A survey done for Health Canada suggests that 39 per cent of people who admitted to smoking pot say they've driven within two hours of smoking up.

The survey was conducted last spring when the government was preparing to table legislation not only to regulate the sale of legal marijuana but also toughen up Canada's impaired driving laws.

Respondents were asked more than 70 questions about whether they used marijuana, when and why. There were also a series of questions around pot use and driving.

For instance, the survey shows that among pot users age 20 to 24 years, 43 per cent admit to having gotten behind the wheel within two hours of smoking the substance. For 16-to 19-year-old users, 27.8 per cent of those say they've driven under the influence of cannabis.

About half of the users in those two age categories said they had done so within the past 30 days.

The results from March 13, 2017 to May 24, 2017 are based on online responses from 9,215 respondents aged 16 years and older across all provinces and territories. The survey has a margin of error of 1.6 percentage points, 19 times out of 20.

The results showed that 28.7 per cent of all respondents had used cannabis in the past 12 months for either non-medical or medical purposes

They also showed that 21.7 per cent of all respondents had used cannabis in the past 12 months for non-medical purposes only, a number that peaked among 20 to 24 year olds, at 44.7 per cent.

The survey also suggested it was more common for respondents to get in a vehicle with someone else who has used cannabis in the two hours before driving.

Overall, 39 per cent of all respondents to the survey said they had done this. But among pot users, that number jumps to up to 78.5 per cent — and 82.1 per cent among 20 to 24 year olds.

Even among non-pot users, more than a quarter have been in a vehicle driven by someone who has used the drug in the past two hours — 27.2 per cent.

Half of users don't think cannabis affects driving

The survey does hint at why those numbers are so high.

When asked if cannabis impairs the ability to drive, about 75 per cent of all respondents said yes.

But among pot users specifically, that number dropped to 50.2 per cent.

And among 16- to 19 year olds, only 41 per cent said they believed cannabis affects their ability to drive.

When al respondents were asked how long a person should wait before driving, the most popular response was "it depends" (35.7 per cent) followed by "don't know" (23.5 per cent).

Guidelines released earlier this year by a panel of health experts recommend cannabis users wait at least six hours before driving, maybe longer, depending on the specific product used.

Securetec's DrugWipe® 6 S saliva-testing device can detect a number of drugs including cannabis, amphetamines, methamphetamines, cocaine, crack and opiates including morphine and heroin. The Senate is considering proposed legislation to help police deal with drug-impaired drivers. (CBC)

For alcohol, government guidelines generally note that even one drink can impair the ability to drive. It is illegal in Canada to drive with a blood alcohol level of more than 0.08 (80 milligrams of alcohol per 100 millilitres of blood) — but most law enforcement officers will issue a warning to drivers with blood-alcohol readings as low as 0.05.

While blood alcohol levels are affected by factors such as the speed of consumption and body weight — and can vary between men and women — Mothers Against Drunk Driving (MADD) Canada says that, for most people, one or two drinks with dinner or after work would not put them over the level of 0.05.

Those guidelines, however, are for alcohol alone, and experts have warned that combining alcohol and cannabis leads to greater impairment.

The survey found 15 per cent of cannabis users had driven after consuming both substances.

Impaired driving bill before the Senate

The data, which was posted online by Health Canada earlier this week, comes as the Senate is considering the federal government's proposed legislation to help police forces deal with drug-impaired drivers.

When the senate rose for the Christmas break, C-46 had just passed second reading and been sent to the Legal and Constitutional Affairs Committee.

That committee will begin hearing witnesses in the new year.

In December, MADD Canada sent a letter to all senators urging them to pass C-46 quickly.

"Drugged driving is a problem right now; too many Canadians are being killed or injured in crashes where drugs are present," the letter says.

It refers to 2013 statistics that show a positive drug reading was found in about one third of all car crashes that resulted in a death, yet very few of all impaired driving charges were drug related.

That finding is supported by the new Health Canada survey. Of those who participated, only 2.4 per cent of people had an interaction with law enforcement while driving under the influence of marijuana.