Recreational use of marijuana became legal in California on January 1, as a result of the passage of Proposition 64 in November 2016. It is still a crime to drive under the influence of marijuana, but unlike with blood alcohol level, California has not established a per se level of THC (the active ingredient in marijuana) in the bloodstream at which it is presumed that a driver is under the influence. In civil auto accident cases, there is a big question whether a driver’s THC level is relevant at all and, if so, under what circumstances.
UC Berkeley law professor Andrea Roth has written an incredibly thorough review of the law and science surrounding marijuana and its effects on driving and has concluded that any per se limit based on THC level is inappropriate because:
“The well-acknowledged truth is that there is no known relationship between THC blood levels and increased relative crash risk . . . and no known relationship between a driver’s THC blood level and his level of driving impairment.”
In other words, looking at a driver’s THC level alone and concluding that he or she was impaired (as a number of states now do) is not valid. Nor can one convincingly say that a driver with a higher THC level is more impaired than one with a lower level.
The AAA Foundation for Traffic Safety came to the same conclusion in 2016 when it analyzed data from motorists arrested for driving under the influence of marijuana. The AAA found no relationship between a person’s THC level and his or her performance on field sobriety tests. That is not to say that marijuana use does not affect performance on these tests, but that a higher level of THC is not a reliable predictor of a worse performance.
It is important to understand that the body processes marijuana much differently than alcohol. THC builds up in the fatty tissues and is released slowly over time, so a frequent user may have a relatively high THC level a day after he last used marijuana and may not be impaired in any significant way at that point. In contrast, an infrequent user may be impaired at a much lower THC level. Also, drivers under the influence of marijuana tend to behave differently than drunk drivers. They tend to drive slower and more conservatively than normal.
The significance of all this information for civil auto accident cases is that a driver’s THC level, by itself, says little or nothing about whether the driver was under the influence. If a THC level is not corroborated by independent evidence of impairment (erratic driving, poor performance on field sobriety tests, etc.) that tends to establish a causal connection between the marijuana use and the crash, then the THC evidence ought to be excluded. There is a strong Evidence Code section 352 argument (particularly in the case of a frequent marijuana user) that the prejudicial impact on the jury of knowing that the driver used marijuana outweighs the limited probative value of THC level alone. See Hernandez v. County of Los Angeles (2014) 226 Cal.App.4th 1599, 1614-15.
Of course, if the California legislature ignores the science and data, and establishes a per se THC level for proving impairment in criminal cases, there will be an obvious carryover into civil cases. It would become considerably more difficult to convince a judge that evidence of a driver’s THC level is irrelevant if that level is high enough to result in a criminal conviction.
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