Obscenity has a specific legal meaning in the US:
the average person, applying
contemporary community standards finds
that the work, taken as a whole,
appeals to the prurient interest; the
work depicts or describes, in a
patently offensive way, sexual conduct
specifically defined by applicable
state law, and the work, taken as a
whole, lacks serious literary,
artistic, political or scientific
Obviously, people aren't constrained by the legal definition of words any more than by the dictionary definition, but personally, I try to hew close to the legal definition whenever it might seem I am offering an opinion on a legal matter.
So traffic might be "murder" and my boss's salary might be obscene, but a death in a traffic accident is "manslaughter" at worst and "shit happens" is merely vulgar.