Questions about the impact of legalized marijuana on intoxicated driving accidents began before California voters even passed Proposition 64.

Now, as state and local governments prepare for a future where adults will be able to buy and consume marijuana legally, one Central Coast lawmaker is looking to address concerns of drugged driving in a post-Proposition 64 California.

On March 3, Assemblymember Jordan Cunningham (R-San Luis Obispo) introduced a bill that would direct the California Highway Patrol (CHP) to study and identify options for setting a legal limit for marijuana-related drugged driving.

Currently, the Control, Regulate and Tax Adult Use of Marijuana Act already provides the CHP with $3 million per year over the next five years to establish protocols to determine whether a driver is operating a vehicle while impaired due to marijuana use, and establish and adopt marijuana impairment-related protocols and best practices to assist the state’s law enforcement agencies. If Cunningham’s bill passes, the CHP would use a portion of that funding to study the viability of setting standards for marijuana impairment, similar to the .08 percent blood alcohol content threshold used to determine alcohol-related intoxicated driving.

“With the recent legalization of recreational marijuana, it is imperative that the state take action on this issue,” Cunningham said in a written statement issued shortly after he introduced the bill. “For road safety, we need to see whether the science supports a legal limit for THC while driving.”

Currently, there’s little consensus between the 24 states that have some form of legalized marijuana, be it medical or recreational. Of the 24, only five have set blood level-based marijuana intoxication limits, also called “per-se limit laws.” But even between those states, that level can differ. For example, while Colorado and Washington—both of which have legalized recreational marijuana—set their limit at 5 nanograms of THC per milliliter of blood, Pennsylvania and Nevada—which allow only medical marijuana—set their limits at 2 and 1 nanograms of THC, respectively. Other states, like Oregon, which also allows recreational marijuana use, have no set limit for marijuana-related driving offenses.

But finding and setting that legal limit is proving difficult when it comes to marijuana, and some researchers are questioning the reliability of implementing such limits. In a 2016 study from the AAA Foundation for Traffic Safety, researchers examined hundreds of lab results from impaired driving arrests and determined that there was no evidence of an objective limit for marijuana impairment.

“Based on this analysis, a quantitative threshold for per se laws for THC following cannabis use cannot be scientifically supported,” the report states.

The report found that unlike alcohol, the amount of THC in an individual’s system didn’t necessarily correlate to their level of impairment. Drivers with relatively high levels of marijuana in their system might not be impaired, while others with low levels may be unsafe behind the wheel, the report stated. In addition, active THC levels in an individual’s blood can drop significantly over the course of a few hours, making it unlikely that police or investigators will get an accurate estimate of a driver’s intoxication with a blood draw, which usually doesn’t occur until two or more hours after an arrest.

CHP Sgt. Glen Glaser Jr. noted similar difficulties when it comes to impaired drivers and marijuana. Glaser is the statewide coordinator for the CHP’s Drug Recognition Expert training program. He said that many variables can impact a person’s level of impairment when it comes to marijuana, including the potency and type of marijuana, how it’s ingested, and even the tolerance level of the individual.

“It doesn’t affect any two people the same way,” Glaser said. “Someone who has been smoking marijuana for 20 years is going to react differently than someone who smoked for the first time.”

Instead of attempting to find out the THC levels in an individual’s blood, Glaser says officers rely on standard and alternate field sobriety tests. These tests ask suspected impaired drivers to do a number of physical and mental tasks to judge what kind of substances may have impaired their driving. When it comes to a specific legal limit, Glaser said the science “just wasn’t there.”

“Until the science catches up, we are going along with those standards,” Glaser said.

Should Cunningham’s bill fail to pass, California would join Alaska, Oregon, and Washington D.C. in having legalized recreational marijuana but no per se legal limit for driving. Cunningham’s bill is still working its way through the legislative process. The proposed law may go to a committee as early as March 19.

Staff Writer Chris McGuinness can be reached at [email protected], or on Twitter at @CWMcGuinness.