The same applies to various aspects of transport data collected worldwide. Definitions of parameters (e.g. accident fatalities) vary from country to country or region to region, hence it is difficult to present global statistics without extensive disclaimers. In research, there are no straightforward conclusions because of all the assumptions one makes regarding other researchers' methodologies. It is good that this blog highlights the challenges faced in trying to obtain data when records are incomplete or non-existent, and also when definitions of parameters differ across the board.