Posted By Tom W. Hello Graham This will be very much dependant upon circumstances, RIDDOR, paragraph 10 details some information about the movement of a vehicle on the road..... But, there could be so many contributing factors to the cause, physical and mental state of the individual at the time of accident, the prevaling weather conditions, the road conditions ( both surface, configuration, and layout)Traffic flow rate. Add to that the relms of PUWER... How was the competence of the individuals driving abilities determined, what was the condition of the vehicle being used, with vehicles only slight variations in mechanical condition are required in order to cause unresponsive handling characteristics or operating conditions, such as varying type pressures, tyre tread depth, uneven and insecured loads being carried, (yes even in a car),mechanical component wear due to lack of maintenance, low system lubricant and collant levels which result in overheating and/or localised heating... with a knock on effect to the well being and opereration of a vehicle. These are just a few of the factors that you would have to take into account when conducting an investigation into the cause of the accident, add to this site visit and examination, operator interview, witness interview (all beiing conducted independently of each other), investigation summary to determine fact from fiction (the opinion that a person form in the time frame fron the point of accident to point of interview)... and only then will you perhaps be able to make a determination! Well grahem, perhaps not the answer you sought, but just a few points from my experience as a Unit Road safety Office, where this was part of the daily routine.