In my experience, visibility is key, not separation from other traffic. But visibility depends on terrain and speed as well. On larger roads with higher speed limits, drivers don't have as much time to spot you and react to you as on city streets at lower speeds. And of course there's other traffic, intersections, the number of lanes etc. On the other hand, on quiet residential streets people can be very wandery and inattentive, so that fast bike riding is not a good choice. And my choice of route is usually a compromise between the fastest and the safest route. All in all i can't say that in general I would choose a bike lane over the road, depends on too many factors. Really good and separate and efficient bike paths are wonderful, and about as rare as white rhinos.