As a practical matter, I agree completely with yoijimboguy:
As an ethical matter (and in reply to the thread’s subject), in most cases it isn’t your place to try and stop someone else from breaking the law. The only time that I would consider it your responsibility is (a) when you have a sufficient grasp of what’s going on in a situation, and (b) you have a responsibility to the person who might be harmed.
For example: if you see two people brawling in the middle of the street, you don’t have an immediate ethical obligation to break it up. You don’t know what the fight is about; some guy you see getting beat up just might deserve it. Contrast this with the rape case Goo mentioned; in that situation it was pretty clear what was happening, and assuming that what was happening was clear from your vantage point, you might have an ethical obligation to step in (or at least, it would not be unethical for you to).
You might also have an ethical obligation in that case because the girl is a person who, presumably, it is within your power to help; generally, I would say you have an ethical obligation to help another person who is in serious jeopardy unless you or some other innocent person might suffer serious harm in attempting to do so (or unless there are other special or mitigating circumstances). In a rape situation, it’s clear whose rights are being violated by whom. In the brawl case, that is not clear, and since your involving yourself might place you in the situation of actually violating someone else’s rights mistakenly, it would be unethical of you to do so.
I would argue that for my first requirement; for the second, I would say that there are some people you owe a responsibility to, and some that you don’t. You probably would have a responsibility to protect any innocent person that you encountered from death or serious injury (again, barring mitigating circumstances), but beyond a situation that obvious, your responsibility is limited. Would you report someone you knew to be cheating on her taxes? Would you tell someone not to loan his stuff to someone else you knew to be irresponsible? Is it your responsibility to tell your aunt not to lend your cousin money because you know he’ll just use it to get high?
So generally, to determine whether or not you have an ethical responsibility in a situation, you need to be sure you know enough about it to be able to make a sound judgment, and it has to be a situation where it’s actually your business to be doing anything at all.
As for the question about driving, I would say you have no responsibility whatsoever to move if you are driving the legal limit and someone wants to go past. You might want to do it for practical considerations (road rage, personal safety, etc.), but not because you’re ethically required to do so. In that sense, I don’t agree that the objections presented would force you to move.
For the objection about the law requiring slow traffic to move over, I would agree that that is meant for traffic moving under the speed limit. If you’re travelling the speed limit, you are the “faster traffic.” If traffic coming faster than you is riding your tail, they’re violating the law; you’re not mandated to give up your right-of-way to someone who legally doesn’t have it. (Again, practical considerations may make you want to do this, but I would argue that the law doesn’t.)
Also, in no way does faciliating speeding by moving out of the way make you responsible for something that might happen as a result. YOU aren’t speeding. If you weren’t there at all, they would still be speeding. Your presence is entirely incidental to their speeding or what happens as a result of it; YOU are not responsible for the consequences of an act you had no willing part of, ethically or otherwise (their speeding, not your allowing them to)—at least, not in a case like this. As was mentioned, you aren’t law enforcement. Taking that responsibility onto yourself is not a requirement and almost assuredly is more trouble than it’s worth.
For a similar reason, just because you don’t know WHY they’re speeding does not mean you must give up your right-of-way to allow them to do it. There IS no way for you to know, unless they’re waving some kind of sign or something. Without the system of right-of-way driving would be chaotic; you are not under any obligation to yeild it to someone because they MIGHT have a good reason to be in a hurry.
I would definitely agree with Goo: