We present a simple leader follower tracking controller for autonomous vehicles following straight paths. The difficulty of this problem lies in the fact that the system is not controllable. We show that global tracking may be achieved with a controller which has a property of persistency of excitation, tailored for nonlinear systems. Roughly speaking the stabilization mechanism relies on exciting the system by an amount that is proportional to the tracking error. Then, the method is used to solve the problem of formation tracking of multiple vehicles interconnected on the basis of a spanning-tree topology i.e., each vehicle communicates with one leader and one follower only.