There's a new front in the "War on Drugs" and its name is DUID.

DUID, short for "driving under the influence of drugs," is the latest buzzword among politicians and police -- however, in this case, words can be deceiving.

Though billed by its sponsors as a necessary tool to crack down on "drugged driving" offenses,1 in reality, DUID laws -- in particular "zero tolerance" per se laws -- have little to do with promoting public safety or identifying motorists who drive while impaired. Rather, the enactment and enforcement of "zero tolerance" DUID legislation improperly defines many sober drivers as "intoxicated" solely because they were found to have consumed a controlled substance -- particularly marijuana -- at some previous, unspecified point in time.

There are various types of DUID laws, some more pernicious than others. Today, every state has DUID legislation on the books. These laws fall into three distinct categories:

Most state DUID laws are "effect based" laws. This legislation forbids drivers to operate a motor vehicle if they are either "under the influence" of a controlled substance, or if they have been rendered "incapable of driving safely" because of their use of an illicit drug. In order for a defendant to be convicted under this statute, a prosecutor must prove that the driver's observed impairment and/or incapacity was directly associated with the ingestion of an illicit substance. To do so, prosecutors typically rely on evidence gathered by law enforcement officers at the scene of an accident (i.e., a driver's failure to pass a field sobriety test, evidence that the motorist was driving at an excessive speed, etc.), testimony from a Drug Recognition Expert (DRE), and/or a positive result from a blood or saliva test indicating recent consumption of a controlled substance. For the most part, this is a multidisciplinary standard that focuses on the totality of circumstances -- most importantly, whether the driver is observably impaired -- and accordingly punishes motorists who drive while impaired from having recently used illicit drugs.

Per se laws prohibit drivers from operating a motor vehicle if they have greater than a set level of a drug or drug metabolite present in their system. Most Americans are already familiar with the most common driving-related per se laws: those governing drunk driving which define a driver as legally impaired per se if their blood alcohol level tests above .08%. Similar per se laws with strictly defined cut-off levels (a designated level of an active drug constituent or metabolite above which a sample is considered to be "positive" for a specific drug) are uncommon for DUID legislation.2 This is because, according to the US Department of Transportation: "Forensic toxicologists generally have failed to agree on specific [per se levels] that could be designated as evidence of impairment. The lack of consensus about per se levels of drugs where impairment could be deemed makes it difficult to identify, prosecute or convict drugged drivers in most states."3

Politicians and police have a simple, if unscientific, response to researchers' failure to define per se standards for DUID offenses: to enact "zero tolerance" per se laws. In their strictest form, these laws forbid drivers from operating a motor vehicle if they have any detectable level of an illicit drug or drug metabolite present in their bodily fluids.

This approach is not based on science but on convenience. In essence, "zero tolerance" per se laws define a new, driving-related offense that is, in the words of one of its chief proponents, "divorced from impairment." Under this standard, any driver who tests positive for any trace amount of an illicit drug or drug metabolite (i.e., compounds produced from chemical changes of a drug in the body, but not necessarily psychoactive themselves), is guilty per se of the crime of "drugged driving," even if the defendant was sober. In the case of marijuana, these laws are particularly troublesome. THC, marijuana's main psychoactive constituent, may be detected at low levels in the blood of heavy cannabis users for 1-2 days after past use.4 Marijuana's primary metabolite THC-COOH, the most common indicator of marijuana use in workplace drug tests, is detectable in urine for days and sometimes weeks after past use5-- long after any psychoactive effects have ceased. Consequently, under "zero tolerance" per se laws, a person who smoked a joint on Monday could conceivably be arrested the following Friday and charged with "drugged driving," even though he or she is no longer impaired or intoxicated.