We propose a new functional form for the 3D-MFD. The 3D-MFD links the number of cars and buses in an urban network to the total travel production. The parameters of the functional form are derived from the structure and topology of the road and bus network. The physical interactions of vehicles are described by a single parameter. We apply the methodology to two empirical data sets from London and Zurich, and discuss policy relevant applications of the functional form
