Law enforcement agencies are preparing for what they fear will be an increase in the number of people driving under the influence of recreational marijuana as California preps for the sale of the drug in the new year.
Unlike laws governing alcohol use and driving, there will be no set legal limit for THC in the body under Proposition 64, partly because there is little scientific evidence linking THC levels and level of impairment, and partly because there is still no reliable biological way to test for the drug.
"Alcohol is kind of a special case, because with alcohol the amount you drink and the amount of alcohol in your blood are very closely related," said Dr. Igor Grant, head of the UC San Diego Center for Medical Cannabis Research.
But while alcohol is water soluble, THC – the active compound in marijuana – is fat-soluble and it is metabolized very differently.
When a person smokes cannabis, they get a very high level of THC in the blood and body fluids within 30 second to a minute, but they likely won't be impaired yet.
By the time they are, most of the THC has gone into the tissues and organs like the brain where it has its psychoactive effects. Very little will remain in the body fluids.
"So there can be a disconnect between the actual blood levels and how impaired you really are," said Dr. Grant.
Experience level with the drug also affects one's THC level, with habitual users retaining persistent low levels in the blood even when they have no impairment. That's why drivers arrested in Colorado over that state's legal limit have been able to successfully argue they weren't actually high at the time.
On the other hand, an inexperienced user could experience a great deal of impairment and still have very low THC levels.
The degree to which THC impairs driving has also been contested. THC has certainly been shown to affect skills needed for safe driving – automatic functioning, perception of speed and depth, divided attention and distractibility.
But driving simulations have shown many users are able to compensate for those effects, often by driving more slowly and cautiously. Unlike alcohol, which increases risky behavior, cannabis tends to make users more risk-averse and aware of their impairment.
Dr. Grant adds there's no doubt that impairment from cannabis is a danger on the road. One study suggested it doubles one's risk of crashing, although compare that to alcohol use where drivers were seven times more likely to crash.
Because of THC's non-linear metabolism and low levels in body fluid, it is much harder to test for than alcohol. In states that do currently have a legal limit, it is defined as a level in the blood.
But blood testing presents challenges in timing since it requires subjects to go to a lab potentially hours after they were pulled over, and it is very difficult to extrapolate from the results when the cannabis was actually used and whether it was causing impairment at the time.
Some law enforcement agencies have been experimenting with a roadside saliva test such as the Draeger 5000, but it shares many of the same problems with blood testing in terms of correlating THC levels to impairment. Other factors such as mouth dryness can also affect the test.
Several companies are now developing a roadside breathylizer test which is sensitive enough to detect very small amounts of THC in the breath. The benefit would be that because THC is present in breath for a much shorter time than blood or saliva, it correlates more closely to the period when impairment is likely: in the first hour or two after smoking.
But Dr. Grant cautioned that technology has not been tested yet. He believes it may be more helpful to improve behavior-based testing, a modification of the standard Field Sobriety Test.
"Could you develop like an iPad app or something that actually would be a better indicator of impairment than, say, having you walk in a straight line or see how much you sway and those kinds of things?" he said.
Meanwhile, the California Highway Patrol is putting its emphasis on training officers to spot the signs of impairment. The agency says it will have all 7,000 officers trained in basic roadside drug evaluation by the end of the year, and it's upping its more advanced drug recognition expert trainings throughout the state.
The L.A. Sheriff's Department is also stressing increased training, but it believes the most important piece will be a public safety campaign by the state Office of Traffic Safety.
"If we can educate the drivers out there that high driving is impaired driving that will get the word out," said LASD Sgt. Robert Hill.
Hill and others in law enforcement are particularly concerned about a potential increase in new cannabis users who may not be as familiar with the effects.
"You have all these people that always wanted to try it, but it's been illegal so they haven’t. So now they will have the ability to buy marijuana and see what it’s all about," he said. "You’re going to have first time users using and getting behind the wheel not knowing how it’s going to affect them."
Officer Glen Glaser, a drug recognition expert with the California Highway Patrol, also worries about the effects of combining cannabis with other substances, especially alcohol.
"In some cases one plus one equals two, but when you mix cannabis and alcohol it's like one plus one equals five," he said. "When you get a low amount of alcohol and a low amount of THC, it's very synergistic and it compounds the person’s impairment."
California will use a portion of the revenues generated by taxing recreational marijuana to further study the issue and develop best practices around traffic enforcement and cannabis.