We report a route to highly aligned, vertical arrays of GaN nanowires in which the degree of vertical alignment is improved via collisions between nanowires during growth. An investigation of the initial growth process indicates that in addition to vertically aligned nanowires a significant fraction of tilted nanowires also nucleate, the density of which appears to sharply decrease with growth time. We attribute this decay in the density of tilted nanowires during growth to collisions with vertical nanowires, which terminate the growth of tilted nanowires shortly after nucleation. The experimentally observed tilted nanowire density evolution agrees well with a Monte Carlo model developed to simulate the collision process. The results show that at high nanowire densities this collision mechanism rapidly terminates the growth of tilted nanowires, leading to highly aligned, vertical nanowire arrays. (c) 2008 Elsevier B.V. All rights reserved.