> But regularly bikers decide, for no discernible reason at all, to use the main road anyway, slowing down traffic.

Bicycles are vehicles. They have the same right to use the road as cars. Cars have no right to a minimum speed on surface streets. Cars have an exclusive right to freeways. High rates of speed are poor etiquette on bicycle routes, in my neighborhood only the fast cyclists ride in the street but at speeds that would be inappropriate on the bike path in that area.

Bicycle infrastructure tends to be an afterthought, even in the best cases. Can you describe this bike lane in more detail? Is it completely isolated from traffic? Is it mixed use with pedestrians?

Here in Seattle the divided bike lanes run parallel to roads with a curb and parking between. This seems good but at intersections bike lanes still cross side streets. That creates an opportunity for (especially right) turning traffic to not see cyclists and turn into them when the bicycle is crossing, even with dedicated bicycle lights. Riding in the street is your best chance to be seen, and on a bicycle being seen is your best chance to stay alive.