The common theme when moving up the 'stack' (whether it be technology or academic disciplines as in the graphic) is that complex systems (whether OS or biology) are not just a sum of their parts and have emergent properties you cannot predict just by looking at the building blocks.
You can set the maximum speed to the max. While this doesn't directly control on-ramp flows, the cars already on the highway will be moving much faster than cars on the on-ramp due to acceleration/distance constraints. Then a jam ensues