2: Slow traffic needs to pull off the road.
Problem: Terrible accidents happen when people get stuck behind slow traffic such as a tractor or a 50 km/h vehicles.
People will want to overtake, take risks, misjudge and the resulting head on collisions are usually very severe.
Solution: When slow traffic is required by law to pull off the road when the have accumulated 5 or cars or more, at the first safe available possibility, then the drivers behind them know that they will never be stuck for a very long time. It will therefore not make a lot of sense to try a high risk move.
After all the cars have passed the tractor can continue on its way, accumulating a new set of cars, buses and trucks.