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Utahns who drive to Colorado for legal pot are in for a drag when they get back: The moment they re-enter state boundaries, they're probably breaking the law.

Sober or not.

Under a Utah law popularly known as the "metabolite statute," drivers may not operate a vehicle with a controlled substance in their system, and a component of tetrahydrocannabinol (THC) can be detected in blood and urine for days and even weeks after ingesting marijuana. The state lab indicates on toxicology reports that the inactive THC metabolite "has no pharmacological activity," but that doesn't rate: The prosecution isn't required to prove that a driver was influenced by the drug.

Needless to say, Utah defense attorneys don't go around wearing giant foam fingers that read "Metabolite Statute is No. 1!"

"There's really no impairment when you're driving with a small amount of metabolite in your system," says Salt Lake City attorney Clayton Simms. "It seems a little bit unfair to penalize somebody at the same level as if you'd drunk 20 beers."

Simms isn't exaggerating. "Driving with any measurable controlled substance in the body" and DUIs result in nearly identical penalties: a class B misdemeanor and automatic suspension of a driver's license.

But Utah's law isn't particularly unusual. According to a 2009 report by the National Highway Traffic Safety Administration, 17 states had metabolite statutes (also known as "per se" standards), accounting for 40 percent of drivers on U.S. roads.

The state code provides three defenses: 1. The marijuana was involuntarily ingested, 2. It was prescribed, and 3. It was "otherwise legally ingested."

You might think that last one is a Get Out of Jail Free card for Colorado tourists who kept their reefer receipts, but as Colorado Gov. John Hickenlooper cheekily cautioned after Amendment 64 passed this November: "Federal law still says marijuana is an illegal drug, so don't break out the Cheetos or Goldfish too quickly."

The Utah Attorney General's Office did not respond by deadline to requests for clarification on the question of federal law, but Salt Lake City defense attorney Kelly Fowler says it's likely to be a strong argument for Utah prosecutors. Still, the defense might persuade a jury to buy the "otherwise legally ingested" argument from a recreational Colorado user.

"The reality is, it's very common for juries, if they can be educated, to go contrary to the law," she says.

And there might be an even better defense.

A Salt Lake City Justice Court judge recently agreed with public defenders that the law (Utah Code section 41-6a-517) violates equal protection rights guaranteed by the U.S. Constitution. The case's two defendants were pulled over for traffic violations and tested positive for marijuana metabolites in 2011, and rather than argue the merits of their consumption, their attorneys said the mere existence of those defenses created two classes of people. And that's a no-no, they say.

The case has been re-filed by Salt Lake City in 3rd District Court, where prosecutors introduced new evidence and the defense reinstituted ancillary claims the justice court judge struck down: that the statute is "cruel and unusual" and that it violates due process rights.

One of the public defenders (who prefers to remain anonymous) said both sides have a vested interest in appealing the district court's ruling to a higher court, whose order would be binding on state police agencies.

The Georgia Supreme Court struck down its metabolite statute in 1999 on the same grounds. Today, there is no distinction in Georgia — should you test positive for a marijuana metabolite, it doesn't matter if it was ingested legally or not: You are guilty of (in Georgia law) driving under the influence.

Statewide numbers on metabolite statute convictions were not immediately available, since both the Utah Department of Public Safety and the Driver License Division do not distinguish in year-end statistics between DUIs and metabolite arrests.

Legislator Paul Ray, R-Clearfield, says he's heard talk of reviewing the statute on the Hill since it came to light that Utah Highway Patrol Cpl. Lisa Steed had repeatedly found symptoms of marijuana use in drivers who — tests showed — had only inactive metabolites in their system.

"If you're not impaired when you're driving, even if you may have the metabolite, you have to wonder, 'Why?'" Ray says.

Fowler says that's it exactly — that DUI laws are adequate for ensuring public safety and that drug tests are perfectly capable of detecting active ingredients in THC.

"It's criminalizing a behavior that isn't the purpose of the law," she says. But, "getting it to change in this state is an uphill battle, big time. Nobody wants to appear soft on driving impaired."

Twitter: @matthew_piper