Authorities in south China's Guangdong Province have launched an investigation into the collapse of a highway ramp under the weight of four overloaded lorries on Friday, an incident in which one person was killed and four others injured.The accident happened at 3:40 am in Heyuan City. The vehicles, loaded with porcelain clay, plunged to the ground.

The Guangdong provincial bureau of transport said on Sunday that it has commissioned a third party to conduct the investigation in order to ensure impartiality.

The highway, opened to traffic in December 2005, connects Guangdong with east China's Jiangxi Province. The 130-meter one-way ramp leading up to the highway collapsed under the four lorries' estimated total weight of 100 tonnes.

"The weight surpassed the ramp's maximum load, causing the rupture," said Ao Daochao, chief engineer with the Guangdong Highway Co. Ltd., the parent of the Guangdong Yuegan (Guangdong-Jiangxi) Highway Co. Ltd., which put up the money for and operates the highway.

Ai explained that the ramp was designed to carry a vehicle of up to 50 tonnes preceded and followed at a distance of no more than 10 meters by vehicles of up to 20 tonnes.

He said the four lorries were being driven too close to each other.

However, many have said the designers should have expected more congested traffic on the ramp, which is close to a toll station. Critics have also questioned how a ramp with a designed service life of 100 years could collapse so soon, even if overloaded.

Another issue is over the responsibility of the toll booth agents -- how could they have waved through vehicles with weights surpassing the ramps' capacity?

The Guangdong transport department has ordered tightened inspections of vehicle weights at toll booths.