking at three piers.

About a mile away on Burrard inlet -- on the northern edge of the city center -- stands Canada's national pavilion, Canad Place. Separate from the rest of Expo but linked to it by SkyTrain, a rapid transit rail line, Canada Place is Expo's most dramatic-looking structure. It is a permanent complex that includes a stunning new luxury hotel, the Pan Pacific Vancouver; a world trade center; and docking facilities for cruise ships. Many of the Pacific Coast cruises to Alaska plan stopovers at Expo this summer.

Sporting five giant white fiberglass sails, Canada Place looks much like a wind-powered cruise ship about to sail from port. The residents of Vancouver appear to have embraced the gleaming white complex, and it is quickly becoming a much-photographed emblem of the city -- the way the Eiffel Tower identifies Paris.

There's no question this fair is a big one, much larger than the two most recent expositions -- Knoxville in 1982 and the financially troubled New Orleans fair in 1984. In fact, Expo 86 has att6 has attracted 54 foreign exhibitors, about triple the ;number that participated in New Orleans.

It is impressive international roster that includes a rare Big-Three lineup of the United States (which will emphasize U.S. space exploration), China and the Soviet Union. These three haven't even appeared together in the past two Olympic Games, and Expo officials hint of competition among them for the best exhibit.

Canada built the pavilions for the international participants, with the idea, said Reid, that the visiting countries put their money into the exhibits rather than the construction of grandiose buildings. The pavilions are all-white modular blocks, which means that the architecture of Expo is far less interesting than that of earlier fairs. But countries are decorating the pavilion exteriors, and some are quite colorful. Cuba's is adorned with a bright tropical mural of palms, sun and sea.

All of the countries, large and small, have been instructed to adhere to the Expo theme, said Reid, though most won't pass up the opportunity to advertise their tourist attractions to the visiting public. Indeed, Vancouver, a cosmopolitan Pacific port city with a mild year-round climate and a magnificent setting between sea and snow-tipped mountains, hopes Expo 86 will put it on the tourist map. The city is a jumping-off place for magnificent mountain and coastal wilderness trips.

Canadians see the fair as a chance, too, for Americans to become acquainted with their country. They say they know a lot about the United States, but most Americans know very little about Canada.

Most of Canada's provinces and territories have built pavilions. Saskatachewan offers a 10-story tower that resembles a grain elevator, a common sight in the wheat-growing province. You can ride up to the top to see how a grain elevator works, and the trip back down demonstrates the operation of a potash mine.

The entrance to the British Columbia pavilion is a transplanted forest of evergreens, representing its lumbering industry -- but also a shady place to rest if there's a waiting line. The Northwest Territories exhibit reflects the ice and cold found in the country's northern reaches. You can step momentarily into a novel cold-climate room for sample of an Arctic winter.

So far, every indication is that the fair will be a success. The figures for advance ticket sales are awesome. To meet its billion-dollar budget, Expo needs to attract 13.7 million paying visitors, and Reid. Even before the gates are open, however, more than 10 million tickets already have been sold. Estimates now are that Expo will draw between 15 million and 16 million visitors, a great many of them from the United States.

Vancouver hotels already are heavily booked for the busy midsummer weeks, though additional rooms will be available for rent in hundreds of Vancouver homes at inexpensive rates.

Officials expect it will take visitors at least three days to explore all of the pavilions, though, of course, that schedule includes time out for meals from fancy dining to quick snacking at dozens of food establishments. The Soviets are expected to bring 12 chefs to operate a Russian restaurant. McDonald's has built a fast-food outlet on a floating barge, dubbed "McBarge" and docked at the fair site.

Here are tips for seeing Expo 86:

*When to go. The fair runs from May 2 to Oct. 13. Opening day, weekends and mid-June through mid-August are expected to be the busiest times. Operating hours are daily from 10 a.m. to 10 p.m., through three cabarets will remain open until midnight for "Expo After Hours."

Most exhibits will be open throughout the fair, but certain aspects of the transportation and communication theme will be highlighted for periods of a week or two only. Special exhibits include polar transportation, aviation, modern railways, marine commerce, search and rescue and urban transit. Dates and details can be obtained from Expo (see information sources below).

*What it will cost. A one-day ticket to Expo for adults and children 6 and older is $15 in U.S. currency ($20 Canadian). A three-day ticket during the run of the fair is $34.95 for adults and $17.50 for children 6 to 12 and seniors 65 and older. Until May 1, the three-day ticket can be ordered for $29.95 adults and $14.95, children and seniors. Children 5 and under are admitted free. Tickets can be ordered by phone and charged to VISA, MasterCard or American Express: (604)660-3976.

The ticket includes admission to the fair site, all exhibits and on-site transportation. The fair's five carnival rides will cost extra, about $2 a ride.

United Airlines is currently listing a Washington-to-Vancouver roundtrip ticket (via Chicago) of $359 in May, based on 30-day advance purchase, travel on Tuesday or Wednesday and a Saturday stayover. The rest of the week the fare is $399. In June, those rates climb to $379 Tuesday and Wednesday and $419 the rest of the week.

After April 22, Northwest will offer a Washington-to-Seattle round-trip ticket of $278, based on 30-day advance purchase and travel on Tuesday or Wednesday. The fare is $298 the rest of the week. Connecting air fares to Vancouver on another airline will add $70 to $106 to those fares, depending on travel dates.

The U.S. dollar is strong compared with the Canadian dollar, which mens travel expenses within Canada are something of a bargain. A Canadian dollar costs about 75 U.S. cents. In Vancouver, you will be quoted hotel rates and read menu prices in Canadian dollars that appear to be about what you might expect to pay for similar dining or lodging in Washington. However, when translated into U.S. dollars, those prices are about 25 percent less.

U.S. currency is widely accepted in the Vancouver area, which is just 30 miles north of the Washington state border, but you probably will get a better exchange rate at banks and other currency exchange offices. There are exchange booths at Seattle and Vancouver airports as well as on the Expo site.

*Where to stay. Accommodations range from luxury, water-view hotels at $100 a night or more to budget motels, bed-and-breakfast inns, campgrounds, youth hostels and rooms in private homes. Most downtown facilities are located within a half-dozen blocks or so of Expo 86.

Among the fanciest of the downtown hotels are the brand-new Pan Pacific Vancouver, where the Cascades lounge is a super place for harbor and mountain views; the Four Seasons Hotel; the exquisite 2-year-old Mandrin Hotel; the intimate Georgian Court -- closest to Expo; ;the towring Hyatt Regency; the small Wedgewood Hotel; the new Meridien Hotel; and the Hotel Vancouver, the city's stately, very British grand old hotel. The Pan Pacific's rates begin at $150 Canadian a night for a double ($112 U.S.); the Mandarin charges $205 Canadian a night ($150 U.S.)

Away from city center, the prices drop considerably. Vancouver has an excellent bus and ferry system and a new rapid-transit line, which makes getting into the city fairly convenient. Bus fares are $1 Canadian (75 cents U.S.)

For a $5 service charge, ResWest, British Columbia's official accommodation agency, can book lodgings for you at hotels, motels, bed-and-breakfast inns, campgrounds, trailer parks and hostels. If there's no space at any of these commercial facilities, visitors can be placed in private Vancouver homes at a rate of $35 for a couple and $5 each for children. For information or a reservation: (604) 662-3300.

For additional travel information: Expo 86: Expo Info, P.O. Box 1800, Station A, Vancouver, B.C. Canada V6C 3A2, (604) 660-3976. The city of Vancouver: Greater Vancouver Convention & Visitors Bureau, P.O. Box 11142, Royal Centre, Vancouver, B.C., Canada V6E 4C8, (604) 682-2222. The province of British Columbia: Tourism B.C., 1117 Wharf St., Victoria, B.C., Canada V8W 2Z2, (604) 387-1642.