Rule 2(b) of the Navigation Rules (aka the General Prudential Rule) says that a vessel may depart from the Rules to avoid a collision only when two conditions are met:
(1) the circumstances must be special and
(2) the danger must be immediate.
The Rules don’t define “special” or “immediate.” Interpretation is complicated and argued by lawyers in marine accident court cases. Read Rule 2(a,b) at www.navcen.uscg.gov/?pageName=navRulesContent, and see the 11/11/13 post.