Chen Shunpan claims his father, who drove a bus from Hunan province to Shanxi province, froze to death on a freeway on Feb 21 in Shanxi. [Shao Xiaoxin / for China Daily]

Autopsy reports show that a bus driver, who his son claimed froze to death on a freeway in Shanxi province, was actually poisoned by carbon monoxide, traffic police said.

The autopsy was conducted by the police authority in Shanxi.

But the driver's family still blamed his death on the traffic police's "inhuman" law enforcement, which became a heated online topic on Tuesday.

Chen Haibo, the 48-year-old driver, was found dead on Feb 21 in the coach he drove carrying 22 passengers, including workers from a fireworks factory, from Hunan province to Yangquan, in Shanxi, to participate in a local fireworks performance.

Chen Shunpan, the son of the driver, told China Daily that they arrived at Jiuguan toll station in Shanxi at 9:30 pm on Feb 20. Though the bus was only several kilometers away from an expressway exit, it was not allowed to pass the station, Chen said.

He said he and his father begged a traffic police officer named Bai Xuejie many times to let the bus pass or return to the nearest service area for rest, but was refused.

According to road traffic management regulations in Shanxi, coaches from outside the province that have seven or more seats are forbidden from driving on freeways in Shanxi between 8 pm and 6 am.

After half an hour, the bus was guided to stop at the side of the toll station, Chen said.

"I led passengers off the freeway and found a hotel in Yangquan to stay. My father was left in the bus to watch the vehicle and our baggage because the weather was cold and we could not carry the baggage," he said.

Shanxi freeway traffic police said on Tuesday that when they arrived at the scene Chen was unconscious with a burning stove left in the bus.

A publicity officer with Shanxi's freeway traffic police authority, who refused to be named, said that the actions of the traffic police at the Jiuguan toll station were correct.

"According to the regulation, the bus was not allowed to pass the toll station. As far as we know, the driver started the trip at 5 am on Feb 20 from Hunan province and if he continued to drive to the destination, it would be very dangerous as the bus had only one driver," the officer said.

The bus was not allowed to return to a service area because it violated traffic rules, the officer explained.

Wang Limei, deputy director of the China Road Transport Association, said the rule was in accordance with national regulations.

"The rule considers the safety risks of passengers. The country released such a regulation after an accident on an expressway in Henan province on the early morning of July 22, 2011 that killed 41 people," Wang said.

However, Chen Shunpan said they actually did not know the regulation in Shanxi province and they only knew a national road regulation that forbids buses driving onto freeways between 2 am to 6 am.

Chen later learned that according to another Shanxi road regulation, the traffic police should guide bus passengers to the nearest service center for rest. But the traffic police did not implement the regulation, Chen said.

Wang Peng, a lawyer based in Beijing, said the performance of the Shanxi traffic police in this case was improper because they did not consider the situation of passengers on such a cold night.

"For buses from other provinces coming to Shanxi province, traffic police should inform the drivers clearly before they enter Shanxi," Wang said.

jinhaixing@chinadaily.com.cn