HERE'S SOMETHING you profligate grass-smokers might call a "buzzkill":

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Yes, recreational pot's going to be legal in Oregon—if not this fall (and it's likely this is the year), then really soon. And yes, that ought to herald a green-tinged golden age for stoners tired of hiding a responsibly enjoyed pastime from the long arm of the law.

Except that's not how it's going to work. Not exactly.

Cops in the two states who've already gone where Oregon's headed—Washington and Colorado—haven't necessarily given up criminalizing pot use, or using it as a pretext for other kinds of police work.

They've just switched tactics. Welcome to the era of the marijuana DUI stop. Aided and abetted by blood tests that may or may not reveal whether someone's actually stoned, but still lead to an arrest, law enforcement's realized it still has one last front to keep fighting the drug war.

Consider what's happened across the Columbia River. Legalization was sold alongside a tough and controversially low benchmark for a driver to be considered "under the influence": five nanograms of THC, the active ingredient in pot, per milliliter of blood. And seizing on that benchmark, cops there have dramatically stepped up DUI-related blood tests.

Washington State Police stats showed 745 drivers tested just in the six months after pot was legalized, according to media reports, with about half of those drivers actually over the limit. In the 12 months before legalization, only 1,000 tests were ordered.

A spokesman for the state police said officers were purposely testing more drivers.

That's troubling for advocates in a couple of ways. For one, a limit as low as five nanograms may not actually mean you're still stoned. Unlike alcohol, the active chemical in pot lingers for days after it's been ingested.

"The science just isn't there to determine what [level of pot in someone's blood] actually constitutes impairment," says Morgan Fox, a spokesman for the Marijuana Policy Project. "Patients who use marijuana regularly will have a much higher tolerance, and there will always be a higher amount in their blood, whether or not they're impaired."

Fox says that imprecision means cops shouldn't shy away from regular field-sobriety tests—which have long been the standard for deciding if someone's too messed up to drive.

"They've been around since automobiles have been around," he says.

Colorado, after initially relying on blood tests alone to determine impairment, has changed its model. Slightly. Drivers who test positive at five nanograms can point to the successful completion of field tests as an acceptable defense against DUIs.

But field tests can also be misleading. A 2012 study published in Psychopharmacology, cited by the New York Times this winter, found less than a third of people under the influence had failed one popular field test: the ability to stand on one leg.

The legalization measures vying for Oregon's ballot this fall do not include a blood test benchmark. They keep intact Oregon's current provisions for driving under the influence of drugs—provisions that rely on field tests before blood is drawn.

"They've arrested plenty of people without a blood test," says Liz Kaufman, the venerable political consultant helping New Approach Oregon with its pot measure, saying the tests aren't "scientific." "There are some people who regret that in Washington."