Neb. Rev. Stat. § 60-6,365: Diesel-powered motor vehicle; smoke; shade, density, or opacity.

No one shall operate a diesel-powered motor vehicle on any highway in this state in such a manner that smoke discharged from the exhaust is of a shade or density equal to or darker than that designated as Number 1 of the Ringelmann Chart or equivalent opacity of twenty percent for ten consecutive seconds or longer.