There are several proximate causes of motor vehicle accidents. But one must take a step back and look at the overall picture of the accident. Was it a driver error, mechanical problem of the vehicle, defective roadway, weather issues, lighting problems, traffic control issue(s), e.e. traffic sign(s), traffic signal(s), etc.
It is the experienced engineer not a retired police officer that will be a key element to determine the proximate cause of the traffic accident in question.