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Officials are cautioning Californians that the state won’t be devoid of marijuana regulations when the drug becomes legal for recreational use on Jan. 1.

Messages reminding drivers, “Drive high, get a DUI,” began popping up on Amber Alert display signs along freeways statewide on Wednesday as part of a campaign launched days before California officially becomes the sixth state to fully legalize weed.

Driving under the influence of marijuana has always been illegal in the state, but law enforcement will face new challenges in regulating such offenses once pot use inevitably becomes more commonplace.

There is no widely accepted method for detecting marijuana impairment, and officials have not disclosed standard procedure field sobriety tests. Unlike with alcohol, there is no legal threshold for the amount of marijuana that can be in a driver’s system.

Authorities have previously announced plans to test cannabis breathalyzers and mouth swab tests, according to the Los Angeles Times, but statewide standards or practices have not been implemented.

In Colorado, officials can use a blood test to detect whether a person has more than 5 nanograms of active tetrahydrocannabinol, or THC, per milliliter of blood. Officials there say 17 percent of DUI arrests last year involved marijuana intoxication.

In a 2016 survey, more than half of marijuana users in the state believed it was safe to drive while under the influence of marijuana, according to the Colorado Department of Transportation.

Much of the determination may be left up to whether there are cannabis products in the vehicle. It is currently a ticketable offense to consume pot while driving or to have an open container of marijuana or weed product in the car — which could constitute anything from a blunt to a vape pen.

While alcohol is still the intoxicant that leads to the most fatalities, authorities have seen an increase in drivers involved in fatal crashes with other substances in their system, according to a California Office of Traffic Safety news release.

“It has taken more than 35 years to convince the vast majority of the public that driving under the influence of alcohol is dangerous, illegal and socially unacceptable,” the agency’s director, Rhonda Craft, said in a statement. “With more dying on our roadways every day, we can’t afford to take that long when it comes to driving under the influence of prescription medications, marijuana, illicit drugs and even some over-the-counter medications.”

On Christmas Eve, a California Highway Patrol officer was killed in a crash in Hayward involving a motorist who officials say was under the influence of both alcohol and marijuana.

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