Section 4. Whenever on any way, public or private, there is not an unobstructed view of the road for at least four hundred feet, the driver of every vehicle shall keep his vehicle on the right of the middle of the traveled part of the way, whenever it is safe and practicable so to do, except that the department of highways may alter this provision by the use of restrictive pavement markings in areas of limited sight distance, at intersections and at obstructions in the highway, on state highways, on ways leading thereto and on all main highways between cities and towns; and may by permit, revocable upon notice, authorize cities and towns to alter said provision by the use of such restrictive pavement markings; provided, that such markings shall be in accordance with accepted standards of engineering practice; but, notwithstanding the foregoing provisions, every driver of a slow moving vehicle, while ascending a grade shall reasonably keep said vehicle in the extreme right-hand lane until the top of such grade has been reached.