Imagine, as Maine continues to grapple with how, when and where to allow the legal consumption of marijuana, that an expert on impaired driving sits down with state lawmakers and makes an ironclad prediction.

“Over the course of a year,” he says, “between two and three dozen people in Maine will get high, get behind the wheel and die in a crash.”

“Nationally,” our expert continues with absolute certainty, “if cannabis is legalized coast to coast, marijuana-impaired driving will account for more than 10,000 deaths annually, or 29 per day, or one every 49 minutes.”

Now for a reality check: The numbers are real. But they pertain to alcohol, not pot.

They’re tabulated each year as a fact of life – or, more accurately, of tragic and wholly preventable death – and most people don’t even bat an eye.

Last week, in a report optimistically titled “Getting to Zero Alcohol-Impaired Fatalities: A Comprehensive Approach to a Persistent Problem,” the prestigious National Academies of Science, Engineering and Medicine recommended that states drop their drunken driving thresholds from 0.08 percent blood-alcohol content to 0.05 percent.

“Based on the available evidence … an individual’s ability to operate a motor vehicle (including a motorcycle) begins to deteriorate at BAC levels well below 0.05%, increasing a driver’s risk of being in a crash,” the report says.

In other words, even as we’ve whittled the blood-alcohol limits down to 0.08 percent over the past three or four decades, we still have it wrong.

Which brings us back to marijuana. And, more specifically, driving while stoned.

For all the good work that the Legislature’s Committee on Marijuana Legalization Implementation has done since Maine voters legalized cannabis in November 2016, impaired driving still looms as one of the most vexing challenges facing policymakers and police alike.

It’s high on Gov. Paul LePage’s list of concerns with legalized pot, and rightfully so.

It also figured heavily in the legislative committee’s prudent decision last week to postpone the creation of marijuana “social clubs” until 2023.

I don’t know about you, but the more I hear the phrase “marijuana social clubs,” the more I think about bars.

And the more I think about bars, especially in light of last week’s drunken-driving report, the more I see what happens when public policy fails to place adequate boundaries around dangerous human activity.

People die.

“We are certainly against any kind of impaired driving under any substance,” David Boyer, Maine political director for the Marijuana Policy Project, said in an interview Friday.

Hear, hear. Yet, when it comes to marijuana as opposed to alcohol, it’s currently near impossible to show scientifically when a driver is impaired and to what degree.

“Obviously, the rub is, when is someone (who uses marijuana) intoxicated?” Boyer agreed. “The only thing that anyone agrees on is that there is no foolproof way to test it.”

Some point to an emerging standard of 5 nanograms of THC, the active ingredient in cannabis, per milliliter of blood. Others counter that such a metric ignores the fact that THC remains in the body far longer than alcohol and can thus give a positive reading on a driver who is in no way impaired.

As the National Academies of Science, Engineering and Medicine noted in a separate report last year on the use of cannabis and cannabinoids, “Unlike other substances whose use may confer risk, such as alcohol or tobacco, no accepted standards for the safe use or appropriate doses are available to help guide individuals as they make choices regarding … when, where, and how to use cannabis safely.”

Here in Maine, for better or worse, that leaves it to law enforcement to make the call – not based on any scientific measure, but rather on firsthand observation.

According to Maine State Police spokesman Steve McCausland, the tricky task falls to a network of state, county and local police officers certified as “drug recognition experts.”

“They’ve had additional training for the telltale signs of drug impairment,” McCausland said. “If you (a patrol officer) know something is wrong and you can’t detect any alcohol, many times these specialists are called in.”

The drawback: They’re not always nearby. And the longer it takes for one to show up, the more time the suspected driver has to, shall we say, get his or her act together.

So, setting aside the ongoing conflict with federal drug laws, what’s a state with legalized marijuana to do when it comes to establishing the rules of the road?

Be very careful, for starters. And look to alcohol as a cautionary tale of how, over time, society numbs itself to carnage that is as foreseeable as it is avoidable.

“You’re correct,” said Sen. Bill Diamond, D-Windham, in an email Friday. “If we had the hard numbers showing the increased fatalities that will result from OUI/marijuana caused accidents – we would not be so eager to implement this new law without more stringent regulations.”

Diamond, who presided over Maine’s driving public as secretary of state for eight years and now sits on the Legislature’s Criminal Justice and Public Safety Committee, admits to feeling “hypocritical in some ways” as he and other legislators labor to implement legal marijuana – knowing fully well that a few misguided Mainers, at least, will take this newfound freedom and drive dangerously with it.

“It doesn’t take a genius to figure out that our roads will be less safe with the legal introduction of marijuana use,” he predicted. “Obviously, pot use is prevalent today, but legalization will no doubt expand its use especially by those who didn’t want to violate the law.”

But Diamond takes issue with the notion that highway safety hinges solely on limiting – for alcohol or marijuana – the measurable amount of intoxicant we can legally ingest before reaching for the keys.

Creating or reducing such thresholds, he said, “will only make us feel like we’ve done something – it’s a simplistic answer to a much more complicated problem.”

Far more important, Diamond suggested, is constantly educating drivers, particularly the younger ones, that operating under the influence is neither cool nor harmless. In reality, it’s nothing short of insanity.

Just look at the numbers.

Bill Nemitz can be contacted at:

bnemitz@pressherald.com