It may be that these trucks are moving far slower than other cars, because they are on a steep grade and trucks are less able to go fast up a hill. In that case the car should accept the risk, otherwise it should remain in the cluster. The driver of the car has a speed relationship with other cars that is brokered by the intermediary trucks. The truck ahead may be matching its speed with a car ahead of it, but then again it may not. The relationship between the car in front of the truck and the car behind the truck is a combination of the relationship between the two pairs.
Transitivity determines how the direct relationships are combined into an indirect relationship.
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