If actress Amanda Bynes was accused over the weekend of drunk driving instead of drugged driving, she'd likely be toast. Eight out of 10 drivers facing DUI allegations are convicted, according to the California Department of Motor Vehicles.

But suspicion of driving under the influence of drugs alone is a much harder case to prove—so much so that some prosecutors elect to drop the charges altogether. “Since there are no firmly established per se levels of impairment for drugs, these cases are more likely to settle, not be charged, or be dismissed,” a DMV study found.

See also: Tips for Drivers Stopped by the LAPD for Weed

The problem is that, while California has a clear line in the sand for blood-alcohol content, .08 percent, there is no such benchmark for drugs, prescription or otherwise. And that's a whole can of legal worms:

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DUI and traffic law attorney Mark A. Gallagher says juries looking at documented evidence that a driver had blood-alcohol content of .08 or higher are very likely to convict, whatever the circumstances of the arrest might be. Even sitting in a parked vehicle with blood-alcohol above the limit can be enough to secure a conviction.

But without a benchmark for drugged driving, juries have to rely on cops' subjective observations from the scene of the alleged crime: Was a driver breaking traffic rules? Was she off-balance or slurring her words. Did she pass a field-sobriety test?

“It's definitely harder to prove a DUI case with drugs than it is with alcohol,” Gallagher says. “They'll have to prove it was affecting her ability to drive.”

Bynes was arrested Sunday at 3:45 a.m. at Riverside Drive and Van Nuys Boulevard in Sherman Oaks, according to the California Highway Patrol.

Passing a red light, Bynes Mercedes GL 450 SUV “stopped in the middle of the intersection,” the CHP alleged in a statement. She “appeared to be under the influence,” the agency stated:

Officers administered a series of field sobriety tests, which she was unable to perform satisfactorilary [sic]. 

Bynes was taken into custody and looked over by a drug recognition expert who determined she was under the influence, the CHP said. 

CHP Officer Leland Tang told us the actress consented to a blood test, the results of which won't be known for at least 30 days. At that point the results will be sent directly to the Los Angeles City Attorney's office for its consideration regarding whether or not to file charges, he said.

For now, Tang said, authorities believe she was allegedly under the influence of some sort of “stimulant.”

“We know she was impaired,” he said. “We know she was under the influence.”

We asked if a drugged driving case like this would be harder to take to court. The officer argued that the particular circumstances of Bynes' arrest could make this an easy conviction:

She ran a red and stopped in the middle of the intersection. She did not do well on the field sobriety test. Based on those two factors alone it shows she was impaired. Any measurable amount of drugs [when the blood test results come back] works to our benefit.

But Gallagher notes that drugs don't work like alcohol, which has a well documented window of rising and falling inebriation that can be charted against blood-alcohol and breathalyzer tests.

“It's hard for the jury to say you're over the line when there's no line,” he said. “The field sobriety test might be seen as nitpicking. That might not be enough.”

Drugs like marijuana can leave markers in your blood long after a high has worn off, which is why pro-pot activists have fought successfully against blood-alcohol-like limits for cannabis that they say could discriminate against medical weed users.

Credit: Up in Smoke

Credit: Up in Smoke

A proposal earlier this year that would have established a 2 nanograms per milliliter THC blood limit for the state's drivers was nixed in the California legislature.

See also: LAPD Targeting Stoned Drivers With Roadside Weed Tests

Forensic toxicologist Henry S. Greenberg, who testifies as a hired expert in DUI cases, says that a test that indicates a driver had THC metabolites in her system doesn't necessary point to a stoned driver.

“The only thing that can be said is that the individual smoked at some earlier time,” he told us.

Even if Bynes were to come up positive for a stimulant, it could be complicated, Greenberg said: “It's an issue of what's there and what is the quantity. What you're talking about now is a theoretical effect.”

The DMV, in its study of DUI convictions, noted that there was a “general lack of scientific impairment evidence for both illicit and prescription drugs” and that “jurors tend to be more sympathetic toward prescription drug users who take the drugs as prescribed by their doctors.”

The department also said that “most of the prosecutors” it surveyed for the study “felt it was difficult to obtain DUI convictions” in drug-only cases:

The greater complexity of the effects of drugs, and the difficulty in determining impairment levels because of wide variation of effects at different doses, make per se laws for drugs more difficult to enact and enforce than those for alcohol. … Prosecuting attorneys stated that it is difficult to obtain DUI convictions for offenses involving only drugs because there are an insufficient number of qualified drug recognition experts in law enforcement. 

However, prosecutors have many tools in their arsenal. Gallagher said that Bynes could possibly be charged with suspicion of being under the influence of an illicit drug if toxicology results lead prosecutors in that direction.

Just walking around high is technically illegal, and such an allegation could be “easier to prove at trial,” the attorney said.

In any case, Bynes should probably be prepared to write a nice-sized check to her legal representative if she plans to fight authorities on this one.

She already has a conviction for reckless driving based on a 2012 DUI arrest in West Hollywood in which she allegedly sideswiped a sheriff's patrol car while behind the wheel. And yeah, that will probably come up if this latest case makes it to a court of law.

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