GAP, France — The aficionados rank the mighty Col du Galibier, which the riders at the Tour de France tackle on Wednesday’s Stage 16, as the toughest climb in the world’s greatest bicycle race. But a combination of circumstances have conspired to make today’s ride up the L’Alpe d’Huez the event’s showpiece occasion.It’s the “Gala Day”that every major sport must have, when every fanatic attends as a matter of course and sponsors bring out the silver cutlery and serve only Champagne to clients in their hired ski chalets and roadside cafes.

What makes the L’Alpe d’Huez so special? Firstly, it is a natural arena with upwards of 800,000 spectators shoehorned along its 13.8 km route, one of the biggest single daily attendances for a sporting contest in the world. French police estimate that more than a million people watched the time trial in 2004 — Lance Armstrong’s last competitive appearance on the mountain.

The infamous 21 hairpin turns, situated approximately every 700 meters, are everybody’s idea of a picture-perfect Alpine ascent and provide natural staging posts up the 1,060-meter climb.They are numbered in descending order, except for the first bend, which by tradition or neglect — nobody is quite sure — has always remained unnumbered.

Unlike many climbs, there is no gradual increase to ease the warriors into action; the road rises from flat to 9% in an instant and the breakaway group is invariably scattered to the winds.After that it’s every man for himself.

The 15th hairpin, where the incline rises to a gut-wrenching 14%, is where riders suffer the most and, naturally, where the biggest and most animated crowd gathers, normally around the mobile disco the Dutch fans erect to party the days away as the Tour approaches.

Because of the compact nature of the mountain, it has become the TV broadcast highlight in a sport that is not always easy to cover visually. Even casual sports fans, who will not watch another stage of the Tour on television, will tune into a L’Alpe d’Huez ascent.

L’Alpe d’Huez is also a cul-de-sac, meaning it is always the finish to a stage and there is no descent for the average climbers to regain lost ground.

The pressure to perform — and, more dangerously, over-perform — is massive, and that’s the X-factor that sets the mountain apart: There is so much energy and adrenaline on the mountain that the over-riding temptation is always to go for broke, which is exactly what riders should not do when racing over such a climb.

Earlier today in the murderous 187 km stage between Gap and L’Alpe d’Huez, the riders will also have to climb the massive 2,360-meter Col d’Izoard — one of the most beautiful as well as demanding climbs on the Tour — followed by the Col du Lautaret. Fearsome propositions, but long stretches will be comparatively quiet — lonely, even — and riders can set their own pace.

No such chance on the bedlam that is L’Alpe d’Huez. Other forces take over, not always for the good. Heart rates soar way over 200 beat per minute, the mythical red zone, and even world-class riders who badly overcook their charge will be reduced to walking.

Those who tend to downplay the L’Alpe d’Huez should pause for a moment and listen to the formidably fit Chris Hoy, a world and Olympic time-trial champion, who completed it last Monday to raise money for charity.

“It was the worst day of my life,”an exhausted Hoy said afterward. “I suffered like a dog in the heat on the [climb] and was down to 9 kmh or less, barely walking speed.It was the hardest thing I have ever done. I won’t be doing it again, thank you very much, but I am very proud to have completed it once.”

It should be added that Hoy, ever the competitor, nonetheless managed to defeat his Great Britain colleague and fellow Olympic gold medalist Jason Queally by some distance.

There is always a little drama and intrigue on the L’Alpe d’Huez. Michel Pollentier won in 1978 only to be caught trying to avoid a doping test and stripped of the stage win; Beppe Guerini won in 1999 despite being knocked off his bike by a spectator who stepped out to take a photograph; and Armstrong mounted an extraordinary charge to victory through chaotic crowds in 2004 despite a very specific death threat the night before announcing where and how he would be killed on the mountain.

Casual viewers should take note of who wins the stage today. On 19 occasions, out of 24, the rider in the yellow jersey after the L’Alpe d’Huez stage has won the tour de France. It tests everything — physical capability, nerve, composure, competitive instincts, and tactical flexibility — and only the best survive.