A woman who admitted to using marijuana before getting into her car has been acquitted of impaired driving, with the Saskatoon judge saying he was not convinced her ability to operate a vehicle was affected.

The case, recently published to an online legal database, concerned the arrest of a woman on June 19, 2011, who was pulled over during a routine traffic enforcement program.

According to the judge, the officer directed her to pull over even though she was driving appropriately and in her proper lane of travel.

Unlike drunk driving cases, where .08 per cent is the legal limit for a person's blood-alcohol level, there's no equivalent for marijuana-impaired driving. Instead, officers must rely on a lengthy list of tests to establish impairment.

In the Saskatoon case, when the officer went to the woman's car, he detected what was described as an "overwhelming odour of marijuana."

He decided to investigate further and learned from the woman that she had smoked some marijuana earlier in the evening.

With that, the officer decided to have the woman perform a number of roadside tests, including walking heel-to-toe and touching the tip of her nose with her finger.

Missed her nose

"She was only successful in touching her nose on one of six attempts," the judge noted in his decision. "On the other five attempts, she touched her face right under her nose."

The officer noted that the woman had reddened eyes, something consistent with marijuana use.

Tests done on suspect in Saskatoon marijuana case: Horizontal gaze nystagmus test.

Lack of convergence Test.

Pulse check.

Walk and turn test.

One leg stand test.

Romberg balance test.

Blood pressure test.

Temperature check.

Pupilometer eye Test.

Finger to nose test.

Urine sample. Source: Court documents

A urine sample was also taken and confirmed that the woman had used marijuana.

However, the judge was not convinced there was any evidence that the woman's driving ability was impaired.

In his decision, the judge said he was left with several unanswered questions, including:

What signs of impairment would one expect to see in someone who has been using marijuana? How long after using marijuana would you expect to see these signs and how long would they last? Can the results of drug evaluation tests taken over 1½ hours after the time of driving be reliably related back to the time the woman was stopped? Was the woman's performance in some of the tests an indication of poor balance or poor co-ordination?

On the other hand, the judge found there was plenty of evidence to suggest the woman was not impaired, noting:

The officer did not observe any problems with her driving as she came to the check stop, when she was directed into the check stop or when she was directed to drive out of the line of cars to a nearby parking lot. She had no problems understanding the officer or answering his questions and did not slur her speech. She was able to provide him with her licence without any problems and had no difficulty following the officer's instructions or getting out of her vehicle. When he asked her to take her hand off her vehicle and step away from it, she did so without problem. She did not have to hold on to anyone or anything for balance and after he handcuffed her, she had no problems walking to his police car and getting into the back seat. She was polite and co-operative with the officer.

The judge added he was not sure how the finger-to-nose test and the walking heel-to-toe test related to one's ability to drive a car.

"I would have appreciated some evidence as to how these observations related to the accused’s ability to drive a motor vehicle," the judge said.

Ultimately, the judge said he was left with reasonable doubt on the charge and the woman was acquitted.

The prosecutor on the case told CBC News that Crown lawyers were still deciding whether to appeal.