Two wounded street cleaners were left on the road for about two hours Thursday morning in Shanghai before an ambulance arrived.

A sanitation truck hit four dustmen on Jungong Road in the morning. Two were rushed to hospital but the other two were left on the road waiting for the city's 120 emergency service, said Xuan Kejiong, a reporter for Shanghai TV, on his Sina Weibo account.

He also published a photo of two men lying on the road, who looked to be in pain.

The cause of the accident is still under investigation.

Shanghai authorities have no target time for ambulances responding to emergency calls. According to authorities, in more than 75 percent of cases a paramedic will be on the scene within 12 minutes, but it can take longer due to traffic.

The city has about 2,275 ambulance workers, according to the Shanghai Health and Family Planning Commission. Most are drivers and nurses, with less than 30 percent of them trained doctors, and that number has been steadily dwindling.

Low salaries and a high-stress working environment go some way to explain the shortage and why complaints about the city's emergency response service have been common in recent years.

Last year, for example, the Shanghai Medical Emergency Center, which has more than 110 ambulances covering the large downtown area, recruited 31 doctors, but 55 resigned during the same period.