The Team Sky rider was slowing down after the road was blocked due to a huge crash by Orica-GreenEdge’s Mitch Docker when he was hit by the motorbike, who had approached the sector at speed.
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Viviani was forced into the barriers at the side of the cobbles as fans took evasive action. A spectator managed to catch the clip on camera and uploaded it to Twitter.
Also on the social network, Viviani updated his well-wishers with pictures of the crash and his bandaged-up injuries.
“Today I was unlucky but at the same time the most lucky of the world! A big Chest trauma and two cut (chest and hip),” Viviani wrote on Twitter.
Sky’s sports director Servais Knaven said: “If you go in the Arenberg there should not be any motorbikes near the riders. He was in the second group with [Peter] Sagan and [Fabian] Cancellara – at the moment [Viviani] is in hospital. He looks all right, nothing serious we think but you never know.”