Everyone has their own sensitivity to traffic, road surface, terrain, remoteness, willingness to travel to the start, point-to-point vs loop, etc. So it's hard.
Strava global heatmap tells you where people ride, but more importantly where they don't ride (don't go there). Generally the heavier the line the safer, but not necessarily so.
I use RWGPS and Strava, but ultimately just go ride.