I. The driver of any vehicle when traveling upon a downgrade, other than motorcycles or private passenger automobiles, as defined in RSA 259:80, I, traveling under 15 miles per hour, shall not coast with the gears of such vehicle in neutral.
II. The driver of a truck or bus when traveling upon a downgrade shall not coast with the clutch disengaged.
III. The fine for a violation of this section shall be $100.