Whenever any highway has been divided into 2 roadways by an intervening unpaved or otherwise clearly indicated dividing space or by a physical barrier constructed to substantially impede crossing by vehicular traffic, the operator of a vehicle shall drive only to the right of the space or barrier and no operator of a vehicle shall drive over, across, or within the space or barrier except through an opening or at a crossover or intersection established by the authority in charge of the maintenance of the highway, except that the operator of a vehicle when making a left turn to or from a private driveway, alley, or highway or making a U-turn may drive across a paved dividing space or a physical barrier not constructed to impede crossing by vehicular traffic, unless the crossing is prohibited by signs erected by the authority in charge of the maintenance of the highway.