One fundamental question to properly apply an MFD-simulation is the proper definition of the trip lengths for the vehicles inside each reservoir. In the literature, it is usually considered a common average trip length for all vehicles traveling inside the same reservoir. This is a strong assumption and clearly impacts the simulated traffic states. In the literature, there are no studies that discuss the calculation of the macroscopic trip lengths based on information of trips on the microscopic network. We then propose to investigate different approaches to calculate the macroscopic trip lengths based on information of trips on the real (microscopic) network. These approaches differ from each other based on how we filter the microscopic trips at the reservoir level.