For example, when you're on a freeway or highway and there are multiple lanes where traffic could flow freely, drivers tend to adapt their speed to the others around them, resulting in clumps of cars that shouldn't necessarily be clumped up. (Different from jams caused by traffic lights, accidents, and so forth.)
I was discussing this with someone a while back and they said they thought it was passive-aggressive; an attempt to prevent others from passing them. I think it's probably more caused by our unconscious motivation to fit into the surroundings/ adapt to those around us- an inborn social trait. I can't find any actual scientific research on this though.