> By this logic they should let you drive drunk until you hit someone too.
I have said no such thing. The only implication of my position for drunk driving is that the law should not be able to punish you for it if you cause no harm. But that in no way means that, for example, a cop who sees you driving erratically can't pull you over, give you a breathalyzer test, find out that you're too drunk to drive, and lock your car, making sure that it's safely parked, and then ask you who to call to come pick you up and take you home. He just can't write you a ticket that forces you to either come to court or pay a fine.
It also in no way means that other people have to let you drive drunk. Bars don't have to sell you drinks if you're going to drive. Friends don't have to let you drive drunk. And in a sane society of responsible adults, those kinds of preventions work better than any law possibly can. The fact that our society has so many nanny state laws that micromanage all aspects of life is a sign that we don't live in a sane society of responsible adults.
> I think doing things in your car that have substantial and easily preventable risk to other people should not be allowed
I think you are way too ready to give power to the government, which will abuse it, and the costs of that abuse of power will far outweigh any possible benefit from such laws.
I am not sure why a cop pulling you over for driving erratically is any different in principle than a cop pulling you over for traveling at an unsafe speed (again, for the purposes of this example, imagine a truly unsafe speed, not 5 mph over).
First, if a cop pulls you over for speeding, it's not for "traveling at an unsafe speed". It's because you were going faster than the posted limit. The law says nothing whatever about such a speed being unsafe. In practice, cops usually don't pull people over just for speeding, but only for going significantly faster than the general flow of traffic. But that's only usually; cops in most jurisdictions have monthly or quarterly ticket quotas, and if it's getting close to the end of the quota period and they're behind on tickets, it's simple to go out to places where people routinely speed and start pulling everyone over. I have seen this happen.
"Driving erratically", unlike "going faster than the posted speed limit", is always a subjective judgment on the cop's part (in most jurisdictions it's something like "reckless driving" or "driving in a clearly unsafe manner"), but the whole point is that it's supposed to be rare; by definition most people on the road will not be driving recklessly and this kind of law can't be abused in the way I described above just to make a ticket quota.
However, the important difference I was talking about was not between speeding and "driving erratically", it was about under what circumstances the cop can write you a ticket that forces you to either pay a fine or come to court. Under our current laws, a cop can do that whenever he pulls you over--there will always be some traffic law you were violating. ("Driving erratically", or something similar, in itself is a traffic violation under current law.) Under the libertarian system I have been describing, the cop could not write you a ticket that forces you to either pay a fine or come to court unless you actually caused harm. He could take steps to prevent you from driving if you hadn't caused harm but were clearly not capable of driving safely, by an objective test (such as a breathalyzer), but he could not impose a fine or court appearance on you.
My entire point here is that just because the system now allows the cops to be capricious doesn't mean we have to throw the baby out with the bathwater and say there is no speed that is so unsafe that it warrants punishment.
I have said no such thing. The only implication of my position for drunk driving is that the law should not be able to punish you for it if you cause no harm. But that in no way means that, for example, a cop who sees you driving erratically can't pull you over, give you a breathalyzer test, find out that you're too drunk to drive, and lock your car, making sure that it's safely parked, and then ask you who to call to come pick you up and take you home. He just can't write you a ticket that forces you to either come to court or pay a fine.
It also in no way means that other people have to let you drive drunk. Bars don't have to sell you drinks if you're going to drive. Friends don't have to let you drive drunk. And in a sane society of responsible adults, those kinds of preventions work better than any law possibly can. The fact that our society has so many nanny state laws that micromanage all aspects of life is a sign that we don't live in a sane society of responsible adults.
> I think doing things in your car that have substantial and easily preventable risk to other people should not be allowed
I think you are way too ready to give power to the government, which will abuse it, and the costs of that abuse of power will far outweigh any possible benefit from such laws.