Abstract
Probing a die-hard traffic congestion controversy, this paper scrutinizes two key variables, density and flow, under equilibrium versus optimal states. We find that optimization requires flow in equilibrium to decrease under mild congestion, but flow must increase under hyper-congestion. However, while under hyper-congestion flow should be increased by decreasing density, a mild congestion requires flow to be decreased by decreasing density. Thus, in flow of vehicles should always be discouraged to either increase or decrease flow of vehicles for economic efficiency. Moreover, even when optimal policy, given traffic demand, requires flow to increase, the optimal flow itself must decrease eventually as demand increases beyond a critical level.