Here dedicated DUI enforcement is done by targeting high crash areas. The state loses money on DUIs for the indigent. The state pays for the jailing fees, pays the public defender, pays the prosecutor, pays the cop, pays the court staff, etc. The convicted gets declared indigent, has fines and court fees waived, pays a $185 drug and alcohol fee for mandatory alcohol/drug rehab sessions...and that's it. If it was about revenue, you'd just target upscale bars. People who aren't indigent can pay fines instead of doing jail time. There are only so many ways to influence human behavior, and since we aren't going to start caning people, financial loss or jail time is about all we have.
The 80% stat is way out there. Most crashes don't involve any impairment at all, other than "head up butt" syndrome or misjudging distances. The two leading causes of crashes are failure to yield on left turn and following too closely. I don't have any breakdown on fatal vs non-fatal.
Like I said, the issue with prescription drugs is one of detection. Alcohol is just easier to enforce. You can smell it on the breath, it produces changes in the eyes that are easily observed, breath machines make for quick and accurate measurement, and there are a butt ton of studies about how impaired someone is at a certain level. Prescription drugs are odorless, only detectable via blood test, AND require medical training to detect. That's why fatal crashes here are mandatory blood draws. Its just unrealistic to expect a cop to have the knowledge and equipment to detect prescription meds (although there are training classes called DRE (Drug Recognition Expert) that does this, a work up takes about 2 hours for one driver). Absent statues that make a blood draw mandatory, probable cause must be developed, and given the limitations described its very difficult to do so.