Everyone agrees that you shouldn't drive while high. It is certainly safer than driving drunk or (worst of all) driving while tired, but it isn't good. The point that it isn't clear what the effects are is true. We don't know what the net result is. It could be, that now that it is legal, more people drive high. It could also be that idiots that were going to drive high were already driving high and so nothing has changed. It could be that now that it is legal, you can run a proper non-laughable PSA campaign to try and reduce the rates of driving while high.
Regardless, unless there is a sudden shocking and unexpected change in the statistics, it is clear that if there is any more pain from idiots driving, it is a small price to pay to not murder citizens in the drug war, not ruin peoples lives with felonies, and defund violent groups of their primary source of income. Not to mention, the end of prohibition is simply a better that a just and free society that loves liberty.
We have tried this experiment before with with one of the most dangerous drug that kill more people than any other drug, alcohol. The results then were clear, and history is simply repeating itself now that we are doing it again with a far safer drug.
So yes, there are going to be negative externalities, and we should find ways to deal with them. It would be wishful thinking to think that there will be literally no consequences to ending prohibition, but those negative externalities pale in the face of the unquestionable horror, suffering, and death that prohibition brought us.