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In 2000 Colorado voters approved a constitutional amendment allowing patients suffering from debilitating medical conditions to use medical marijuana (see Article XVIII, section 14 of the Colorado Constitution).  In 2012, Colorado voters approved a constitutional amendment allowing persons over the age of 21 to use recreational marijuana (see Article XVIII, section 16 of the Colorado Constitution).
Driving under the influence (DUI) or driving while ability impaired (DWAI) by any intoxicating substance, including marijuana (cannabis, THC, weed, pot) has long been a violation of law.  Historically, drinking and driving laws have included a presumption of intoxication based on a drivers blood or breath alcohol content level.  Colorado, like most states, have not had a scientific measurement indicating when a person is considered under the influence of marijuana.  
In 2013, House Bill 1325 was passed creating, similar to the presumption of intoxication for blood alcohol content, a “permissible inference” of intoxication when a driver has 5 nanograms (NG) of marijuana in a blood test.  This allows the trier of fact (a judge or jury) to infer a person is under the influence of marijuana, or more correctly THC (tetrahydrocannabinol, Delta 9-THC, the psychoactive element in marijuana) based on a blood test result of 5 NGs or more in the blood.
The 5 NG threshold does not necessarily indicate that a person is under the influence of marijuana, but merely is evidence in a case.  It basically gives judge or jury, permission to assume a person was too high to drive without necessarily requiring the prosecution to introduce additional facts.
When a person is charge with a DUI or a DWAI offense for any intoxicating substance it is important to consider all the evidence in a case.  Although the district attorney (DA) may get a conviction for DUI THC (or blood or breath alcohol content) based solely on the amount of the substance found in a person’s blood, certainly the DA will be introducing additional evidence in a driving under the influence of marijuana case that is favorable to obtain a guilty verdict.
The concern is that this 5 NG permissible inference level may not be an accurate measurement of a driver being impaired.  The National Highway Traffic Safety Administration, in a 2004 report, discussed that it does not take much to bring a driver’s THC level to what is now Colorado’s threshold level, 5 NGs.  The study revealed that a single hit of marijuana from a joint can be enough to elevate THC levels over three times Colorado’s 5 NG threshold.  Also, THC can stay in the bloodstream in measurable quantities for hours and even weeks. Furthermore, the study concludes that a driver’s level of impairment can depend on a person’s experience with marijuana, as opposed to just a level of THC in the blood stream.  According to NHTSA “It is inadvisable to try and predict effects based on blood THC concentrations alone.”
The Colorado court system will no doubt have a series of legal battles revolving around this 5 NG law as many experts believe the NG limit is an arbitrary number solely created to ease the burden for prosecutors to obtain a guilty verdict.  The problem with this is that defendants may be found guilty when in reality they are not under the influence of marijuana, but merely have used cannabis sometime in the recent past.  The blood test may show elevated levels long after the psychoactive effects of the drug have worn off.  Many experts agree that THC can show in a person’s blood days or weeks after use.
THC remains dissolved in the fatty tissues of the body and can be released in the blood over an extended period of time. This is why blood tests can show THC for weeks after the person has consumed marijuana.
Tests have been conducted on drivers with 15 nanograms of THC in their blood, which is 3 times the legal driving limit.  These drivers had no trouble driving whatsoever. The concern is that these test subjects could have been easily convicted in a Colorado criminal trial with a prosecutor explaining to the jury that the driver is 3 times the limit set in Colorado.  It will be beneficial to have an expert testify that the mere presence of THC in the system is not conclusive to determine if a driver was impaired. 
The attorneys at Black & Graham do not believe that the arbitrary 5 NG  limit set by the Colorado legislature is a true indication of a person’s impairment.  To defend a client charged with DUI or DWAI for marijuana, or any other substance, additional facts during the law enforcement contact stage need to be shown to the jury.  The lawyer should carefully review the police reports and question witnesses about various tasks the driver completed that would indicate the driver is not impacted by the alleged intoxicant.  For example:
Purpose for traffic stop:  Was the driver driving erratic, weaving, running a red light, or cause an accident?  Was the driver merely pulled over for speeding, parked or the victim of an accident?
Officer contact:  Did the office notice bloodshot or watery eyes, slurred speech, seemed confused?  Is there an explanation for red eyes like the driver’s employment (being around dust or on a computer all day, sports related) or was the driver merely tired?
Vehicle exit:  Did the driver exit the vehicle in an unsteady manner or exit as any other person would?
Roadside tests:  In Colorado roadside tests are voluntary, see link to roadside tests.  Did the driver complete the roadside tests in a satisfactory manner or show signs of intoxication?