On a 2-lane highway where passing is unsafe because of traffic in the opposite direction or other conditions, a slow-moving vehicle, behind which 5 or more vehicles are formed in line, shall turn off the roadway wherever sufficient area for a safe turnout exists, in order to permit the vehicles following to proceed. As used in this section, a slow-moving vehicle is one which is proceeding at a rate of speed less than the normal flow of traffic at the particular time and place.