  We employ an effective kinetic description, based on the Boltzmann equation
in the relaxation time approximation, to study the space-time dynamics and
development of transverse flow of small and large collision systems. By
combining analytical insights in the small opacity limit with numerical
simulations at larger opacities, we are able to describe the development of
transverse flow from very small to very large opacities. Suprisingly, we find
that deviations between kinetic theory and hydrodynamics persist even in the
limit of very large opacities, which can be attributed to the presence of the
early pre-equilibrium phase.
