(a) No person shall operate a motor vehicle at a speed lower than forty miles per hour on any limited access divided highway and no person shall operate a motor vehicle on any other highway at such a slow speed as to impede or block the normal and reasonable movement of traffic except, in either case, when reduced speed is necessary for safe operation or in an emergency, or in compliance with the law or the direction of an officer. The provisions of this section shall not apply to (1) maintenance vehicles or equipment of the state or any municipal highway department, or to such vehicles or equipment of a contractor under contract with any such department while engaged in maintenance operations; (2) any motor vehicle with a commercial registration which while traveling on any limited access divided highway is unable to maintain the minimum speed limit of forty miles per hour due to the gradient, or to any such vehicle which while traveling on any other highway is being driven at such a slow speed as to obstruct or endanger following traffic, provided the operator thereof employs flashing lights on such motor vehicle.

(b) The operator of any motor vehicle having a gross weight of more than twenty-five thousand pounds shall also employ flashing lights when the vehicle is traveling on a limited access divided highway and maintaining a speed higher than forty miles per hour but lower than the speed of the traffic on the highway due to the gradient.

(c) Violation of any provision of this section shall be an infraction.