I set my speed based on those signs unless conditions warrant slower. I'm not in rush to get into an accident - Yes, people are annoyed I travel at or bellow the limit, but I drive in the right lane or use pull offs if five vehicles stack up behind me on two-lane roads.
Someone that feels they are exempt from following emission restrictions, should have their vehicle impounded. Fines should be set high enough that arguments about higher mileage or increased engine longevity will be negated by the penalty imposed and should cover environmental reclamation from the pollution released.