I don't know where you're from, but cannabis is certainly very available in many parts of the U.S. And much easier for minors to obtain in areas where it's illegal!
I mean I am hardly ever further than a couple minutes away from a place where I can buy alcohol.
As for minors, I'd argue the problem lies more in the late access to alcohol in the US.
Then as of the effects of cannabis itself, it increases reaction time. So you could definitely pull the brakes one second too late in front of that kid and his balloon. But yeah, it might not make you as prone to think killing 3 people is an acceptable activity.