Separating different types of traffic could be a good idea, but there's no free lunch in physics. Higher speeds require more energy, period. Heavier loads require more energy, period. Wasting momentum due to curvy roads or traffic is inefficient, but higher speeds don't somehow make up for that.

Maybe speed limits are the wrong idea - you probably do want trucks to be able to speed up above their cruising speed before going up a hill. But dedicated lanes and tele-op could set up an incentive structure that favors a lower speed without any laws.