The circumstances leading up to the occurance of road accidents are so complex that a reliable assessment of accident causation can only be made with detailed knowledge of the many aspects of road environment, vehicle and road user which may have contributed to the accidents. This report describes in detail the techniques adopted for an on-the-spot investigation into accident causation starting of the contributing factors. The organization, staffing, investigating and analytical procedures used are described; difficulties encountered and possible improvements in technique are also discussed