Data like this? We definitely need more, but there's a lot of data already out there.
The prevalence of both alcohol and cannabis use and the high morbidity associated with motor vehicle crashes has lead to a plethora of research on the link between the two. Drunk drivers are involved in 25% of motor vehicle fatalities, and many accidents ...
www.ncbi.nlm.nih.gov
Speaking anecdotally, I don't know anyone who has used THC products and
NOT become impaired. I know plenty of people who have one or two drinks and are still just fine to function like a normal human being. Not mention the fact that you can contract a secondhand high from being around people blazing the chronic but you can't become secondhand drunk. I just don't see how THC usage could be tenable with military service.
Also, the reason the Navy uses such a terrible test (It produces a lot of false results) is largely driven by money. If you consider that by instruction, the entire service has its pee tested at least once a year (this isn't actually true) this a large expenditure and the Navy cuts costs wherever it can. I don't think there would be an appetite for a more accurate test.