I'll talk first of all about this issue of tolerance between the heavy users and the occasional users, which is the fundamental part of your first question. We know that some studies show that people do develop tolerance to all drugs: alcohol, opioids, THC, and so forth.
The level of tolerance that can be developed with THC is on about the same order of magnitude as what can be developed with alcohol, according to Dr. Harold Kalant, a professor at the University of Toronto, so there is some tolerance for THC. What we find is that those people who are chronic daily users of cannabis develop a level of THC in their body that is there durably, and they are impaired for an extended period of time even when they stop taking cannabis. Studies have shown that these people can remain impaired over a three-week period of total abstinence, even when they show zero THC in their blood.
The issue is impairment. If you have somebody who is an addict, basically, which is what these people are, they develop a tolerance, they will be impaired and, yes, they should be banned from driving.
On your question on the issue of tandem per se, I've put forth a concept that needs to be fleshed out and based upon Canadian laws, norms, and values. It's just a bare-bones concept at this point. It is very similar to the zero tolerance laws that are already in place in many states in the United States.
The difference is that zero tolerance laws typically require reasonable grounds to collect a blood sample, and if a person has any level of these impairing substances, that person is then guilty of a violation. What I'm proposing is not reasonable grounds but rather probable cause, which is a little higher level, and also requiring that the probable cause be based upon behaviour and impairment assessments, not simply on finding some weed in somebody's glove compartment. That would not suffice as probable cause.
What I'm suggesting is a concept. It is very similar to an extension of the zero tolerance laws that are already in place and have been working for many years in many states in the U.S.