Determining travel times on arterials is crucial for urban traffic management. Travel times strongly depend on traffic conditions and tend to highly increase when congestion appears. Most existing works consider congested situations that occur due to peaks of the upstream demand and not to downstream spillbacks. Only the second option leads to fully congested situations on the whole arterial. This paper explains that considering imaginary voids instead of vehicles is more natural in congestion. Solving the problem becomes then easy. Voids travel toward update stream. They move at the backward wave speed when passing each other a traffic flow at capacity, stopping in absence of vehicle, and traveling at a reduced speed when encountering a generic traffic flow. Using this concept, an existing method of travel time computation is extended to congestion. In free-flow conditions, this method consists in determining the separation between vehicle groups at each signal of the arterial. The main point to apply this method in congestion is to consider the voids instead of vehicles. Besides, this paper generalizes the Flow-Platoon Diagram (FPD) concept introduced in a previous work. It aggregates all possible travel times of vehicles in all possible stationary conditions on an arterial. Moreover, it easily leads to an exact estimation of the Macroscopic Fundamental Diagram. Input data needed to use the proposed approach are fundamental diagram and signal timings. After that, FPD provides travel times according to the flow from loop detectors.