I correct people doing the speed limit in the left lane regularly with my horn.
Seriously, though, my rule of thumb is to mind my own business. I did break it one time when a very young and very pregnant girl was lighting up outside a restaurant. I doubted it would do any good but I did it anyway.
My husband once pulled over to, ahem, correct a guy who was slapping a woman around right in the middle of a sidewalk. I doubt that did any good either.
Overall I’d say even with best intentions you’re unlikely to make a difference in most situations. You can influence people with whom you have a rapport, not strangers.