Collision velocity between d₁ = 2 and d₂ = 5 km planetesimals as a function of the semimajor axis a for the different values of model parameters varied one at a time (with respect to the fiducial set of parameters) and shown in the subplot legends. The fiducial value of the varied parameter in each subplot is shown with the solid black curve. The dotted line in panel (a) shows the collision velocities in the limit that σ_i = 0.