3.2 Multiple causes
Now we will explore multiple causes using an example familiar to us all – road accidents. The deaths of about 10 people each day on the UK's roads are less dramatic than, for example, the capsize of the Herald of Free Enterprise, but one feature that links them both is the element of risk associated with everything we do – and even with inaction.
We have just seen that many factors contribute to the risks which result from the inherent hazards associated with something we do.
Look at the following hypothetical account of a road accident, which is not atypical of many we see in the newspapers or hear about on news bulletins:
Two vehicles were involved in an accident on the Anytown bypass late last night. X was travelling north and was in collision with another vehicle turning right from the southbound carriageway into a petrol filling station. The driver, Y, of the second vehicle and its passenger, Z, were both thrown from the vehicle, and X was taken to the Hereshire hospital in a critical condition.
A detailed investigation may conclude that a variety of factors contributed to the incident, including:
Driver X was exceeding the speed limit.
The braking behaviour of driver X's vehicle was poor.
Driver Y failed to yield right of way and turned right incorrectly.
The vehicle occupants Y and Z were not wearing seat belts.
The doors of driver Y's vehicle were not securely closed and resistant to impact.
Rain had made the surfaces wet and so reduced the coefficient of friction of vehicles on the road.
Telegraph pole and road signs were too near the road and compounded the injuries.
Driver X has a history of careless driving incidents.
and so on.
We conclude that there were many factors contributing to the cause and the severity of the incident. Figure 6, showing the effect of the four factors Environment, Engineering, Education and Enforcement (the four Es) on road accidents, is actually a very simplified multiple cause diagram, drawn from the perspective of, say, the highways engineer.