Along I10 in Louisiana, these signs pop up every now and then. There doesn't seem to be any rhyme or reason why they would want trucks in the left lane. Is everyone supposed to coast in the left lane and pass on the right?
I think it is the short way of saying: "The right lane up ahead is deteriorating badly and we need you to move your heavy axle vehicle to the left lane while we save enough money to fix the highways (assuming we do not spend it on a football recruit first)"
It's probably in areas where temporary widening has been done for construction phasing. They need to get the axle weight off the temporary areas as they're unraveling.