The motion of atomic steps was studied using ultraflat Si(lll) substrates which have flat terraces of up to 50 mu m in width separated by monoatomic steps. The motion is revealed in time lapse movies made by in situ scanning electron microscopy. When samples are heated to high temperatures (similar to 1000 degrees C) the surface erodes in a step-flow mode: steps retreat in an orderly fashion, one after the other. Here, steps are forced to collide constructively, providing a means to probe step interactions. Diffusion theory is used to explain the collision dynamics, allowing the extraction of physical parameters. After colliding, double steps continue to move at speeds simply related to the precursor steps' velocities.