mean field theory prediction [@Farrell2012], stating that the flocking threshold does not depend upon $\text{Pe}$, should not be expected.

Clustering {#sec:cluster}
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![Illustration of the clustering mechanism due to alignment interactions (left) and active brownian motion in the presence of excluded volume interactions (right). The grey shaded area around the particles in the left panel represents the alignment-interaction area (of radius $R_{\theta}=2\sigma$). []{data-label="aline"}](cartoon.pdf)

After the description of the phase behaviour of the model in the $(\mbox{Pe},\,g)$-plane, we turn now into the characterization of the different self-sustained structures emerging in the system, in particular in the MC regime, where band- and lane-like patterns coexist.

As it has now been widely reported, active particles show a tendency to aggregate in the absence of inter-particle attractive interactions. Clustering in our model comes from the natural competition between self-propulsion, alignment interaction and excluded volume. As illustrated in Fig. \[aline\], two different aggregation mechanisms are involved: a first one driven by velocity alignment, as in Vicsek models, and a second one dominated by the self-trapping of particles because of the persistence of their motion.

Let us focus first on the high coupling regime ($\text{g} \gg 1$). In this regime, when two particles are close-by, they 