Time evolution of the timescale for planetesimal–planetesimal collisions for the nominal HL Tau simulation. The collision timescale was calculated using Eq. (20) and using an initial planetesimal mass of 281 M_⊕, following the results from simulation #4 of Paper I. The dotted line marks where the collision timescale equals the time of the simulation. Even though the collision timescale for one planetesimal can be as low as 10⁴ yr, it is still roughly an order of magnitude larger than the actual simulated time, meaning that planetesimal-planetesimal collisions can be safely ignored in our simulations as a source of dust production.