While I won't promote driving stoned (or drunk), field sobriety tests are BS, half the time, a sober person can't pass them...
My fav being "say the alphabet backwards quickly", or rerunning the follow-the-pen test over and over until they get the result they want.
The real test is the blood test, all else is BS.
As for piss tests and weed, its bunk, because piss tests only say that a person has used it within a large time window (usually weeks), and not if the user was currently high when driving etc...
unfortunately, we are more concerned with putting people away than using logic... but no surprise there.
PS I have never gotten a DUI but I have gone through the process due to over-zealous cops.