by the numerical simulations of @YoudinJohansen2007 (hereafter YJ07) and @JohansenYoudin2007. @BaiStone2010a confirmed that in the nonlinear regime, dust densities up to three orders of magnitude above the background could be attained, which @Johansenetal2007 argued would suffice to instigate gravitational clustering. As relatively large (10 cm - 1 m) particles are needed for significant clumping [@BaiStone2010a], it remains to be demonstrated whether solids can have grown this large beforehand [@Leeetal2010b].

Although much attention has been devoted to the streaming instability in the past years, its physical interpretation in the *linear* phase remains somewhat obscure. For example, as YJ07 point out, the idea that radial drift slows down in overdense regions and leads to a sort of ‘traffic jam’, does not explain the onset of growth in the linear phase —such reasoning leads, in fact, to stable wave propagation. Such traffic jams, however, appear to be relevant for the nonlinear phase (JY07). JY07 showed that drafting, analogous to pelotons in bicycle races, also was not necessary for the initial generation of particle overdensities, although it could enhance growth if present. CY10 discuss a one-dimensional vertically integrated toy model drawn from @GoodmanPindor2000, in which a (single) fluid is subject to a “collective drag” 