GAP, France — The road descending from La Rochette into Gap is picture-book pretty. A valley dotted with red-tile roofs and tawny wheat fields opens before it. The snowy crags of the high Alps rise behind. Rustic farm houses and purple wild flowers line its shoulders.

For hikers, it is idyllic. For drivers, tricky. For racers on bikes traveling 40 mph, terrifying. The road is twisting and steep, too narrow for compact cars to pass, with switchbacks that arrive shockingly fast. Innumerable ridges and barely filled potholes jolt even heavy vehicles.

It was here during the 2003 Tour de France that a top rider, Joseba Beloki, let fly, hoping to drop the race leader, Lance Armstrong. Instead, his front wheel slid on tar softened by the July heat and sent him tumbling. The crash effectively ended Beloki’s promising career and provided a stark reminder of an old saying: Races are rarely won on descents, but they can be lost.

On Tuesday, as the 16th stage of the Tour came storming through these Alpine foothills, the dangerous descent from La Rochette almost took another big victim: Chris Froome of Sky, the race leader, who went off the road while swerving around the third-place rider, Alberto Contador of Saxo-Tinkoff.

In a near replay of the 2003 crash, Froome had been chasing Contador after the Spanish rider had tried to pedal away before the summit of the Col de Manse, hoping to trim his deficit and, perhaps, force the leader into a rash move. But when Contador momentarily lost control on a hairpin turn, Froome was forced into the grass.

Neither rider was hurt and they leaped back onto their bikes to finish with another group of contenders who, following Tour etiquette, had slowed to let them catch up. As a result, the top three places in the race remained unchanged, with Froome retaining the yellow jersey, followed by Bauke Mollema of Belkin 4:14 back and Contador 4:25 behind.

After the race, however, Froome made clear his displeasure with Contador for taking chances on an unsafe descent.

“I personally think teams are starting to get desperate now and therefore taking uncalculated risks,” Froome told reporters.

Rui Costa, a Portuguese rider with Movistar, won the stage after joining a breakaway of about two dozen riders that built a lead of more than seven minutes on the main field. Costa, who has won the past two Tours of Switzerland, accelerated away from the group on the final climb and won by 42 seconds.

Today, a hilly time trial is expected to play to Froome’s strengths. But the stage is also likely to see a fierce battle among the riders trying to reach the podium in Paris. Then on Thursday comes perhaps the most awaited stage of the Tour: a double ascent of towering Alpe d’Huez.

Between the two climbs, the riders will face another treacherous descent, off the Col de Sarenne along a back road that has never been used in the Tour before. Several riders, including Tony Martin of Omega Pharma-Quick Step, have raised concerns about the safety of the road, which is bumpy and lacks guardrails.

“It is a very dangerous descent,” Froome said Tuesday. “It’s not smooth, that’s for sure. There aren’t any barriers on the corners. If you go over the corner, you will fall down a long way.

“Like we’ve seen today, this race is far from over. One incident, one mechanical, or one crash in the wrong moment and your Tour can be over.”

Indeed, at this point in the three-week race, when the rider in the yellow jersey is trying to stay safe while his rivals are trying to pressure him into dangerous mistakes, descents become all the more crucial. Yet descending remains the forgotten stepchild of bike racing, with far more attention given to climbing, sprinting and time trialing.

To casual observers, riders flying at speeds in excess of 50 mph downhill may look as if they were taking a break after hard climbs. But far from it. With so many dangers to worry about — from gravel to holes to wet spots to unexpectedly sharp turns — the riders must remain intensely focused. A crash would not just cost a rider time; it would probably end his race.

“Descents are tough on the riders,” said Michael Barry, a former Grand Tour rider from Toronto who retired from the sport last year. “It’s not like you can relax and coast. The television doesn’t capture the speed the riders are descending at, how close they are to each other, and how fast they accelerate to catch each other.”

Although Beloki’s crash is the best-known cautionary tale about descending, it is not the only case of a downhill affecting the course of a race. In the 2011 Tour, Andy Schleck, a top contender not known for his descending skills, lost time on the climb to La Rochette and then even more on the descent. He finished second that year to Cadel Evans.

This past May, the reigning Tour de France champion, Bradley Wiggins, lost time while descending with great caution after a crash during the Giro d’Italia. Wiggins, who is not riding in this year’s Tour, dropped out of the Giro.

But descents can also work in favor of those sufficiently bold and skillful to attack them. On a stage of this year’s Tour of Switzerland, Peter Sagan of Cannondale regained valuable seconds that he had lost on a climb during an aggressive descent. That put him into a position to win the stage in the final sprint.

Sagan is best known as a sprinter, but he is also widely considered among the best descenders in the Tour. Weight — as in more of it — helps. At about 160 pounds, Sagan carries 10 pounds more than some of the lightest climbers. His strong bike-handling skills are also essential.

But other Tour riders point to an intangible as the most important factor in strong descending: fearlessness.

“A huge part of it is up here,” said Andrew Talansky, a 24-year-old American rider with Garmin Sharp, while pointing to his head. “People like Sagan are just not afraid, and part of that is confidence. If you tap your breaks at the wrong time in a corner, you crash. If you think you’re going to crash, you crash.”

Talansky, riding in his first Tour de France, said he found it difficult to practice descending because, even on roads closed to car traffic, a rider never experiences the pure fear, desperation and desire that one feels in the heat of a race.

“You just wouldn’t push it quite the same, because there is nobody forcing you to do that,” he said. “There’s not this: ‘I have to be in the front here.’ ”

Riders on straight descents will try to tuck themselves into the most aerodynamic position possible to reduce wind resistance, at times getting out of their saddles to sit directly on their bike frames.

On turns, they will search for the straightest possible lines, starting wide and then slicing across the road at the bend to exit wide on the other side. They will barely move the handle bars, turning by leaning instead, while tapping the brakes as lightly as possible.

Staying relaxed is a big part of the skill. Fearful riders are usually stiff riders, making them more likely to make the sorts of herky-jerky movements that can destabilize their bikes.

Sagan said that riders not comfortable on screaming downhills are the most likely to crash.

“When you see fear in his eyes, then he’s rigid,” Sagan, of Slovakia, said in an interview before the race. “Then he don’t make a good turn. Maybe when he want to go more faster, it’s risky for him.”

Asked to summarize the essence of his descending technique, Sagan, for whom English is a third language, thought for a moment before answering.

“Not risk too much,” he said, “and come down first.”