The point isn't that his routes are dangerous, it is that apparently no one has told him.
When I plan my own routes for rush-hour traffic I will go well out of my way to make sure left turns are at lights, or stretches of road so long and flat and straight I can see anyone ... because I am pretty sure the drivers are reading their texts.
On the Sunday slow ride group route there is one unavoidable stop sign at the bottom of a hill---but the route goes straight, and there is space enough for a group to wait. There is another left at the top of a hill, but it is a very low traffic zone with great lines of sight and again, room for a group to wait, and to be easily seen by cars approaching from behind.
Maybe on the next ride, crash into him at the bottom of the descent and push him into traffic ... I mean Tell the guy that his route-planning is occasionally dangerous. Maybe he just hasn't realized how much more important that stuff is with a group ("What's the matter? I ride that road solo every day.")