Full disclosure: I have been pulled over and arrested for "driving while ability impaired" (essentially the non-alcohol DUI)
When I was going to school in upstate NY, I was pulled over as I drove my friend home. The cop thought we smelled like weed (fair enough, my friend had some on him, I was sober and not holding). So he pulls me out of the car and makes me do a "field sobriety test" which consisted of the following (no joke):
-Standing on one leg for 10 seconds
-Walking in a straight line in the dark while he flashed a light at you like a strobe
-Having to close your eyes and then open them when you "thought" 30 seconds had passed
-Having to recite as much as the alphabet backwards as you can
I'm not a coordinated person in general but I had never been pulled over or issued a traffic ticket before. But they then use this circus act as the basis to say "Hey, time for a blood test because you might be high" and then they take you to a hospital. You can refuse the blood test but you get your license suspended for like a month immediately and I needed to drive to work and school. I hadn't smoked in a bit so I thought I'd be good for the blood test and that I'd be fine in court. Of course, it's fallacious to use the blood sample because it can have trace THC from days ago. So you, like I did, can get slapped with a charge based on the fact that you smoked last weekend and then - how dare you - decided to drive days after the fact.
I had to go to court and then was found guilty based on this erroneous evidence. I had to go to "addiction counseling" where the psych told me I exhibited absolutely no addictive symptoms yet I still had to attend. I got one of my scholarships revoked simply based on the charges against me. I had to pay fines and then sign statements at the end of my "counseling" that I would refrain from drug use and be a productive member of society (it actually was worded like that).
I'm not really sure what the solution is. It will be interesting to see what the accident rates / fatalities from stoned drivers are in the coming years. All I know is the current system is a farce and only used to drive revenue to the state.