  An excessive demand of vehicles to a motorway bottleneck leads to traffic
jams. Motorbikes are narrow and can drive next to each other in a lane, or
in-between lanes in low speeds. This paper analyses the resulting traffic
characteristics and presents numerical scheme for a macroscopic traffic flow
model for these two classes. The behavior included is as follows. If there are
two motorbikes behind each other, they can travel next to each other in one
lane, occupying the space of one car. Also, at low speeds of car traffic, they
can go in between the main lanes, creating a so-called filtering lane. The
paper numerically derives functions of class-specific speeds as function of the
density of both classes, incorporating flexible lane usage dependent on the
speed. The roadway capacity as function of the motorbike fraction is derived,
which interesting can be in different types of phases (with motorbikes at
higher speeds or not). We also present a numerical scheme to analyse the
dynamics of this multi-class system. We apply the model to an example case,
revealing the properties of the traffic stream , queue dynamics and class
specific travel times. The model can help in showing the relative advantage in
travel time of switching to a motorbike.
