By Martyn Herman

RIO DE JANEIRO (Reuters) - Rider misjudgments not Rio's scenic Olympic road race course were responsible for a series of bone-crunching crashes over the weekend, according to UCI chief Brian Cookson.

Dutchwoman Annemiek van Vleuten remained in hospital on Monday with three fractured vertebrae and concussion after a horrific-looking accident on the Vista Chinesa descent close to the finish at the Copacabana beach.

On Saturday, Vincenzo Nibali, who like van Vleuten was leading the race on the final, high-speed descent, broke both collarbones after he and Sergio Henao crashed heavily on the same stretch of road. Henao fractured his pelvis.

Britain's former Olympic track champion Chris Boardman criticized the course, specifically the Chinesa descent on which Australian Richie Porte broke his right shoulder in another crash on Saturday, describing it as "dangerous".

Only 63 of the 144 men who started the men's 237.5km road race finished but Cookson, speaking to Reuters at the velodrome after visiting van Vleuten in hospital, said riders had to take responsibility.

"We will have a look at things and do a review, but a large number of riders managed to go down those hills without crashing so maybe some people took too many risks," he said.

"Every rider has to make judgments and sometimes they get it wrong. We are concerned for safety but ultimately riders have a responsibility to make judgments under the circumstances because it takes place in a real environment."

He expressed sympathy for van Vleuten, however, and was happy to report that the Dutch rider would make a 100 percent recover despite being battered and bruised.

"She was doing the ride of her life and was on fire but yeah, staying on the bike is part of the game and if some riders misjudge things that's unfortunate," he said.

"She is obviously disappointed because she was pretty much on for the gold medal."

BEST 'EVER SEEN'

Cookson said the Rio road course, starting and finishing on the Copacabana beach, and taking in loops of Rio's spectacularly hilly coastline, had provided the backdrop for arguably the best Olympic road races ever seen.

Belgian Greg van Avermaet snatched men's gold after reeling in Poland's Rafal Majka less than 2km from the finish while the women's race was even closer when solo leader Mara Abbott was caught by Anna van der Breggen 200m from the finish.

"We had two epic races that will go down in history, perhaps the most exciting Olympic road races we have seen with the results in doubt right up to the line," Cookson said.

"The course planning was fantastic. They had everything -- winds from the coast, cobbles, hills, exciting and challenging descents. We were very careful in the planning of the routes.

"We will be talking about these races for years to come."

Cookson's comments were endorsed by Germany's Roger Kluge, who has swapped the road for the track in Rio.

"The risk is up to ourselves," the IAM Cycling rider, who will compete in the omnium here, told Reuters.

"The road was good. If they want to go too fast, it's up to the riders. And after six hours in that heat, mistakes happen. It was no fault of the organizers. The parcours was good."

Van Vleuten's crash, when she skidded and went over her handlebars into a stone kerb, shocked television audiences but she posted an update on Twitter on Monday.

"Still in the hospital. Waiting for some research and hope I can leave today. Knowing that this chance is one in four years doesn't make it easy," she said.

Road racing concludes on Wednesday with the men's and women's individual time trials which will be raced on the less problematic Grumari section of the road race course.

(Editing by Clare Fallon and Mark Lamport-Stokes)