This paper is concerned with the traffic flow control using the concept of flatness. The proposed approach exploits the bond-graphs formalism in the macroscopic modeling of the traffic flow. The main advantage is that it allows to strongly simplify the analysis techniques, offering a physical comprehension of the dynamical behavior of the studied system. Indeed, the obtained model is on the form of space discretized continuous lumped parameters. Starting from the mathematical equations deduced directly from the bond-graph model, we propose a flatness based control algorithm in order to regulate the flow at the mainline section of road. The same method can be used to develop isolated and coordinated ramp metering algorithms