The set of changes to Canada's impaired driving laws, which was introduced alongside the sexier marijuana legalization bill, sounds fairly benign.

It includes harsh penalties for drug-impaired drivers and supplemental rules about breathalyzer tests to detect drunk drivers. In essence, the new rules crack down on driving under the influence of alcohol – and driving under the influence of anything else you shouldn't be taking before getting behind the wheel.  

Keeping roads safe is a legitimate government function, and it's responsible to make sure that removing criminal penalties for using pot doesn't leave people free to engage in reckless "high" driving with no consequences. But the details of the changes are problematic.

Saliva tests

Police can already take roadside saliva samples to test for marijuana intoxication, but the legislation now links such tests with punishment as serious as a decade in jail. (If the saliva test is positive, a blood test may follow, and if certain levels of THC are detected therein, significant penalties can now be imposed. In the past, the saliva samples very rarely led to convictions.)

The problem? We're actually not very good at accurately figuring out if people are currently high on marijuana, or how high they might be.

The saliva tests can deliver a positive result based on marijuana that entered a person's system up to a month before the test, which is rather unhelpful when trying to determine if the guy in the sports car who just blew a red light did so because he was stoned, or simply a terrible driver.

Two different people who smoked the same amount of marijuana can end up with significantly different levels of THC in the blood depending on how their particular bodies metabolize pot, and now, they can also end up with significantly different criminal sentences.

In time, these methods will be, if not perfected, at least fine-tuned enough to be extremely reliable. But it would have been nice if the government had waited for that accuracy before piling on the punishments.

It's especially hard to tell how dependable we can consider the drug testing of drivers when the legislation doesn't even specify what specific method should be used. Ideally, that would be spelled out in advance of locking people in prison for 10 years.

Breathalyzer tests

The other troubling change is the circumstances under which police can demand a roadside breathalyzer test.

While police currently have to have a "reasonable suspicion" that a driver is intoxicated by alcohol in order to administer a test, the new legislation allows officers to order a breathalyzer for anyone they've lawfully pulled over, even if it's for something as innocent and unrelated as a burned-out tail light or an expired registration sticker.

The government says ditching the "reasonable suspicion" condition is necessary because it has allowed too many drunk drivers to get away without being caught. That is a difficult claim to understand; if the suspicion of these drivers wasn't strong enough to be considered legally reasonable to administer a breathalyzer, how can it be, at the same time, strong enough for the government to somehow know these people  "got away" with drunk driving?

Potential legal issues with proposed impaired driving laws6:37

But perhaps more importantly, we have Charter rights that protect us from unreasonable search and seizure. How can a mandatory breathalyzer for no good reason – or even no reason at all – not violate those rights?

And it's not enough for the government to simply say (as it has) that it wants to reduce legal action over whether officers had "reasonable suspicion" of alcohol impairment. That's like saying: "You know that thing about giving people a trial before convicting them of a crime? That's a real pain in the behind, and it leads to annoying legal wrangling. We'd really reduce the burden on the courts if we just did away with it and put people directly in jail."

Government doesn't get to dispose with legal protections of citizens just because the protections are inconvenient. The inconvenience is an indication the protections are actually working.

RCMP impaired driving roadblock in Coquitlam, June 27, 2014

We can't let "getting tough on impaired driving" be a sufficient criterion for this legislation's success. (CBC)

The crux of the debate is this: Are the impaired driving law changes as carefully thought-out and crafted as they should be? If they almost immediately result in a major constitutional challenge – as seems inevitable given the sanctioning of arbitrary breathalyzer demands – the answer is probably "no."

They fail to strike the appropriate balance between protecting crucial civil liberties and protecting people from harm inflicted by others — and the Trudeau government doesn't even appear to have made much of an effort to balance the scales, which is a disappointing indication that it will be no better than the Harper government about giving proposed legislation careful constitutional scrutiny.

We can't let "getting tough on impaired driving" be a sufficient criterion for this legislation's success. Due process still matters.

It would be an ironic shame if the small win for individual freedom afforded by cannabis legalization were cancelled out by sloppily crafted, broad new criminal penalties for intoxicated driving.

This column is part of CBC's Opinion section. For more information about this section, please read this editor's blog and our FAQ.