A woman was in a critical condition last night after a runaway 11-tonne rubbish truck hit her as it careered down a hill in Happy Valley. The truck, which rolled away 'like a cannon ball' while its driver loaded rubbish from behind, left a 160m trail of destruction. It ploughed through traffic signs, flattened a phone box, destroyed railings and slammed into a carpet shop in Shan Kwong Road. Pedestrian Ma Wan-ming, 52, the wife of Professor Nelson Chow Wing-sun from the University of Hong Kong's social work department, suffered head injuries. She was in critical condition in Queen Mary Hospital. The accident occurred at about 8.50am when driver Chan Lap-kwan, 32, parked his truck on a slope in Shan Kwong Road opposite Po Kok Girls' Middle School. The section of road has a gradient of one in five. The driver told police he put on his hand-brake but left the key in the ignition before getting out and unloading rubbish into his vehicle, which belonged to a private company. The truck rolled forward and mounted the pavement, wrecking two traffic signs and hitting a metal canopy before it knocked down Mrs Chow near the junction of Yuk Sau Street as the driver chased after it. At the junction of Yik Yam Street, the Mitsubishi truck climbed a safety island where it flattened 10m of railings and two more traffic signs. It then mounted the pavement on the other side of the road, mowed down railings and a telephone booth and ploughed into the carpet shop. It was not open at the time. 'There was a big bang like an explosion when it rammed into the shop,' a witness said. Superintendent Ho Wing-sun said: 'The truck careered down the slope like a cannon ball at 70 to 80km/h. 'Drivers should not stop and leave their heavy goods vehicle unattended on a slope. If so, they should jam wood under all four wheels,' he said. The truck was taken to the Quarry Bay government vehicle plant for an examination.