Whenever any highway has been divided into two or more roadways by leaving an intervening space or by a physical barrier or clearly indicated dividing section so constructed as to impede vehicular traffic, every vehicle shall be driven only upon the right-hand roadway unless directed or permitted to use another roadway by official traffic-control devices or police officers. No vehicle shall be driven over, across or within any such dividing space, barrier or section except through an opening in such physical barrier or dividing section or space or at a crossover or intersection as established, unless specifically prohibited by public authority. For clarification, a left turn across a painted median is authorized unless prohibited by an official traffic-control device.