One thing you can say about Los Angeles - beyond the fact that its basketball team is still the best in the world - is that the city's inhabitants do know how to celebrate.

No sooner had the Lakers become the first team in 19 years to win back-to-back NBAchampionships by beating the Detroit Pistons 108-105 Wednesday night than Los Angeles fans by the thousands were putting on their "Back-to-Back" T-shirts and making their way out of the Fabulous Forum - the scene of the dream - and out to the parking lot. There they got into their several thousand cars and immediately proceeded to do some serious partying.

They had a celebratory traffic jam.

Towns tend to do what comes natural. Detroit likes to loot and riot when its teams win championships. New Yorkers write graffiti and take home individual-sized portions of the playing field. In San Francisco they tend to go out for cracked crab dinners and toast the championship with expensive wine.

In L.A. they do everything in their cars.

What better way to toast their Lakers than circling the block? And then circling it again?

Toyotas and BMWs and Mercedes Benzes and Mazda pickups and old VW buses and every other make of car listed in the blue book were joined by a wide assortment of limos and motorcycles as the victory parade kept making its way around and around the Forum Tuesday night.

The scene suggested the Harbor and Santa Monica freeway interchange at the height of rush hour, but no one was uptight. No one was trying to get to Orange County. No one was trying to make it home for dinner. Cars were letting other cars in front of them. Drivers were honking at each other - and smiling, and waving huge fingers - of the index variety.

Everybody was No. 1.

This was a happy pileup. Nobody wanted out of this jam. Cars were coming from Culver City and Westchester and Westwood just to get in. Occasionally people would spill out of their cars when things got to a total standstill - which was often - and jump into one another's vehicle.

Everybody was having a nice day.

On Crenshaw and Manchester and Arbor Vitae and Century - on all the main arteries in Inglewood that service the Forum - they circled the scene of the Lakers' latest greatest triumph.

Little old ladies from Pasadena and young girls in T-Birds having fun, fun, fun were part of the throng.

Many radio stations were turned up loud and tuned to 57 KLAC, the station of "Your World Champion Los Angeles Lakers." Every so often KLAC would replay Chick Hearn's countdown of the last few dramatic seconds of the Lakers' win that was now hours old - "A. C. Green lays it in! . . . Now Magic has the ball! . . . It's over! The Lakers are world champions!"

Every time they played the tape it ended the same. Every time it ended it got an ovation of horns and screams.

The radio station was flooded with requests for the song, "Mamas, Don't Let Your Babies Grow Up To Be Pistons," a tune specially written two weeks ago to welcome Detroit to town that had become quite popular on the local commuting scene.

"Don't go away," said the DJ on "Your World Champion Los Angeles Lakers Station." "We promise we'll play `Mamas, Don't Let Your Babies Grow Up To Be Pistons' again in the next half hour."

Nobody was going away. This was a captive audience. Happier here than if they were in the fetal position. Behind the wheel with something good to honk about.

It had been a long time coming, this second half of the Lakers' "Back-to-Back" feat. And even when it looked for all the world that those mangy Pistons from Detroit had been stomped for good - down by 15 with less than a quarter to play - they had stormed back to make it a contest right to the last pit stop.

It was the kind of finish that didn't call for a fashionable early exit from the Fabulous Forum or for a sedate ultra-cool postgame celebration at a quiet restaurant.

No, this one called for a street party of the first order. The kind only Los Angelenos could produce. The final gun was a cry for a little road work. Gentlemen, and ladies, start your engines.

What's a party without your personalized plates and your car stereo jacked to the max?

The game ended just before 9 p.m. By midnight the Forum was still being circled. Your World Champion Road Rally was still going strong, and the best part was that it didn't have to end. For anyone planning on going to work Wednesday morning, they could look forward to doing it all over again.