Not empirical because there's no money to fund such studies, but: Mandatory veganism would almost certainly significantly reduce the healthcare (and food) costs for a society by a large fraction. Banning cars would make the air cleaner, remove a bunch of health problems, and facilitate more social forms of travel which would help form a more cohesive society and probably would reduce a lot of problems which arise from modern urban isolation.

I'd personally like to live in a society where people were mostly vegan, there were no cars, and there were no firearms.

Whether or not we should force that is another question, but it sure sounds nice.

Mandatory veganism isn't going to prevent kids from being murdered in their schools, which is the salient issue.

It might, due to the reduction of testosterone.

Yes, as I pointed out in an answer to a sibling comment, the effects of global veganism and car-lessness could indeed be positive—that's why they are lofty ideals. My answer was challenging the suggestion that “the same argument” from the original commenter could be used to argue for mandatory veganism or the banning of cars. In other words, the forced bit.

Not advocating for it but _eventual_ transition to veganism would definitely be a net positive (lower carbon emissions, land and water use, horrors of factory farming, mental health of the factory workers, exploitation of immigrant labor, etc.). Most of the arguments I've seen from the Veganism movement seem to have sufficient scientific backing.

Yes, that is surely the case. Likewise, a great reduction in the need for cars and their usage would also be a net positive for society and the environment. However, what I was challenging was the assertion that mandatory veganism or banning cars were comparable to effective gun regulation as policies based on empirical evidence instead of lofty ideals.

Veganism to reduce carbon emissions and animal suffering, a car-less society to improve communities and public health, and the right to bear arms for the security of a free state are all lofty ideals. However, we have no examples or reasons to believe that enforcing the first two by decree is even feasible, nor can we imagine the unpredictable side effects. On the other hand, there are many examples of states where gun ownership is heavily regulated, restricted, or virtually banned which however remain secure and free and with no obvious downside.