this doesn't say whether the drivers were found to be at fault, just that they died while under the influence. and it was conducted with 246 cases in a single county in Ohio. it's hard to find a causal link without know a lot more.
one interesting takeaway, though, is:
The high rate of THC positivity remained consistent over six years and was unaffected by the state’s legalization of recreational cannabis during the study period.
so, we have evidence that legalization doesn't necessarily lead to an increase of related DUIs. although, this could just as easily indicate that, since legalization, public consumption by those who drive high hasn't increased, and that all that changed is where they buy their weed.
hopefully, this will lead to larger, more well-rounded studies on this subject.