  We consider distributed consensus and vehicular formation control problems.
Specifically we address the question of whether local feedback is sufficient to
maintain coherence in large-scale networks subject to stochastic disturbances.
We define macroscopic performance measures which are global quantities that
capture the notion of coherence; a notion of global order that quantifies how
closely the formation resembles a solid object. We consider how these measures
scale asymptotically with network size in the topologies of regular lattices in
1, 2 and higher dimensions, with vehicular platoons corresponding to the 1
dimensional case. A common phenomenon appears where a higher spatial dimension
implies a more favorable scaling of coherence measures, with a dimensions of 3
being necessary to achieve coherence in consensus and vehicular formations
under certain conditions. In particular, we show that it is impossible to have
large coherent one dimensional vehicular platoons with only local feedback. We
analyze these effects in terms of the underlying energetic modes of motion,
showing that they take the form of large temporal and spatial scales resulting
in an accordion-like motion of formations. A conclusion can be drawn that in
low spatial dimensions, local feedback is unable to regulate large-scale
disturbances, but it can in higher spatial dimensions. This phenomenon is
distinct from, and unrelated to string instability issues which are commonly
encountered in control problems for automated highways.
