In 2013, after two failed attempts, Colorado approved a stoned-driving standard of 5 nanograms of THC per milliliter of blood.

Critics of the proposal pointed out that there was an almost total lack of scientific evidence to suggest that this limit would be accurate for the vast majority of people, as is the alcohol intoxication level of .08 BAC, owing to the different ways the separate substances are metabolized — and how THC tends to linger in the system for longer periods of time.

Note that medical marijuana critic William Breathes tested at nearly three times over the standard when sober.

Now, a study from the University of Iowa suggests that the 5 nanogram limit is far too low.

The research, synopsized in a document on view below, argues that 13.1 nanograms per milliliter is the actual equivalent to the .08 BAC alcohol intoxication level.

The study, conducted by UI's National Advanced Driving Simulator, didn't only consider the effect of marijuana on drivers. Researchers also looked at the combination of cannabis and alcohol — and they found that drivers who use them together "weave more on a virtual roadway than drivers who use either substance independently," according to a university release.

"However," the release continues, "the cocktail of alcohol and marijuana does not double the effect of the impairment" — and "participants who consumed only alcohol weaved more during a 35- to 45-minute simulated driving test than those who consumed only vaporized cannabis."

Eighteen people — thirteen men and five women — between ages 21 and 37 took part in the experiment. They were given ten minutes to consume a mixed drink (some were the genuine items, others were placebos), after which they "were given ten minutes to inhale a placebo or vaporized cannabis using a vaporizing system designed in Germany called 'Volcano Medic.'"

Then, they climbed into the simulator — a 1996 Malibu sedan mounted inside a 24-foot diameter dome. Here's a look at the outside of the dome....

...and the inside.

The drivers were then assessed by several measures: weaving within a lane, how often the car left the lane, and the speed of the weaving.

The study revealed that only drivers with alcohol but no cannabis in their system displayed impairment in all three areas.

In contrast, those who had vaporized cannabis but consumed no alcohol had problems in just one area — weaving within the lane.

Pro-cannabis reports like this one have interpreted the results as showing that the study showed "virtually no driving impairment under the influence of marijuana," which is definitely a stretch.

But the data showed that 8.2 nanograms of THC per milliliter was the equivalent of .05 BAC — a measure under the legal intoxication level. It took 13.1 nanograms of THC per milliliter to approximate .08 BAC.

Clearly, more studies like this one need to be conducted in order to determine if a definitive THC intoxication can be set. But the results definitely argue that Colorado's 5 nanogram limit is far too low.

Here's the complete study.