Maxwellian distribution of relative speeds for mm-sized particles (τ_f = 0.003, pressure support Δ = 0.05 and solid concentration Z = Σ_solid/ Σ_total = is 0.074). For particles separated by distance d< 10^-5H (H is the disk scale height) the Maxwellian distribution has characteristic speed â ~ 0.3 cm/s (red). Extrapolating as d → 0 we obtain a conservative estimate of â₀ = 0.05 cm/s (blue). For reference, we include the sticking, bouncing and fragmenting regions from Güttler et al. (2010), and the root-mean-squared speed v_rms for all the particles in the simulation. Note that the shape of the curve is distorted by the log scale; in particular, the great majority of collisions are in the bouncing regime. Also note that v_rms is a poor estimate for the the collision speeds. The value â is obtained by the maximum likelihood method.