As a large bonus: it's healthier for the user and those around them too.
Not to mention the stench of weed smokers...
For this specific reason, I expect higher variation from weed — though I also wouldn't trust either to drive a car.
Try 6 beers vs 1 entire joint and maybe you're getting somewhere close to equivalence in level of intoxication.
Most people don't smoke an entire joint in one sitting. All it takes is about 3 or 4 puffs and that's typically enough for quite a while. That's the equivalent of about 2 beers. And I'd trust someone more behind the wheel that had a few puffs of a joint vs. someone who just drank 2 or 3 beers.
The kind of intoxication is very different between alcohol and THC. There is no way to do a 1:1 comparison as far as effects on driving ability. They each act on very different pathways, and produce different results.
Marijuana also never made anyone pass out while driving from smoking too much, but alcohol definitely can do that.
By other means it's true that it's less harmful than alcohol in average, but there's a big caveat for people with a psychiatric background in their family, as the risk of drug induces mental health issue (schizophrenia) can be very high in that case.
Even then it's not. Alcohol is remarkably bad for you.
That doesn't mean alcohol isn't bad.
I found out, years after deciding to cut back on my coffee intake, that my dose was in that range: when the only thing you drink is instant coffee (as in, not even plain water) and you're making it will multiple table spoons per cup instead of teaspoons…