Dozens of motorbikes suddenly crashed to the ground in Hanoi's Kim Lien Tunnel Monday for no apparent reason.

A severe traffic jam ensued and no injuries were reported, but new questions have arisen about Kim Lien's safety. The bridge has been hailed as the most modern and advanced in Vietnam before it opened to massive flooding in its first few days of operations last year. Complaints of cracks and leaks in the tunnel also followed, but experts found no cracks and said the leaks were normal.

Many of the motorbike victims said they inexplicably lost control of their vehicles in the middle of the tunnel without understanding why.

Others said the fact that motorbikes in front of them collapsed made them brake urgently or crash into falling vehicles.

The domino effect caused a traffic jam in rush hour on Monday morning.

Hanoi Traffic Project Company, which manages the Kim Lien Tunnel, said the falls were not linked to the road's design or any other technical issue.

The firm said rainwater in combination dust on the road made the ground slippery, so that motorcyclists who drove too fast easily fell down.

It had been raining in Hanoi and rainwater entered it via motorbike tires.

Dr. Khuat Viet Hung, deputy director of Transport Management and Zoning Institute, said relevant agencies should wash the road regularly to reduce dust.

He also suggested installing speed bumps to minimize falls.

Kim Lien Tunnel, which links Dai Co Viet and Dao Duy Anh streets of Hanoi, was inaugurated in mid-June last year.