If looking for the principal "cause" of fatal accidents you only have to compare the proportion of over limit drivers in fatal accidents to the proportion of these in random breath test campaigns - 25-50% vs 0.1-0.5%.
Having said that, no accident has a single cause - if a drunk driver is speeding while texting on a narrow winding dirt road - what is the "cause" of the accident?
If you look for real life data (as opposed to theoretical studies), consider that over the last fifteen years, as the death and accident rate on Australian roads has steadily decreased, mobile phones have gone from rare to ubiquitous, and their use while driving has certainly increased.
Similarly, in the USA, some states have made phone use while driving illegal, some have not. If phone use were a significant factor in accidents and deaths, you would expect this to show up in statistics as a readily observable difference in the rates, or at least the trends, as the use of phones increased. It is not possible to see this difference in the figures!
The conclusion, to my mind, has to be that phone use is not a major factor, let alone "cause" of fatal accidents.
John

