This spring, 16 state patrol officers from Colorado and Wyoming took a couple days off their usual work schedule to do something special. They assembled in a hotel conference room in Denver. As instructed, they wore street clothes for their first assignment: going shopping at nearby marijuana dispensaries.

“It’s a brave new world,” said instructor Chris Halsor, referring to the years since Colorado legalized recreational marijuana.

There are now more marijuana dispensaries in Colorado than there are Starbucks shops, said Halsor, a Denver lawyer and former prosecutor. And though consuming cannabis is legal across the state, driving under its influence is not.

The cops in that conference room, with their buzz cuts and Mountain Dew, are all part of the force charged with keeping the roads safe. But first, they needed a formal pot education — to learn how to identify various marijuana products and paraphernalia when they pull over a driver they suspect is under the influence.

Here’s the rub: Despite the increasingly legal use of cannabis in many states, cops still don’t have the equivalent of a reliable alcohol breathalyzer or blood test — a chemically based way of estimating what the drug is doing in the brain. Though a blood test exists that can detect some of marijuana’s components, there is no widely accepted, standardized amount in the breath or blood that gives police or courts or anyone else a good sense of who is impaired. [Read more at NPR]