The devil is in the details. Some back streets can be dangerous with cars pulling in and out of drive ways, car doors opening in front of you, clueless people walking into the street between parked cars, etc. Some highways have nicely paved shoulders or bike lanes that are pleasant enough even with the cars speeding by. So it really depends on the specifics of the road and the rider to determine the best route and not a question of whether back streets or arterials are better. Sure, all things being equal we want to ride on quiet roads with little traffic but if that means a stop sign at every intersection we may be willing to put up with more traffic to avoid all the stopping. What I dislike the most is not traffic but poor road surface quality.