Looking at materials from the U.S. Bicycle Route System I noticed one comment about a deteriorated section of side path beside what I take from the context to be a busy road. The commenter suggested shifting the route to a regular road with lower traffic volume because the bike path never gets repaired. A road would.
Lower traffic volume does not always equal more safety. In many cases the drivers who use it are doing an end-run around congestion elsewhere. They may be zooming because they can, or to make up for the extra distance they might have to travel to use the less congested road.
On a beautiful little road in Hiram, Maine one day, I saw one car in probably six or eight miles. It was literally airborne off a small rise. This small, sporty VW product was doing about 80 on a road the driver obviously knew well and used frequently. The next time I rode the road, at roughly the same mid-day hour, I saw the same car driven the same way. Perhaps the driver goes to the same place for lunch every day.
The small road section of my commute, between my house and the state highway, can be some of the most nerve wracking. In the morning I'm warming up. In the evening I'm tired. It's a bendy, hilly three miles. Some of the locals drive it like a race course. Nearly everyone speeds, cell phone or beverage in hand.
When devising routes we can only do our best. Lower volume is generally better unless the choice is between enough motor vehicles to clot them up or just enough less traffic to let the meat grinder run at high rpm. And there's no accounting for the random missiles on roads where vehicles are downright sparse.