From a social-science passage: "Economists once treated traffic as a fluid: pour in more cars, widen the channel, and flow improves. The metaphor was seductive because it was tractable—fluids obey equations. Yet drivers are not water molecules; they anticipate, hesitate, and route around what they expect others to do. When a new lane opens, some who had avoided the road return, and the freed capacity quietly refills. The fluid model is not wrong so much as it is innocent: it cannot imagine that the channel might widen the river."