The dangers of drunk driving are manifold, but what about drugged driving? As cannabis products grow ever more popular and accessible, it's about time we knew the risks.

The main psychoactive ingredient in cannabis, delta-9-tetrahydrocannabinol (THC), is known to impact many of the same motor and cognitive skills needed to operate a vehicle. But medical forms of cannabis are typically free of this chemical, relying on cannabidiol (CBD) for therapeutic effects.

Without the inebriating effects of THC, there might be no real risk at all. A small new study has now found that vaping CBD on its own isn't all that different to vaping a placebo when it comes to being able to drive safely.

"These findings indicate for the first time that CBD, when given without THC, does not affect a subject's ability to drive," says Thomas Arkell who researches cannabinoid therapeutics at the University of Sydney.

"That's great news for those using or considering treatment using CBD-based products."

The experiment involved 26 volunteers who were provided with one of four different combinations of cannabis types, giving them either a dose dominated by THC, or mostly containing CBD, or a combined dose, or a placebo low in both.

They then waited 40 minutes before driving for around an hour, covering 100 kilometres (about 60 miles) down a stretch of highway. Four hours later they took a second drive. At various points they were given a few other tests to study their cognition.

In cases where THC was inhaled, whether by itself or with CBD, drivers experienced mild to strong intoxication, which appeared to fade after four hours.

"While some previous studies have looked at the effects of cannabis on driving, most have focused on smoked cannabis containing only THC (not CBD) and have not precisely quantified the duration of impairment," says Iain McGregor, who also studies cannabinoid therapeutics at USYD.

"This is the first study to illustrate the lack of CBD effects on driving and to also provide a clear indication of the duration of THC impairment."

Recently, some public health experts have worried that CBD products might make drivers feel more lethargic, which could impair their driving abilities.

If you're certain that what you're vaping is CBD-only, this study confirms it's unlikely to impede your driving. Unfortunately, there is always a small risk that what you think is pure CBD contains some traces of THC, creating both a legal risk and a safety concern.

The dangers of driving under the influence of THC are far less consistent and pronounced than drunk driving, but that doesn't mean there aren't real risks, and many places around the world have a zero-tolerance approach to this practice.

That's because cannabis intoxication can mildly impair psychomotor skills, although it's still not clear at what dosage that occurs, or how long the effect lasts for.

While some research has shown heavy recreational cannabis users are worse at driving even when they're sober, it's unclear if that has to do more with common traits held by frequent cannabis users or an effect of the drug itself.

Most research to date suggests impairment brought on by cannabis is short-lived, and that's just for smoking. Virtually no study exists for eating cannabis, though with research showing a similar, if delayed, influence, it's safer to assume that the risks of edibles are likely to be similar, too.

More research needs to be done so that governments can implement evidence-based laws and regulations that best protect the public. Formulating these policies is going to be difficult however, especially without an accurate roadside test for THC.

Even if scientists do come up with an accurate way to measure THC in the blood, that might not tell us much in practical terms. Cannabis affects everyone's psychomotor skills slightly differently, and THC blood levels don't always correlate with acute usage or intoxication, making it hard for the law to be fairly enforced.

Even THC products that are 'undercut' by CBD do not make drugged drivers any more safe, although it might make them feel more confident. In the study, a mixed dose of THC and CBD produced the same amount of driving impairment, even as participants felt less stressed and more assured in their ability to drive.

But the study is limited in that it did not examine chronic cannabis users, who might have different responses to the drug, and the CBD dosage used was lower than what is often clinically prescribed.

"While symmetry analysis suggested no difference in the proportion of impaired vs improved drivers in the CBD condition, these findings are exploratory and based on a small number of drivers and a single CBD dose," the authors admit.

Until we know more, people should not be driving under the influence of THC for many hours after getting high, and even then, it's questionable. Given that CBD products can contain higher doses than marketed and sneaky amounts of THC, those relying on non-psychoactive forms of the drug should also beware.

Ultimately, regulation and research needs to catch up with the popularity of these products so we can better inform the public and keep people safe on the roads.

For now, if you're not sure, it's always safer to wait.

The study was published in the Journal of the American Medical Association.