As a civil engineer in the manhole business I'm not totally familiar with practices in China, but if they are anything even remotely like the US the chance of a cover being dislodged by driving over it is pretty slim. More than likely, the issues was poor construction and settlement occured around the manhole (the manhole is supported by deeper soils) and the surrounding pavement dropped and the manhole lid then protruded above the roadway as a result. I see this on occasion but rarely it's left unfixed for long in the US. Now any discerning driver would probably notice something sticking up 4 or 5 inches and avoid it but who knows. Lot's to be speculated about this one...