To reiterate most of the points I've made in this thread...
1. Cannabis does slow reaction times and causes an altered state of mind/focus, that negatively affects driving ability.
2. Whether you feel like you are "impaired" or not, is irrelevant. People who regularly drink alcohol often feel like they are ok to drive when they aren't.
3. Tolerance does not necessarily equal less physical impairment. Feeling less of a buzz, or being used to it, doesn't mean that the chemicals in your body/brain aren't causing impairment. Even if you ARE experiencing less effects from a THC, that doesn't mean they are nonexistent.
4. The "lesser of two evils" argument is not a sound one, at all, and should be left out of debate, unless you are specifically debating which of two things is worse. Punching someone is much worse than slapping them, but that doesn't make slapping people acceptable. You shouldn't drive under the influence of anything that can negatively affect your driving ability, be it prescription drugs, exhaustion, alcohol, Cannabis, depression, etc.. Just because some of those things aren't punishable by law, doesn't mean everything else should be legally allowed. That's not sound reasoning. If you had to make them all legal or illegal, they should all be illegal. Nobody needs to be killed because you aren't in a safe-for-driving state of mind.
4. Regardless if you are a recreational user or a medical one, you shouldn't be allowed to drive while under the influence of Cannabis. It doesn't make any difference if it helps you medically, or if you can't drive without it due to seizures or anything else. If you can't drive without being buzzed on Cannabis, you just shouldn't be driving (my seizures are more dangerous while driving than me being buzzed behind the wheel). What if having a drink or two helped lessen your condition? Does that mean you should be able to drive with those drinks in your system? No, it doesn't. Other people should not be put at risk so you can have the ability to get to work or travel. If you are unable to drive due to a condition that Cannabis doesn't treat, you also shouldn't be allowed to just do it anyway and risk others, because you need to support your family, or leave the house. It's an unfortunate fact that some people have diseases/disabilities, but that doesn't mean others should be made to suffer as a result. The only one who has to suffer is the one that is afflicted. It's sad, but that's not me being mean. If you are considered too close to being blind to drive, you can't just do it anyway and risk the lives of others. You're the blind person. You are the one who has to suffer. Again, that's not me trying to be cruel or uncaring. I would love for those afflicted with issues to be given help. I just don't feel like one person's disabilities should put other people in harms way.
5. The same way two people with the same BAC can "feel" different levels of drunkenness, yet still be physically impaired to the same level (not referring to the mental impairment here, but rather the biological changes that occur at each BAC level), I believe that a BTHCC (blood THC content) level that is "safe" for the average person can be agreed upon, and that that number should be enforced. Sure some people might be more or less affected by the same BAC/BTHCC level, due to other biological factors, but you can't make laws based on the exceptions. You have to make them based on the large majority average, seeing as the people who fall into that category will be the ones involved in the most situations.
6. Consciously trying to drive more safely due to knowing you are buzzed (whether for the safety of others or out of fear of being pulled over) is not necessarily a good thing. That's using your attention/focus for trying to be extra careful, which is a distraction in itself. You aren't as aware of the entire situation when you are focused on one thing.
7. Similarly to #6, the increased focus provided by a Cannabis buzz, is not one that is generally going to be beneficial to driving in the open world. Having tunnel vision and laser focus MIGHT help you out if you are playing a video game, or something that doesn't involve potential unexpected occurences (a child running out in front of your car, an animal jumping into the road, another car doing something dangerous, slipping on ice, etc.), but when you have the chance that something unexpected can happen and require ridiculously fast mental processing to avoid an accident, it is a hinderance. The Cannabis buzz focus is good for some things, but driving isn't one of them.
8. Regardless of how used to the way Cannabis usually affects you, the fact that so many factors can cause an unexpected change in how it affects you each time, makes it unsafe to drive while under the influence. What if you get a split second vision flash/blur that happens sometimes? What if you get much higher than expected, or worse, much more impaired physically without feeling much more buzzed? It's usually fine in your home, but not behind the wheel. Just because it's never happened to you, doesn't mean it never will, and being behind the wheel when it does, is not a good idea.
I believe that covers all of the prevailing arguments I've seen in this thread, and the points I've made up until this point.
