I look for routes with lower speed differential (between me and other vehicles) and lower traffic density. If high speed and/or high density can't be avoided, then I look for a shareable width outside lane, preferably one that has not been defaced with a bike lane.
The density and type of junctions (cross-streets, driveways, controlled vs. uncontrolled intersections, etc.) can also be a factor.
If the temperature is below 60 degrees F (which eliminates 90% of the obliviots), if my destination makes it an appropriate choice, and if I'm not in a hurry, I might hop onto the MUP for some of the journey. Being committed to following the rules for drivers of vehicles and enjoying a quiet ride away from car fumes are not mutually exclusive propositions.