The idea that weather conditions and driver- and vehicle-population characteristics affect a homogenous roadway segment's flow-versus-density relationship is explored here. The interaction of third-order polynomial regressions of flow on powers of density with a variety of explanatory variables suggests that driver, vehicle, and environmental attributes significantly influence the flow-density relationship and conform in substantial part with intuitive expectations. For example, higher flows are predicted across most densities for more mature and more male traveler groups as well as for nonrainy conditions with fewer long vehicles and trucks. Moreover, under highly congested conditions, braking is associated with slightly higher flows than those predicted for accelerating vehicles.