  The automaton known as `Langton's ant' exhibits a dynamical transition from a
disordered phase to an ordered phase where the particle dynamics (the ant)
produces a regular periodic pattern (called `highway'). Despite the simplicity
of its basic algorithm, Langton's ant has remained a puzzle in terms of
analytical description. Here I show that the highway dynamics obeys a discrete
equation where from the speed of the ant ($c={\sqrt 2}/52$) follows exactly.
