One factor that you may not be considering is that by routinely posting overly low speed limits, the belief that the posted speed limit is the "moral minimum" s

One factor that you may not be considering is that by routinely posting overly low speed limits, the belief that the posted speed limit is the "moral minimum" speed or that it's okay to exceed it by a certain amount is reinforced. The speed limit is supposed to be the upper bound. By over using low speed limits, we indirectly increase the danger level for vulnerable road users because of the reasons detailed above.