Section 4B. Upon all ways the driver of a vehicle shall drive in the lane nearest the right side of the way when such lane is available for travel, except when overtaking another vehicle or when preparing for a left turn. When the right lane has been constructed or designated for purposes other than ordinary travel, a driver shall drive his vehicle in the lane adjacent to the right lane except when overtaking another vehicle or when preparing for a left or right turn; provided, however, that a driver may drive his vehicle in such right lane if signs have been erected by the department of highways permitting the use of such lane.