Every motor vehicle transporting passengers, for hire, every school bus, and every motor vehicle transporting high explosives, or poisonous or compressed inflammable gases, and every motor vehicle used for the transportation of inflammable or corrosive liquids in bulk, whether loaded or empty, shall, upon approaching any railroad grade crossing, other than a crossing that is specifically exempted from the stopping requirement by order of the division of motor carrier and railroad safety of the department of economic development, be brought to a full stop within fifty feet, but not less than fifteen feet, from the nearest rail of such railroad grade crossing, and shall not proceed until due caution has been taken to ascertain that the course is clear, except that such full stop shall not be required at a streetcar crossing within a business or residence district, nor at a railroad grade crossing protected by a watchman or traffic officer on duty or by a traffic control signal (not railroad flashing signal) giving positive indication to approaching vehicles to proceed, nor when the division of motor carrier and railroad safety has ordered the placement of an exempt sign at the crossing.