N.H. Rev. Stat. Ann. § 266:28

Every motor vehicle and every combination of motor vehicle with trailer or semi-trailer when driven upon the ways of the state shall at a speed of 20 miles per hour be capable, at all times and under all conditions of loading, of stopping on a dry, smooth, approximately level pavement free from loose material, upon application of the foot or service brake, within a distance of 30 feet.