Source: http://www.drugpolicy.org/

Under a decision by a divided Michigan Supreme Court in June, you can be convicted of drugged driving, even if you are not in the least impaired by drugs. In a pair of cases, Derror v. Michigan and Kurts v. Michigan, the Court upheld the constitutionality of Michigan’s zero tolerance per se policy, which allows prosecution for driving under the influence of drugs if a person tests positive for any trace amount of a Schedule I drug or its non-psychoactive metabolite. 

The ruling reverses a Michigan Appellate Court ruling which said that in order to move forward with a DUID prosecution, the state had to demonstrate that the presence of a controlled substance in a driver's body was the proximate cause of an accident.

The Michigan policy has a series of impractical and even harmful consequences.  It is useful to compare the zero-tolerance law to the state's alcohol laws, which permit drivers to have a blood alcohol content of up to .08.  This level is equivalent to about 1-2 drinks per hour for an average size person.  Such a person can legally drive, but who smoked a joint three days earlier cannot. In other words, any casual drug user who responsibly refrains from operating a motor vehicle while intoxicated (typically a matter of hours) must now wait days, weeks, or, in the case of marijuana, even months before operating a motor vehicle in Michigan. If Michigan’s policy were adopted nationwide, the licenses of over 25 million Americans could be revoked or suspended--with absolutely no discernible benefit to public safety.

What is more, Michigan’s zero tolerance law effectively overrides the laws of at least 11 other states-–representing nearly one-third of the U.S. population--which have ratified the medical use of marijuana.

In a scathing dissent, Justice Michael Cavanaugh of the Michigan Supreme Court observed that the decision "now makes criminals out of numerous Michigan citizens who, before today, were considered law-abiding, productive members of our community." 

In short, Michigan’s law bears no rational relationship to concerns for public safety – or any other governmental interest. At the same time, it is likely to wreak havoc on the lives of thousands of otherwise law-abiding, safety-conscious individuals. As a result, the Court’s ruling is destined to end up on the trash heap of stupid and harmful legal decisions. The only question is when.