Gross violations of safety norms led to casualties in the flyover collapse case in Kolkata, an expert on traffic and transport safety, working with the National Highway Authority of India, said on Friday.

"There can be technical issues during construction and so the industry practice is to demarcate any construction zone.

When such a large overhead structure was being constructed how could people and traffic be allowed underneath," IIT Kharagpur Professor Bhargab Maitra said.

His team has been working with the National Highway Authority of India on conducting road safety audits.

He said it is a norm that during such constructions no vehicular or pedestrian movement be allowed underneath.

"I am shocked why was it not followed. This is even there in the manual of the Indian Road Congress. Had that been followed then there would have been no casualties. There has been gross violations," Maitra said.

Even the workers are supposed to wear protective gears like helmets, shoes and gloves.

About 60-metre-long portion of an under-construction flyover collapsed on Thursday afternoon on a congested road intersection here killing at least 24 people.

Another issue, the expert said, is the absence of any disaster management plan.

"The first hour of any accident is known as the golden hour because it is during that time when lives can be saved if the victims are hospitalised. But it took few hours to take them to hospital," Maitra said.

He and other experts from IIT-Kharagpur are ready to work with the government in finding out the technical reasons which led to the tragic accident.

"You cannot call it an 'Act of God'. There has to be some factors which may not be known to us immediately," the expert said.