Highway clumping

This weekend, while driving, as I am wont, five miles per hour below the speed limit, as imperturbably as I can manage, I turned over in my head a “human math” problem that has puzzled me for years, namely: Do automobile drivers clump? In other words, do drivers unconsciously (or consciously) bunch up on the highway, instead of spreading out to take the full rational advantage of the available timespace on the road?

From my vantage, consistently and irksomely five mph slow, it seems as if they do. For five or even ten minutes, our car will be more or less alone, the only vehicle in the stretch of road visible before and behind us. Then we’ll be overtaken by a group, who will spend some time maneuvering around pokey me, until, having overcome the impediment, they zoom off, and there is peace again.

My first question is, Are the cars really bunched up, or do I merely perceive them to be? Perhaps the cars are randomly spaced, and it is my mind that groups them. Presumably this question could be answered with time-lapse aerial photography and some undergraduate statistics.

My second question is, If the cars are bunched up, why? Is the cause traffic lights? They gather cars up and then release them en masse, and maybe they leave a residual “groupiness” in traffic even miles down the road. Or is bunching a way to dodge speeding tickets? Maybe drivers feel that in a group they are less likely to be singled out by a state policeman.

Or is the cause some vestige of evolutionary programming within drivers? Maybe humans like to drive in groups, because they share a bit of mental code with birds that fly in V-formation and wolves that hunt in packs. In that case people may speed up or slow down until they are travelling in a configuration that feels right to them. And perhaps within that configuration they position themselves — in the high-testosterone avant-garde, in the safe middle, or straggling behind — in a way that corresponds to the social role they believe they inhabit.