Oh the madness....
I'm not against making it legal for recreation, but they need to set a legal limit, and here is the kicker...be able to test for it. The last part is where I always find opposition. So far, I'm not aware that there is a way to test for the level of impairment, just that it is still in your system.
I took all of 30 seconds to see what Canada put in place...I think it will satisfy your uncertainty about testing...first the officer has to have a "reasonable suspicion" that a person is under the influence of drugs, not just marijuana, while driving based on "objective facts" which they state are "red eyes, muscle tremors, agitation, abnormal speech patterns", then if the officer can "demand an oral fluid sample". Some of these "objective facts" must be for crack or meth and not cannabis.
"If a driver tests positive on an oral fluid drug screener the positive result confirms the presence of the drug and, combined with other signs of impairment or drug use observed by the police at the roadside, may provide grounds for the investigation to proceed further by making a demand for a blood sample."
There is the issue with cannabis, for those who consume it often, and its how traces of it can remain in your system for a long time even after you have consumed it. Levels have been set by the government that seem to take this into account otherwise the first case before a judge is probably going to end up being challenged.
Illegal distribution of cannabis and other drugs carry stiff fines and jail sentences especially if the drugs are sold to minors and the consequences for selling to minors is far far greater than if one sold alcohol to a minor.
In Canada, impaired driving kills more people than any other crime. Some of those who drink and consume drugs always feel they are able to perform their driving skills as if they were sober. Some of us who drive our vehicles or ride our bikes sober know we make mistakes and they're called accidents.
In Canada, alcohol prohibition was very short and in some provinces like my own it only lasted 4 short years. There was never strong support for alcohol prohibition in Canada. In the US prohibition of alcohol lasted 13 years and Canadian distilleries made large sums of money running booze into the US. Most of the "old money rich folks in Canada got their initial wealth from "exporting" liquor to the US. Cannabis prohibition and criminalization has been with us in Canada since 1923 and finally in 2018 the prohibition and criminalization was lifted.
Getting legal sales up and running has been pathetic in most provinces (each province is allowed to set up its own rules for distribution and sales and each municipality can either allow or not allow sales in their towns). A shit show providing yet another clear example that government's are incapable of running a business. So the illegal market still reigns when it could have been so different. For two years provincial and local governments whined about not having enough time to get it together instead of getting it together. Both Washington State and Colorado were already up and running and clear examples of how to get it done and receive benefits. Most of these clowns couldn't govern their way out of wet paper bag. Cheers.