This is a non-controversy.  Police are empowered to enforce regulations against impaired driving.  They can choose to take your keys away even if you pass the Breathalyzer if you exhibit other symptoms of being impaired.  We don't need a fancy machine for this at the roadside.  People drive impaired by using marijuana now and as dangerous as that may be, no one was making a big deal about this 2 years ago.  We are legalizing usage because it is so widely available and used, that enforcement of rules banning it are: a) more harmful than the drug and b) brought the legal system into disrepute.  Because usage is so widespread, usage levels are unlikely to change just because it is now legal.  People are either going to be responsible and fearful of causing harm to others, or they are not.  Police can still arrest you for impairment.  Non-issue and non-controversy.