Considering there is no way to readily quantify the amount of THC in someone's system (like the breathalyzer for Alcohol), and no real standards for determining intoxication level if there was, this raises the issue of driving under the influence and law enforcement.
While it may be obvious to the nonconsumer that the person is impaired, it is difficult to anticipate that the driver could know just how much impairment is legal and how much is not, in terms of their consumption. Unlike alcohol, where quantities of the intoxicant are regulated and predictable, both in produced BAC and effect on members of the general population, the same does not exist for marijuana, and that does not even begin to take into account the host of adulterants which have been known to exist in nonpharmaceutical grade pot or products thereof.
While the 'drunk' in 'drunk and disorderly' can be quantified and readily measured, just as in DUI, by a screening test, breath test, and blood test, enforcement of any laws regarding marijuana intoxication becomes more complex, and often subjective, rather than objective and quantified), just as anticipation of being in violation of any standard becomes more difficult than say, having three beers at a known body weight.
From:
https://nearu.pro/lawyers/how-many-car-deaths-in-the-us-each-year#screen4
Despite collapsible steering columns, padded dashboards, seat belts, air bags, 'crumple zones', driver's education, and the lot, we still manage to kill over 35,000 people a year since the late '40s. It isn't as if stoned people don't drive now, but until we have a way to know what level of consumption is okay, that leaves zero tolerance.
As if we don't have enough trouble, socially, personally, societally, and on the highways with alcohol, introducing another legal intoxicant to the mix is unlikely to improve anything.
That's my rational argument.
For the rest:
I have worked around people on different jobs, the kind people in the drilling end of the oil patch take to get by when oil prices crash, who get stoned on a regular basis. With rare exception, they do not perform well, either stoned, or with the lingering aftereffects of last weekend. They are more likely to make mistakes which cost time, money, and even cause injury to those around then or themselves. It's the reason (not just wages) you see a lot of relatively recently arrived Mexicans on construction crews, whatever their legal status. They are work-focused and get the job done and done well. This may be unpopular, but as a rule whites of the same age often have neither the work ethic nor focus to get the job done as well in an even close time frame. Some of that may come from having lifted little heavier than a six pack and a game controller, but I give weed credit, too.
