It has always been a dangerous proposition to send nearly 200 aggressive cyclists hurtling down a road at high speed, their featherweight bicycles inches apart.

Do it in the Tour de France, cycling's most prestigious event, and it's a recipe for disaster.

And that's what has happened in the 99th edition of the Tour. Seven days into the race, it is shaping up as the most dangerous in decades, with 20 riders pulling out of the three-week event following crashes.

Early race nerves, the stress of needing to get results to justify sponsors' financial investment, and an overcrowded peloton are some of the reasons riders and team directors give when asked to explain it.

Sunday morning only 180 of an original 198 riders took off from Belfort for the stage across the Jura mountains into Switzerland. Another two, including Olympic champion Samuel Sanchez, abandoned during the race.

"The guys are really nervous the first week," said Rein Taaramae, the Estonian who wears the white jersey as the highest placed rider under 25. He said the crashes had made the riders feel "like soldiers in a war."

The last time so many riders had abandoned this early in the race was 1998, when a whole nine-man team was thrown out of the race for doping. There have been no doping cases so far this year, but 20 riders have abandoned because of injuries ranging from a broken pinky finger to fractured ribs and a punctured lung.

By far the worst day was July 6, the sixth stage from Epernay to Metz.

As the pack picked up speed to chase four breakaway riders with about 16 miles to go, at least two dozen spilled across the rural road. Many were downed and dazed. One rider said it looked like a trench hit by a grenade.

"Lots of blood and screaming. Carnage," Rabobank rider Laurens Ten Dam said on Twitter.

Some riders said the crash happened when one rider swerved while trying to put a teammate's shoe cover into his back jersey pocket.

The crash knocked Giro d'Italia champion Ryder Hesjedal out of the race with massive bruising on his hip and knee. Tom Danielson, his teammate on U.S. team Garmin-Sharp, was briefly knocked unconscious, and rushed to a hospital for hip, collarbone and elbow injuries.

Another rider whose Tour, and possibly career, ended that day was three-time world champion Oscar Freire of Spain. He was hospitalized with a punctured lung and broken ribs.

Yellow jersey holder Bradley Wiggins of Britain's Team Sky lost a key support rider on the third stage, when teammate Kanstantsin Sivtsov crashed and broke his left shin, becoming the first of this year's casualties.

Sky manager Dave Brailsford says the first week in the Tour "is inherently risky."

"It's a question of staying upright," Brailsford said.

Easy for him to say. Sky has been relatively unscathed by the carnage on the roads.

Garmin-Sharp, which was gunning for a historic Giro-Tour double under Hesjedal's leadership, is now searching for a reason to continue the race after also losing Danielson, the team's "Plan B" and the highest ranking American in last year's Tour. Garmin-Sharp is down to six riders after South African Robert Hunter injured vertebrae in Friday's crash.

Swiss rider Fabian Cancellara, himself seriously injured in a crash at the Tour of Flanders in April, is back in top form at the Tour and wore the race leader's yellow jersey for the first seven stages. Cancellara says that this year's Tour lacks a traditional sprinter-led team such as Cavendish had last year with the now defunct HTC team, or the ones featuring Italy's Mario Cipollini in the 1990s.

"The fight is just bigger, everyone is fighting to get the right wheel to put his rider in the right place," Cancellara said.

Tour de France director Christian Prudhomme disputed the claim that there are more crashes than in the past.

"We have no memory, every year people say this," Prudhomme said. But he couldn't remember a Tour that had as many abandons this early in the race.

Another theory to explain the multiple crashes came from Thomas Voeckler. The Frenchman, who finished fourth last year after wearing the yellow jersey for ten days, blames the race radios that riders wear to get instructions from their team managers riding in cars behind the race.

Voeckler said that at certain key points in the race, sport directors from all 22 teams start shouting directives to get to the front of the pack into the riders' earpieces. "The road is seven meters wide, there's not room for everyone," Voeckler said.

Tour veteran Stuart O'Grady, who crashed out of the 2007 race with five broken ribs, had another view.

"There are a lot of young kids out there, and they don't know how to ride their bikes," the Australian told Cycle Sport magazine. "There's a lot of inexperience, a lot of desperation, a lot of nerves. I think everyone needs to chill," O'Grady said.